Window Calculator
Does your bedroom window meet egress code? Is the glass next to your door required to be tempered? Will your replacement project pass IECC U-factor inspection in your climate zone? This free window calculator answers all three per opening — plus a full materials list for sill pan flashing, jamb flashing, head flashing, sealant, low-expansion foam, and roofing nails per ASTM E2112.
Every cost calculator on Google handles windows as a lead-gen page; every egress calculator handles them in isolation. Nobody combines per-opening IRC R310 egress qualification, R308 safety-glazing flags, IECC R402 U-factor compliance by climate zone, FBC TAS 201/202/203 HVHZ impact-glass triggers, and the current §25C tax-credit status (terminated by Pub. L. 119-21 for installs after Dec 31, 2025). This one does — and reports the verdict per window in a single row-level table.
Built on 2021 IRC R308 / R310 / R612, IECC R402 (2021 default, 2024 toggle), ASTM E2112 install practice, NFRC 100/200/500 rating procedures, AAMA/WDMA/CSA 101/I.S.2/A440 NAFS, EPA RRP rule 40 CFR 745, and IRC §25C as modified by the One Big Beautiful Bill Act. Free, no signup.
Window Calculator
Per-opening egress (IRC R310), safety glazing (R308), IECC U-factor compliance, flashing materials, and §25C tax-credit status in one place.
Project context
Not sure? Find your IECC climate zone by ZIP →
Federal §25C Energy Efficient Home Improvement Credit was terminated by Pub. L. 119-21 (One Big Beautiful Bill Act, signed July 4, 2025) for property placed in service after Dec 31, 2025. The checkbox above flips the calculator back into pre-sunset mode for historical installs.
Windows: flashing, replacement type & code
Three things make or break a window job: flashing it in the right order so it never leaks, choosing insert vs. full-frame replacement, and meeting the egress and safety-glazing code rules. These engineering-style diagrams cover each one — use the “see the diagram” links beside the inputs above to jump to the figure you need.
The flashing-order diagram is why the estimate carries sill pan, jamb, and head flashing in a set sequence. Each layer laps the one below, bottom-up, so water sheds out of the wall, and reversing the order sends water behind it. The flashing is small material that decides whether the window leaks, which is why the order is called out.
The replacement-path comparison is why the calculator asks insert versus full-frame. An insert keeps the sound old jambs with no new flashing but less glass, while a full-frame tears out to the studs for a new flashed window with the most glass. Same opening, different materials — which is why the path changes the estimate.
The egress-and-glazing diagram is why the calculator flags certain windows. A bedroom egress window needs a minimum clear opening and sill height, and tempered glass is required near doors, low panes, and wet or stair areas. These are code gates, not options, so the calculator surfaces them before you order.
Calculation Formulas
Operable sash only — the bottom sash on a double-hung, the bottom (or top) sash on a single-hung. Frame jambs eat ~2.5" each side; meeting rail + balances eat ~4.5" off the half-height. A 32"×60" double-hung nets about 4.8 sq ft and often fails IRC R310's 5.7 sq ft minimum.
Example:
32"×60" RO: (32−5) × (60/2 − 4.5) = 27 × 25.5 = 688.5 sq in ÷ 144 = 4.78 sq ft. FAILS 5.7 sq ft EERO.
Full sash swing minus frame and hinge gear. Casements deliver the most NCO per square foot of unit and are the easiest path to egress in small RO openings. Same RO as a failing double-hung often passes as a casement.
Example:
24"×48" RO casement: (24−3) × (48−4) = 21 × 44 = 924 sq in ÷ 144 = 6.42 sq ft. PASSES 5.7 sq ft.
Only half the unit width is operable. Same "slightly less than half" rule applies per Fine Homebuilding's Sept. 2006 Underwood reference.
Example:
60"×48" slider: (60/2 − 3) × (48−4) = 27 × 44 = 1,188 sq in ÷ 144 = 8.25 sq ft. PASSES.
All four conditions must be satisfied. Sill height is measured above the finished floor. The 5.0 sq ft exception applies only to grade-floor and below-grade openings — R310.2.1.
