Beam Span Calculator
This free beam span calculator sizes a floor girder — the beam that picks up floor joists in a basement, crawlspace, or first floor and carries them down to posts, piers, or a foundation wall — the way a plans examiner checks it, not a rule of thumb. IRC R502.5 states plainly that a dimension-lumber girder span cannot exceed the values in Table R602.7(1) or R602.7(2), the same prescriptive tables used to size a door or window header. Tell it your building width, whether the girder also carries roof and ceiling load (an exterior/perimeter condition) or floors only (interior), and it returns the smallest built-up girder from 2-ply 2×4 up to 4-ply 2×12, for a combined No. 2 species column — no species to guess at.
Because a girder is not just a span, it also gives you what a header calculator does not: how many posts or piers you need across the girder’s total run and how far apart to space them, the minimum bearing at each end (1½" on wood or metal, 3" on masonry or concrete per IRC R502.6), and the minimum post size (4×4 wood or 3" Schedule 40 steel pipe per R407.3 — a code floor, not a load rating, which this calculator says plainly rather than pretending to size your post for you). It also applies the same footnote reduction headers use: an unbraced 2×8, 2×10, or 2×12 girder loses 30 percent of its tabulated span.
It covers both IRC tables — R602.7(1) for an exterior/perimeter girder (ground snow 30, 50, and 70 psf, five supported conditions) and R602.7(2) for an interior girder (one or two floors, no snow) — at building widths of 12, 24, and 36 feet. Sizing only — no pricing, no signup. For an opening in a wall instead of a floor beam, use the Header Size Calculator; for the post footing, use the Footing Size Calculator.
Floor Beam / Girder Span Calculator
Size a built-up floor girder — the basement or crawlspace beam that carries joists to posts or piers — plus post/pier spacing, end bearing, and minimum post size, straight from IRC Table R602.7 and R502.6. Free, no signup.
What are you solving for?
Find the smallest girder for the post spacing you want, or the widest post spacing a girder size can carry.
The girder & the load
An interior girder (Table R602.7(2), floors only) or an exterior/perimeter girder that also carries roof + ceiling (Table R602.7(1), any snow load).
Values use the combined No. 2 species column (Douglas fir-larch, hem-fir, Southern pine, spruce-pine-fir), so no species picker is needed — the same table used for door and window headers (IRC R502.5).
Posts & run
Code edition, lateral bracing, and the total run the girder covers (for post count).
Uncheck only if the girder top genuinely has nothing bracing it — the footnote then cuts the span of 2×8/2×10/2×12 girders by 30%.
Bearing & posts
What the girder ends bear on, and the post/column material.
Girder span table — 24-ft building, interior bearing
Maximum girder span (ft-in) by size, per IRC Table R602.7(2) — combined No. 2 species, one floor only. Change the building width or support above to see other columns.
| Girder size | Max span between posts |
|---|---|
| 2-ply 2×4 | 2'-10" |
| 2-ply 2×6 | 4'-4" |
| 2-ply 2×8 | 5'-5" |
| 2-ply 2×10 | 6'-6" |
| 2-ply 2×12 | 7'-7" |
| 3-ply 2×8 | 6'-10" |
| 3-ply 2×10 | 8'-1" |
| 3-ply 2×12 | 9'-6" |
| 4-ply 2×8 | 7'-11" |
| 4-ply 2×10 | 9'-4" |
| 4-ply 2×12 | 11'-0" |
Post/column size is the IRC R407.3 code minimum (4×4 wood or 3" Schedule 40 steel pipe), not a load table — verify the post you use is rated for its tributary load. Verify your locally adopted IRC edition.
How a floor girder actually sits
A girder isn't a header in a wall — it runs through open floor framing, picks up joists from both sides, and lands on posts down to their own footings. The span you enter above is measured post to post; the posts and their footings are what actually carry it to the ground.
How much bearing does code actually require?
The minimum contact length between the girder and its support depends on what it's resting on — wood and metal need less than masonry and concrete. Either way, gravity bearing alone isn't enough; every connection needs to resist uplift and lateral movement too.
Calculation Formulas
IRC R502.5 states the allowable span of a floor girder fabricated of dimension lumber cannot exceed the values in Tables R602.7(1), (2) and (3) — the same table used to size a door or window header. The maximum span is read directly from that table, not calculated.
Example:
24-ft building, 2-ply 2×10, interior girder, one floor → 6'-6" maximum span.
For a target post/pier spacing, the calculator walks the built-up sizes from fewest plies and shallowest depth upward and returns the first whose tabulated span covers that spacing.
Example:
An 8-ft post spacing in a 24-ft-wide house, one floor, needs a 2-ply 2×10 (6'-6" — too short) or a 3-ply 2×8 (6'-10" — still short); a 2-ply 2×12 (7'-7") reaches it.
