Gutter Calculator
How many feet of gutter and how many downspouts does your house actually need? This free gutter calculator gives DIY homeowners and remodeling pros instant linear-foot, downspout, hanger, miter, elbow, and strap counts for K-style, half-round, or West Coast fascia gutters in aluminum, copper, galvanized steel, or vinyl — sized to your roof footprint, pitch, and local 5-minute rainfall intensity.
Most online "gutter calculators" stop at perimeter × 1.10. The real math is sequential: roof plan × pitch factor (SMACNA Table 1-1) gives design drainage area; that divided by local rainfall (Table 1-2) and your downspout cross-section (Table 1-3) tells you how many downspouts; gutter capacity (Table 1-4) confirms the profile will keep up. Get any one of these wrong on a cloudburst storm and water sheet-falls at the corners — every single time.
Built on SMACNA Architectural Sheet Metal Manual 7th Edition Chapter 1, IRC R903.4 drainage requirements, ASTM B370 / A653 G90 / B209 material specifications, AAMA 1405.1 aluminum coil standards, NRCA expansion-joint practice, and Florida Building Code §1514.4.1 / RAS 111 HVHZ provisions. Free, no signup.
Gutter Calculator
Size gutters and downspouts, count hangers, miters, elbows, and straps — built on SMACNA Tables 1-1 through 1-4 and local rainfall intensity.
Roof footprint
Length × width = plan area. Steeper roofs catch more rain per square foot of footprint — the pitch factor compensates (1.10 at 6:12, 1.30 at 12:12).
Gutter runs and corners
Leave blank to use the roof perimeter (2 × L + 2 × W).
1 = continuous wrap. 4 = each eave separate.
Add for L-shapes, U-shapes, or any reentrant eave.
Leave blank: auto-calc (continuous wrap = 0; otherwise 2 per run).
Profile, material, and installation
Seamless requires aluminum coil + on-site roll-forming. Copper, galvanized, vinyl, and half-round (any material) ship as sectional 10-ft sticks.
Rainfall and downspouts
1 in/hr = FHA / HUD MPS residential baseline (yields the familiar '1 sq in per 100 sq ft' downspout sizing). Step up to 5-min peak intensity (NOAA Atlas 14) for SMACNA design: ~3 in/hr Northeast, ~5 in/hr inland, ~7 in/hr Gulf Coast, ~9 in/hr Miami.
Distance the top elbow has to span to bring the pipe back against the wall.
IRC R903.4 — discharge water away from the foundation. Set 0 if hard-piped underground.
Climate and options
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How to Use This Calculator
- Enter roof footprint: length and width in feet, plus pitch (SMACNA Table 1-1 pitch factor 1.00 for ≤ 3:12 up to 1.30 for 12:12).
- Set the gutter run count: 1 for a continuous wrap, 4 for each eave separate. Add outside and inside corner counts (4 outside corners for a simple rectangle).
- Pick profile, material, and installation: 5″ K-style is the national default; 6″ K-style is the new-construction default in moderate-to-high rainfall regions. Aluminum is the only seamless option; copper, galvanized, vinyl, and half-round ship as 10-ft sectional sticks.
- Set local rainfall intensity: ~3 in/hr Northeast, ~5 in/hr inland, ~7 in/hr Gulf Coast, ~9 in/hr Miami. Use NOAA Atlas 14 5-min / 10-yr for typical residential or 100-yr for high-value / coastal projects.
- Pick the downspout: 2×3 (3.94 sq in) for ≤ 600 sq ft per downspout at low rainfall, 3×4 (11.7 sq in) for tree-heavy lots, larger roofs, or any time you want clog-resistance. Round downspouts (3″ or 4″) for half-round systems.
- Set eave height, offset distance from eave to wall, and ground extension (IRC R903.4 requires discharge away from foundation — 4 ft is the industry minimum).
- Pick the climate / code zone: standard (24″ o.c. hangers), Pacific Northwest (22″ o.c.), heavy snow (18″ o.c.), extreme snow (12″ o.c. + below roof line), Florida HVHZ (16″ o.c. hidden hangers, fasteners in rafter tails), or wind-borne debris region.
- Click Calculate: see total gutter LF (sticks for sectional, LF for seamless), downspout count and LF, hangers, miters (inside + outside), end caps, A-style 75° elbows, straps, splash blocks, expansion joints needed, and full installation notes citing SMACNA / IRC / NRCA / FBC standards.
The Sizing Chain — Why Plan Area Alone Is Not Enough
A correct gutter takeoff is a chain of four decisions, not a single multiplication. Step 1: plan area × pitch factor = design drainage area (a 6:12 roof presents 10% more catchment than its footprint). Step 2: design area ÷ (downspout cross-section × 96.15 / local rainfall) = downspout count from capacity. Step 3: total gutter LF ÷ 35 = downspout count from practical max run length. Step 4: take the worst of step 2, step 3, and one-per-run as the actual downspout count. Snow climates bump that up one more. The gutter profile (5″ K, 6″ K, half-round) is then verified against SMACNA Table 1-4 — it must carry the design area at 1/8″/ft slope and the chosen rainfall intensity. Most "gutters overflowing during storms" callbacks trace to a system that passed step 1 but failed step 2 or step 3.