Example:
24"×48" casement: 6.42 sq ft ≥ 5.7 ✓, height 44" ≥ 24" ✓, width 21" ≥ 20" ✓. Passes if sill ≤ 44".
IECC permits area-weighted averaging across multiple fenestration products to satisfy the U-factor requirement. A small high-U feature window can be offset by larger lower-U bedrooms.
Example:
Three windows: 8 sq ft @ U-0.32, 12 sq ft @ U-0.28, 12 sq ft @ U-0.28. U_avg = (8×0.32 + 12×0.28 + 12×0.28) / 32 = 0.29.
Per ASTM E2112 the sill pan extends 6" up each jamb. Pre-formed (DuPont FlexWrap NF, Henry FortiFlash) or self-formed butyl.
Example:
36" RO: (36+12)/12 = 4 LF of sill pan flashing.
Jambs extend 6" above the head to lap behind head flashing. Head flashing extends 6" past each jamb. Both must be shingle-lapped with the WRB.
Example:
36"×60" RO: jamb LF = 2(60+6)/12 = 11 LF · head LF = (36+12)/12 = 4 LF.
Single bead of paintable polyurethane on the interior side, with closed-cell backer rod sized one diameter larger than the perimeter gap. No exterior bead at the sill — leave it open to drain per E2112.
Example:
36"×60" RO: 2(36+60)/12 = 16 LF perimeter sealant.
Manufacturer nailing schedules vary — typically 8" o.c. around the fin with extra at corners. Andersen, Pella, Marvin, and JELD-WEN all publish per-product schedules; verify before fastening.
Example:
36"×60" RO: 2(36+60)/8 = 24 nails minimum.
When a permit is pulled, the IECC version in effect on the permit date applies — regardless of the original window's specs. Like-for-like replacement does NOT exempt the project.
Example:
Zone 5, IECC 2021: U_max = 0.30, SHGC NR. If U_weighted = 0.29, PASS.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| IRC R310 NCO Minimum | 5.7 sq ft (5.0 grade-floor) | Net clear opening minimum for emergency escape and rescue openings. 5.0 sq ft exception applies only to grade-floor and below-grade openings. Net clear height ≥ 24", net clear width ≥ 20". |
| IRC R310 Sill Height | 44" max above finished floor | Maximum sill height for the EERO. Higher sills require a permanently affixed platform / step or relocation of the window. |
| IRC R310 Window Well | 9 sq ft horizontal projection, 36" min projection / width | For below-grade openings. The 9 sq ft is HORIZONTAL projection at grade, NOT interior floor area of the well. Ladder required when depth > 44" — most-cited inspector finding. |
| IRC R308.4.2 Safety Glazing Trigger — Doors | Within 24" arc of a door, bottom edge < 60" above floor | Glazing within 24" of a door swing must be tempered (or laminated, or wire glass) when the bottom edge is less than 60" above the floor. |
| IRC R308.4.5 Safety Glazing Trigger — Wet Areas | Tub / shower / pool, bottom edge < 60" above standing surface | 2021 IRC wording changed from "facing" to "containing or adjacent to" — broader trigger than 2018 IRC. Exception: glazing more than 60" measured horizontally from the water's edge. |
| IRC R308.4.6 Safety Glazing Trigger — Stairs | Adjacent to stairs / ramp, bottom edge < 36" above walking surface | Stairway glazing within 36" horizontally and 36" vertically of the walking surface must be tempered. |
| IECC 2021 R402.1.3 — Zone 5/6 Window U-Max | U ≤ 0.30 | Cool to cold zones. Tightens to U ≤ 0.28 in 2024 IECC. Zones 7-8 (very cold) tighten to U ≤ 0.27. |
| ENERGY STAR v7.0 Northern | U ≤ 0.22, min SHGC ≥ 0.17 | Effective Oct 23, 2023. First time an SHGC MINIMUM is required in the Northern climate to preserve winter solar gain. ENERGY STAR Most Efficient requires U ≤ 0.20 (typically triple-glazed). |
| Federal §25C Credit | 30% up to $600/yr — expired Dec 31, 2025 | Pub. L. 119-21 (One Big Beautiful Bill Act, July 4, 2025) terminated §25C for property placed in service after Dec 31, 2025. Calculator displays as expired by default; pre-2026 install dates toggle eligibility back on. |