Given the total length the girder must run, the calculator divides it into the fewest equal bays that each stay within the achievable span, then spaces the posts evenly — never wider than the table allows.
Example:
A 20-ft run with an achievable span of 7'-7" needs 3 bays (posts every 6'-8"), not 2 bays at 10 ft.
When the top of the girder is not laterally braced by perpendicular framing (subfloor sheathing normally provides this), the tabulated span for 2×8, 2×10, and 2×12 sizes is reduced by 30 percent — the same footnote that applies to wall headers.
Example:
A 2-ply 2×10 interior girder at 12-ft width, one floor: 9'-2" braced → 6'-5" unbraced.
IRC R502.6 sets the minimum bearing length at each end of a joist, beam, or girder by what it bears on. Bearing on masonry or concrete without direct contact needs a 2"-nominal sill plate with at least 48 sq in of bearing area.
Example:
A girder pocketed into a concrete foundation wall needs 3" of bearing; the same girder on a wood post needs 1½".
IRC R407.3 sets these as code MINIMUM sizes only — not a load-capacity table. The actual post needed for a given tributary load is a separate engineering or manufacturer-rating question, which this calculator flags rather than guesses.
Example:
A 4×4 satisfies the code minimum for any wood post regardless of height, but may be undersized for the load it carries — verify separately.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Species basis | Combined No. 2 (DF-L / HF / SP / SPF) | The same single governing No. 2 column the header table uses — no species selection needed. Southern-Pine-only jurisdictions use a separate reduced-value supplement. |
| Building widths | 12 / 24 / 36 ft | The 2018/2021 IRC table's width axis. Interpolation between widths is permitted (footnote c). |
| Snow columns (exterior only) | 30 / 50 / 70 psf | Ground-snow columns for an exterior/perimeter girder that also carries roof and ceiling load. An interior girder skips this — Table R602.7(2) has no snow dimension. |
| Footnote factor | ×0.70 unbraced | Tabulated spans for 2×8/2×10/2×12 are multiplied by 0.70 where the girder top is not laterally braced. |
| Bearing — wood/metal | 1½" minimum each end | R502.6 minimum bearing on wood or metal, or an approved joist hanger rated for the reaction. |
| Bearing — masonry/concrete | 3" minimum each end | R502.6 minimum bearing on masonry or concrete, direct or via a 2"-nominal sill plate with ≥48 sq in of bearing area. |
| Column minimum — wood | 4×4 nominal | R407.3 minimum wood column size — a code floor, not a load rating. |
| Column minimum — steel | 3" dia. Schedule 40 pipe | R407.3 minimum steel column size (ASTM A53/A53M Grade B or approved equivalent) — an adjustable (Lally) column's real capacity is its manufacturer rating. |
| Built-up nailing | 20d @ 32" o.c. staggered | IRC Table R602.3(1) item 28: 2-inch built-up girder/beam layers get 20d common (or 10d box/3"×0.131" nails at closer spacing) at top and bottom, staggered, plus 2-20d common face-nailed at every end and splice. |
| Positive connection | Post cap or equivalent | R502.9 requires a positive connection against uplift and lateral displacement wherever a post or beam bears on floor framing — gravity bearing alone is not enough. |
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 R502.5 — Allowable Girder and Header Spans(IRC R502.5)
View StandardDirects that dimension-lumber floor girder spans follow the same Tables R602.7(1), (2), and (3) used for wall headers — the code provision that makes this calculator's table reuse correct, not a shortcut.
Key Requirements:
- •Girder spans = header/girder span table values
- •No separate girder-specific span table exists in the IRC
- •Combined No. 2 species basis carries over from the header table
IRC Table R602.7(1)/(2) — Girder & Header Spans(IRC Table R602.7(1) (exterior), R602.7(2) (interior))
View StandardThe prescriptive span table itself: maximum span by building width, ground snow load (exterior only), supported condition, and built-up girder/header size, for a single combined No. 2 species column.
Key Requirements:
- •Span read from the table by width and size
- •Building widths 12/24/36 ft; interpolation permitted between widths
- •Exterior table: 30/50/70 psf snow, five supported conditions
- •Interior table: one-floor or two-floor supported conditions, no snow dimension
- •Footnote: reduce 2×8/2×10/2×12 spans by 0.70 when the top is unbraced
IRC R502.6 — Bearing(IRC R502.6)
View StandardSets the minimum bearing length for the ends of joists, beams, and girders — 1½" on wood or metal, 3" on masonry or concrete, or an approved joist hanger, with a sill-plate alternative for indirect masonry bearing.