Frequently Asked Questions
How many feet of gutter do I need?
Total gutter LF equals the roof perimeter (the sum of every eave). For a simple 60 ft × 40 ft rectangle, that's 2 × (60 + 40) = 200 LF. Add 10% waste for sectional installation (cut and overlap on 10-ft sticks) or 3–5% for seamless (on-site formed from coil, minimal cut waste). On 200 LF: sectional ships 22 × 10-ft sticks (220 LF); seamless ships ~208 LF of continuous formed gutter.
How many downspouts does my house need?
Take the worst case of three rules: (1) capacity — design drainage area divided by per-downspout capacity (downspout sq in × 96 / rainfall in/hr); (2) practical max run — total gutter LF ÷ 35; (3) one per independent gutter run. For a 2,400 sq ft single-story at 6:12 pitch in Atlanta (5 in/hr) with 200 LF of perimeter and 2×3 downspouts: capacity = 2,640 / (3.94 × 96 / 5) = 35; run length = 200 / 35 = 6; runs = 1 (continuous wrap). Worst case 35 — but residential practice would step up to 3×4 downspouts and reduce the count to a reasonable 4–8. The calculator does this comparison automatically.
Do I need 5-inch or 6-inch K-style gutters?
5″ K-style with 2×3 downspouts handles most retrofits and inland single-story houses under ~1,000 sq ft per downspout. 6″ K-style with 3×4 downspouts is now the new-construction default in much of the country and required in the Pacific Northwest — the 3×4 downspout carries roughly 2× the flow of 2×3 and resists clogging from leaves and tree debris. Step up to 6″ when (a) drainage area exceeds 1,400 sq ft per downspout at 6 in/hr rainfall, (b) you're in a tree-heavy lot, or (c) your local rainfall intensity exceeds 6 in/hr (Gulf Coast, parts of FL / TX / LA).
What's the difference between seamless and sectional gutters?
Seamless gutters are formed on site from aluminum coil through a portable roll-forming machine and cut to the exact run length — joints only at corners, downspout outlets, and expansion joints. Sectional gutters ship as 10-ft sticks and are spliced at lap joints every 10 ft. Seamless is aluminum almost exclusively (.027″ or .032″); copper, galvanized, vinyl, and half-round are sectional-only because most residential crews don't carry the specialized roll-formers. Seamless eliminates the most common leak path (mid-run laps) and uses less waste (3–5% vs 10%), but adds 10–20% to material cost vs sectional aluminum.
How close together should gutter hangers be?
Per AAMA 1405.1 and SMACNA general practice: 24″–32″ o.c. in standard residential climates (IRC Zones 1–4). Tighten to 16″–18″ o.c. in heavy snow (IRC Zones 5–8, ground snow ≥ 30 psf) and 12″ o.c. in extreme snow (≥ 50 psf). Florida HVHZ requires hidden hangers at 16″–18″ o.c. with corrosion-resistant fasteners landed in solid framing (rafter tails, not just fascia) per FBC §1514.4.1. The calculator picks the spacing automatically from the climate zone you select.
Can I use aluminum gutters under a copper-flashed roof?
No. Copper-ion runoff pits and perforates aluminum within 5–10 years per GAF Technical Bulletin R-107. The same applies to algae-resistant shingles that contain copper granules — they leach trace copper into the gutter water and accelerate corrosion. Use all-copper gutters under copper flashing, or specify pre-painted Galvalume steel as a compromise. The galvanic series ranks zinc (anodic) → aluminum → galvanized → carbon steel → stainless → copper (cathodic); the more anodic metal corrodes preferentially when both are in electrical contact in the presence of rainwater.
Why does my gutter need an expansion joint?
Metal expands and contracts with temperature. Aluminum's coefficient of thermal expansion is 12.9 × 10⁻⁶ in/in/°F — a 50 ft run at ΔT 100°F (winter low to summer high) moves 25/32″ (0.78″). Without a joint to absorb that movement, the gutter oil-cans in the middle, end caps blow out, or hangers pull out of the fascia. SMACNA Fig. 1-6 caps continuous runs at 50 ft without an expansion joint; NRCA additionally specifies that joints be ≤ 25 ft from any fixed corner (downspout, miter, or end cap). Aluminum is sometimes flagged conservatively at 40 ft because of its higher CTE.
How far should the downspout extend from the foundation?
IRC R903.4 requires that roof drainage be directed away from the foundation. The industry minimum is a 4-ft ground extension on flat extensions or a splash block (24″ × 12″ concrete or polymer pad) at minimum. On clay soils, that's often insufficient — hard-pipe to a buried drain tile, dry well, or daylight discharge. Failed foundation drainage is one of the top three causes of basement water intrusion. Rain barrels acceptably replace the ground extension at one downspout per barrel, with the barrel overflow piped to a splash block or drain tile.