| ASTM E2112 Install Sequence | WRB → sill pan → window → jamb flashing → head flashing → WRB lap | Shingle-lap rule: every upper layer laps over every lower layer so water drains outward. Sill pan extends 6" up each jamb and is sealed to the WRB. |
| EPA RRP Rule (40 CFR 745) | Pre-1978 housing — applies to ALL window replacement | The < 6 sq ft minor-repair exception EXPLICITLY EXCLUDES window replacement. Certified RRP firm and Certified Renovator required on site. Containment, HEPA cleanup, work-area signage, 3-year recordkeeping. |
| FBC TAS 201/202/203 (HVHZ) | Miami-Dade NOA required | Miami-Dade County + Broward County + portions of Monroe. Statewide Florida Product Approval is NOT sufficient inside the HVHZ. Impact-rated laminated insulating glass carries a 10-20% premium over standard FBC-approved products. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
IRC 2021 R310 — Emergency Escape and Rescue Openings(IRC R310)
View StandardGoverns egress windows in basements, habitable attics, and every sleeping room of dwelling units. Each sleeping room requires its own EERO; basement sleeping rooms each need one even if the basement has another EERO. Operable from inside without keys, tools, or special knowledge — ASTM F2090-compliant fall-prevention devices are permitted.
Key Requirements:
- •Net clear opening ≥ 5.7 sq ft (≥ 5.0 sq ft for grade-floor / below-grade openings)
- •Net clear height ≥ 24" and net clear width ≥ 20"
- •Sill height ≤ 44" above the finished floor
- •Operable from inside without keys, tools, or special knowledge
- •Window wells (below-grade): ≥ 9 sq ft horizontal projection, ≥ 36" projection and width
- •Wells with depth > 44": permanently affixed ladder or steps (12" inside width, 3" projection, 18" o.c.)
IRC 2021 R308 — Safety Glazing(IRC R308)
View StandardDefines "hazardous locations" where tempered, laminated, or wire glass is required. The 2021 wording in R308.4.5 expanded from "facing" wet surfaces to "containing or adjacent to" wet surfaces — broader trigger than 2018 IRC.
Key Requirements:
- •R308.4.1 — Glazing in doors (swinging, sliding, bifold)
- •R308.4.2 — Within 24" of either side of a door, bottom edge < 60" above floor
- •R308.4.3 — Large panel: > 9 sq ft, bottom < 18", top > 36", walking surface < 36" away
- •R308.4.4 — Guards and railings (any height, any area)
- •R308.4.5 — Tub / shower / spa / pool, bottom edge < 60" above standing surface
- •R308.4.6 — Adjacent to stairs / ramps, bottom edge < 36" above walking surface
- •R308.4.7 — Bottom stair landing, < 36" above + within 60" horizontally
IECC 2021 R402.1.3 — Vertical Fenestration U-Factor and SHGC(IECC R402.1.3)
View StandardMaximum window U-factor and maximum glazed SHGC by climate zone. Area-weighted averaging across multiple fenestration products is permitted per R402.3.1. A small 15 sq ft glazed exemption (R402.3.3) and one 24 sq ft opaque door exemption (R402.3.4) preserve a carve-out per dwelling.
Key Requirements:
- •Zones 1-2: U ≤ 0.40, SHGC ≤ 0.25
- •Zone 3: U ≤ 0.30, SHGC ≤ 0.25
- •Zone 4 except Marine: U ≤ 0.30, SHGC ≤ 0.40
- •Zone 4 Marine and Zones 5-8: U ≤ 0.30, SHGC Not Required
- •Area-weighted averaging permitted per R402.3.1
- •Replacement permit triggers compliance at the time of permit
IECC 2024 R402.1.3 — Updated Fenestration Table(IECC 2024 R402.1.3)
View StandardPublished Aug 14, 2024. State adoption as of May 2026 is still early — Nevada, Colorado (via municipal action in Thornton and Golden), and New York (2025 ECCC built on 2024 IECC) are the early movers. Tightens U-factor in cold zones only; Zones 0-4 unchanged, Zones 5/Marine-4 from 0.30 → 0.28, Zone 6 from 0.30 → 0.28, Zones 7-8 from 0.30 → 0.27. Climate Zone 0 explicitly enumerated.
Key Requirements:
- •Zone 0: U ≤ 0.50, SHGC ≤ 0.25 (new explicit row)
- •Zone 1: U ≤ 0.50, SHGC ≤ 0.25
- •Zone 5, Marine-4, Zone 6: U ≤ 0.28 (tightened from 0.30)
- •Zones 7-8: U ≤ 0.27 (tightened from 0.30)
- •Footnote: U ≤ 0.32 cap applies in Marine-4 and Zones 5-8 above 4,000 ft elevation or in WBDR regions
NFRC 100 / 200 / 500 — Energy Performance Rating(NFRC 100 (U-factor), NFRC 200 (SHGC/VT), NFRC 500 (CR))
View StandardNational Fenestration Rating Council procedures for whole-window energy performance. The NFRC label on every certified window is the only rating that satisfies permit inspection — center-of-glass U-factors from marketing materials do not.
Key Requirements:
- •NFRC 100 — whole-window U-factor (BTU/hr·ft²·°F)
- •NFRC 200 — whole-window SHGC and visible transmittance
- •NFRC 500 — condensation resistance rating
- •Whole-window basis (not center-of-glass)
- •Certified label affixed to every unit
AAMA/WDMA/CSA 101/I.S.2/A440 — NAFS Performance Class(NAFS-2017 / NAFS-2022)
View StandardNorth American Fenestration Standard. Classifies windows into performance classes by design pressure gateway and intended use. Cited in 2021 IBC and 2024 IBC as the required test method for air leakage (ASTM E283), water penetration (ASTM E331/E547), and structural performance (ASTM E330).
Key Requirements:
- •R — gateway DP 15 psf (one- and two-family dwellings)
- •LC — gateway DP 25 psf (low/mid-rise multifamily)
- •CW — gateway DP 30 psf (low/mid-rise commercial)
- •AW — gateway DP 40 psf (high-rise / heavy commercial)
- •ASTM E283 air leakage test minimum: 0.3 cfm/sq ft at 1.57 psf
ASTM E2112 — Standard Practice for Installation of Exterior Windows, Doors, and Skylights(ASTM E2112-19c (revised as E2112-23))
View StandardThe canonical install practice document — shingle-lap flashing integrated with the WRB so water drains outward. Specifies sequence: WRB cut and folded back, sill pan installed first, window set with shims and fasteners, jamb flashing over the fin and integrated with WRB, head flashing last and shingle-lapped behind WRB course above.
Key Requirements:
- •Sill pan extends 6" up each jamb
- •Jamb flashing ≥ 4" wide, lapped over the fin
- •Head flashing ≥ 4" wide, lapped over jamb flashing
- •Sealant on interior side; sill exterior left open to drain
- •Low-expansion foam only (high-expansion bows jambs)
- •Fastener schedule per manufacturer (typically 8" o.c. around fin)
FBC TAS 201 / 202 / 203 — HVHZ Impact and Pressure Testing(FBC TAS 201/202/203 (Miami-Dade NOA))
View StandardFlorida Building Code testing protocols for the High-Velocity Hurricane Zone (Miami-Dade County, Broward County, and portions of Monroe). Statewide Florida Product Approval is NOT sufficient inside the HVHZ — a Miami-Dade Notice of Acceptance (NOA) is required. Risk Category II design wind speed is 175 mph in Miami-Dade and 170 mph in Broward per FBC §1620.2.
Key Requirements:
- •TAS 201 — Large-missile impact: 9-lb 2×4 at 50 fps
- •TAS 202 — Structural / air / water performance
- •TAS 203 — 9,000 cycles of design-pressure cycling
- •Miami-Dade NOA required (statewide FL Product Approval is insufficient)
- •175 mph design wind speed Miami-Dade, 170 mph Broward
EPA RRP Rule — 40 CFR 745 Subpart E(40 CFR 745.80-92)
View StandardEPA Renovation, Repair, and Painting Rule. Applies to ANY pre-1978 housing window replacement — the < 6 sq ft minor-repair exception explicitly EXCLUDES window replacement under §745.83. Certified RRP firm + Certified Renovator required on site, with containment, HEPA cleanup, work-area signage, and 3-year recordkeeping.
Key Requirements:
- •EPA-certified RRP firm + Certified Renovator on site
- •Pre-renovation lead disclosure to occupants (Renovate Right brochure)
- •Work-area containment (6 ft plastic ground cover, plus interior containment)
- •HEPA vacuum cleanup; no dust outside work area
- •3-year recordkeeping of all activities
- •Civil penalties for non-compliance can exceed $37,500 per violation
IRC §25C / Pub. L. 119-21 — Energy Efficient Home Improvement Credit(26 U.S.C. §25C)
View StandardFederal Energy Efficient Home Improvement Credit for ENERGY STAR Most Efficient windows. Through Dec 31, 2025: 30% of cost up to $600/year for windows; $1,200/yr envelope cap. Pub. L. 119-21 (One Big Beautiful Bill Act, signed July 4, 2025) terminated §25C for property placed in service after Dec 31, 2025.
Key Requirements:
- •Eligible only for property placed in service before Jan 1, 2026
- •30% of qualifying cost, capped at $600/yr for windows
- •$1,200/yr aggregate envelope cap (insulation, doors, windows combined)
- •Windows must meet ENERGY STAR Most Efficient certification (U ≤ 0.20 typical)
- •Manufacturer certification statement required for IRS substantiation
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Climate Zone Drives Glass Package
IECC U-factor and SHGC tighten dramatically from south to north
2021 IECC R402.1.3 sets a national floor; state stretch codes (CA Title 24, MA Stretch, NY ECCC, WA WSEC) and 2024 IECC tighten further in cold zones. Replacement projects pulling a permit must meet the version in effect on the permit date.
Regional Examples:
HVHZ Impact Glass — Miami-Dade NOA Required
Statewide Florida Product Approval is NOT sufficient inside the HVHZ
Miami-Dade County, Broward County, and portions of Monroe County are the High-Velocity Hurricane Zone (HVHZ). Windows must carry a Miami-Dade NOA and be tested to FBC TAS 201 (large-missile impact), TAS 202 (structural / air / water), and TAS 203 (9,000 pressure cycles). Risk Category II design wind speeds are 175 mph in Miami-Dade and 170 mph in Broward.
Regional Examples:
Stretch Codes — When State Trumps IECC
CA Title 24, MA Stretch, NY ECCC, WA WSEC are stricter than baseline IECC
Several states publish their own residential energy code stricter than IECC baseline. The calculator surfaces a stretch-code overlay; users in stretch-code jurisdictions must verify U-factor and SHGC against the state-specific table.
Regional Examples:
Pre-1978 Housing — EPA RRP Rule Applies Universally
Window replacement automatically triggers RRP, no minor-repair exception
EPA 40 CFR 745.83 explicitly excludes window replacement from the < 6 sq ft minor-repair exception. Any window replacement in a pre-1978 home by a paid contractor requires a Certified RRP firm with a Certified Renovator on site, plus containment, HEPA cleanup, and 3-year recordkeeping.
Regional Examples:
Egress Trap on Double-Hung Bedrooms
A typical 32"×60" double-hung fails IRC R310 — the most common code violation
The most-flagged window code issue in residential remodels: a 32"×60" double-hung delivers about 4.8 sq ft of net clear opening, below the 5.7 sq ft IRC R310 minimum for upper-floor sleeping rooms. The same RO as a casement passes easily. Builders default to double-hung for aesthetic match; bedrooms need to break that pattern.
Regional Examples:
Replacement Permit Triggers Current-Code U-Factor
Like-for-like replacement does NOT exempt the project from current IECC
Most US jurisdictions require a permit for window replacement, and the IECC version in effect on the permit date applies — regardless of whether the original window was code-compliant. Replacing a 1995 single-pane wood window does not give you a free pass to install a U-0.40 dual-pane unit in Zone 5; the code-of-permit-date applies.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
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How to Use This Calculator
- Set project context: climate zone (1-8 + Marine-4), IECC code year (2021 default, 2024 toggle), and any stretch code overlay (CA T24, MA Stretch, NY ECCC, WA WSEC).
- Toggle project-wide flags: permit pulled (triggers IECC compliance check), built before 1978 (EPA RRP applies), HVHZ designation (impact glass required), install date before Jan 1 2026 (§25C eligibility).
- For each window: pick type (double-hung, single-hung, casement, awning, slider, picture, bay/bow/garden, special-shape), install path (insert / full-frame / new construction), manufacturer, frame material, and glass package.
- Enter rough opening width and height in inches, plus sill height above finished floor (for R310 sill-height check on bedrooms).
- Tag location flags per window: bedroom (EERO required), grade-floor / basement (5.0 sq ft NCO threshold instead of 5.7), within 24" of a door (R308.4.2), adjacent to tub/shower (R308.4.5), on a stair/ramp (R308.4.6).
- Optional: override the default U-factor and SHGC with the actual NFRC certified values from each window's label. Calculator defaults to typical mid-tier values per glass package — explicit NFRC values are required for permit-ready compliance.
- Add more windows (up to 20 per project). Click Calculate: per-window row shows NCO, egress verdict, safety-glazing flags, U/SHGC, install path, and any HVHZ issues. Project totals show area-weighted U-factor and SHGC, IECC pass/fail verdict, full materials list, and §25C status.
Why The Per-Window Row Matters
Window code compliance is per-opening, not per-project. A house can have a perfectly compliant IECC weighted U-factor while failing a single bedroom's IRC R310 egress requirement — and the bedroom failure stops the certificate of occupancy. This calculator surfaces a row-level verdict for every window: net clear opening in sq ft (with the actual height and width), egress pass/fail with the specific reason cited, safety glazing triggers naming the R308.4 subsection, HVHZ impact-glass mismatch, and the per-opening flashing materials. Project totals then aggregate everything. The math approach mirrors how a building official actually reads a plan-review submittal: per-opening first, project-summed second. NCO formulas in this calculator are approximate — Andersen, Pella, Marvin, Milgard, and JELD-WEN all publish per-call-size NCO tables; verify against the manufacturer's spec sheet before placing the order.
Frequently Asked Questions
What size egress window do I need for a basement bedroom?
Per IRC R310, grade-floor and below-grade openings need a net clear opening (NCO) of at least 5.0 sq ft (the exception in R310.2.1); upper-floor sleeping rooms need 5.7 sq ft. Minimum net clear height 24", net clear width 20", and maximum sill height 44" above the finished floor. A 32"×60" double-hung typically delivers about 4.8 sq ft of NCO and FAILS — the meeting rail and the fact that only one sash operates cut the available area roughly in half. A casement at the same rough opening usually passes because the entire sash swings out. For basements, the window well also needs to be at least 9 sq ft of horizontal projection at grade with 36" minimum projection and width, and a permanently affixed ladder if depth exceeds 44".
Do I need tempered glass next to my door?
Per IRC R308.4.2, glazing within 24 inches of either side of a door swing must be tempered (or laminated, or wire glass) when the bottom edge of the glass is less than 60 inches above the floor. The rule covers both swinging and sliding doors. A separate rule, R308.4.5 (updated in 2021 IRC), requires safety glazing in walls containing or adjacent to a tub, shower, spa, or pool when the bottom edge is less than 60 inches above the standing surface. R308.4.6 adds a stair / ramp trigger when the bottom edge is less than 36 inches above the walking surface. The calculator asks about each of these locations per window and flags tempered required by citing the specific R308.4 subsection.
Insert vs. full-frame window replacement — which should I pick?
Insert (also called pocket or block-frame) leaves the existing window frame in place and slides a new unit inside the existing jambs. Math runs on the inside-frame dimension; no nail fin, no new flashing. Faster, less disruptive, lower cost — but you lose roughly 0.5 to 1 inch of glass area per side and you cannot inspect or repair flashing behind the old frame. Full-frame tears out to the studs, installs a new window with an integral nailing fin, and lets you redo the flashing per ASTM E2112. Math runs on the rough opening. Slower and more expensive, but the only path that resolves rot, water intrusion, or aging WRB issues. New construction is the same math as full-frame but with a continuous integrated WRB and sill pan from the start — used in additions and gut remodels.
What U-factor and SHGC do I need for my climate zone?
Per 2021 IECC Table R402.1.3: Zones 1-2: U ≤ 0.40, SHGC ≤ 0.25. Zone 3: U ≤ 0.30, SHGC ≤ 0.25. Zone 4 except Marine: U ≤ 0.30, SHGC ≤ 0.40. Zone 4 Marine and Zones 5-8: U ≤ 0.30, SHGC not required. 2024 IECC tightens U-factor in cold zones — Zones 5 / Marine-4 and Zone 6 move to U ≤ 0.28; Zones 7-8 move to U ≤ 0.27. Pulling a permit triggers compliance with the IECC version in effect on the permit date, regardless of the original window. Area-weighted averaging across multiple fenestration products is permitted per IECC R402.3.1. The NFRC label on each window is the only rating that satisfies inspection — center-of-glass U-factors from marketing materials are not the whole-window value.
Are new windows tax-deductible in 2026?
No. The Federal §25C Energy Efficient Home Improvement Credit was terminated by Pub. L. 119-21 (One Big Beautiful Bill Act, signed July 4, 2025) for property placed in service after December 31, 2025. Through Dec 31, 2025, §25C provided 30% of cost up to $600/year for windows meeting ENERGY STAR Most Efficient certification, with a $1,200/yr aggregate envelope cap. The Inflation Reduction Act had previously extended §25C through 2032; OBBBA cut it short. State and utility rebates may still apply — check your local utility and state energy office. The calculator displays §25C as expired by default; toggle the "install before Jan 1 2026" checkbox to see the historical credit calculation for pre-sunset installs.
How do I measure a window for replacement?
Three measurements per dimension — use the smallest. For width: measure at the top, middle, and bottom of the opening. For height: measure at the left, center, and right. Older homes often have racked or settled openings; the smallest measurement is what the new window has to fit. For insert / pocket replacement, measure inside the existing frame jamb-to-jamb and sill-to-head. For full-frame replacement, measure the planned rough opening AFTER tear-out — the existing trim and casing hide the actual stud opening, which is usually 0.5-1 inch wider in each direction. Andersen, Pella, Marvin, Milgard, and JELD-WEN all publish dedicated insert vs. full-frame sizing tables; pull the right one for your install path.
Andersen vs. Pella vs. Marvin vs. Milgard vs. JELD-WEN — quick orientation?
Andersen 400 Series uses a composite Fibrex frame with a deep custom catalog and a vinyl-clad / wood-interior tilt-wash double-hung that has been the volume leader for decades. Pella 250 Series is vinyl with hidden screens on hung and slider; the multi-chambered fully-welded frame is rated 52% stronger than standard vinyl per Pella's published Architectural Design Manual. Marvin Elevate uses an Ultrex fiberglass exterior with a wood interior — a mid-tier upgrade from vinyl with thinner sightlines and better long-term dimensional stability. Milgard Tuscany V400 is vinyl with the SmartTouch lock (now owned by MITER Brands). JELD-WEN V-2500 is vinyl with broad Lowe's distribution; W-2500 is the clad-wood line in the same SKU family. The calculator does not editorialize quality beyond published specs — use the dropdown for the product line you're actually pricing.
Why isn't my window a standard size?
Standard sizes are in-stock at the manufacturer and ship in 2-3 weeks. Custom sizes are special-order in 1/8" increments and add 4-6 weeks lead time depending on the manufacturer. All five primary brands (Andersen Custom, Pella Special Sizes, Marvin made-to-order, Milgard, JELD-WEN) offer custom sizing. For replacements, the existing rough opening dictates the order — measure carefully and order the closest standard size below your rough opening, then shim out the difference (typical shim allowance is 1/4 to 1/2 inch per side). Older homes with non-standard openings often need custom sizes; new construction can almost always use a standard size by sizing the rough opening to the catalog.