Key Requirements:
- •1½" minimum bearing on wood/metal
- •3" minimum bearing on masonry/concrete
- •Approved joist hanger may substitute for direct bearing
- •2"-nominal sill plate with ≥48 sq in bearing area where masonry contact isn't direct
IRC R502.9 / R407.3 — Connections & Columns(IRC R502.9, R407.3)
View StandardR502.9 requires a positive connection against uplift and lateral displacement wherever post-and-beam construction supports floor framing. R407.3 sets minimum column sizes (4×4 wood, 3" Schedule 40 steel pipe) and requires lateral restraint at the column base — but is not a load-capacity table.
Key Requirements:
- •Positive connection (post cap or equivalent) at every post-to-girder bearing point
- •4×4 nominal minimum wood column size
- •3" diameter Schedule 40 steel pipe minimum
- •Column base restrained against lateral displacement
- •Column load capacity is a separate engineering/manufacturer question — not prescribed by size alone
IRC Table R602.3(1) Item 28 — Built-Up Girder Nailing(IRC Table R602.3(1))
View StandardThe fastening schedule for laminating 2-inch built-up girders and beams from multiple plies.
Key Requirements:
- •20d common nails at 32" o.c., top and bottom, staggered — or
- •10d box (or 3"×0.131") nails at 24" o.c., staggered on opposite sides
- •2-20d common (or 3-10d box) face-nailed at every end and splice
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.
Code Edition
2015 vs. 2018/2021
The girder/header table's building-width axis changed between editions — the 2015 IRC used 20/28/36 ft, while the 2018 and 2021 IRC use 12/24/36 ft. The 2024 IRC expands to widths of 20/24/28/32/36/40 ft.
Regional Examples:
Interior vs. Exterior Bearing
Which table applies
A center girder buried in a basement or crawlspace, carrying only floor joists, uses the interior table (no snow dimension). A perimeter girder that's also part of an exterior bearing wall — carrying roof and ceiling load down to the same posts — uses the exterior table and needs a ground-snow input.
Regional Examples:
Ground Snow Load
Exterior/perimeter girders only
Where a girder also carries roof and ceiling load, heavier ground snow shrinks its achievable span the same way it shrinks a header's. Interior-only girders (floors alone) are unaffected by snow.
Regional Examples:
Foundation Type & Post Base
Slab, crawlspace, basement
How a post's bottom end is restrained (R407.3) and how its footing is built varies with foundation type — a basement post typically lands on a poured footing, a crawlspace post on a pier, and a slab-on-grade post on an embedded or surface-mounted base. Frost depth also drives footing depth regionally.
Regional Examples:
Beyond the Prescriptive Table
When you need an engineered beam
Post spacing wider than the largest tabulated span for a size, girders carrying a concentrated point load (a post, wall, or beam bearing on them), and long open spans all fall outside the prescriptive code. These need an engineered solution — commonly an LVL, LSL, glulam, or steel beam sized by the manufacturer or an engineer.
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
Related Calculators
Header Size Calculator
Size a door or window header in a bearing wall — header size, ply, jack & king studs, and max span, per IRC R602.7. Free, no signup, no pricing.
Joist Span Calculator
Free joist span calculator: max spans for floor and deck joists by species, grade, size, and spacing per IRC R502.3.1 and R507.6. No signup.
Footing Size Calculator
What size footing for a foundation wall? Free calculator for width & thickness by story count, soil bearing & snow load — IRC Table R403.1(1).
Deck Beam Span Calculator
How far can a deck beam span? Free calculator for beam size, ply, post spacing, footing & cantilever from the joist span it carries — per IRC R507.5 / DCA 6.
How to Use This Calculator
- Choose a mode: find the girder for the post spacing you want, or the widest post spacing a girder size can carry.
- Pick the bearing condition — interior girder (floors only) or exterior/perimeter girder (also carries roof + ceiling) — and the building width.
- For an exterior/perimeter girder, set your ground snow load and what it supports.
- Enter the total run the girder covers so the calculator can lay out posts evenly across it.
- Leave "girder top laterally braced" checked for a normal subfloor-sheathed floor.
- Set what the girder bears on and the post/column material, then click Calculate for the girder size, post count and spacing, bearing, and minimum post size.
How the IRC Sizes a Floor Girder
IRC R502.5 doesn’t give floor girders their own span table — it points straight at the same header/girder table used for door and window openings (R602.7(1) and (2)), sized by building width (which stands in for the roof and floor load funneling to that line) and what the girder carries. A girder differs from a header in what happens at its ends: instead of jack and king studs in a wall, it lands on posts, piers, or a foundation wall, and IRC R502.6 sets the minimum bearing there — 1½ inches on wood or metal, 3 inches on masonry or concrete. R502.9 then requires a positive connection against uplift and lateral movement at every post-to-girder bearing point, not just gravity contact. The posts themselves have a code MINIMUM size under R407.3 — 4×4 nominal wood or 3-inch Schedule 40 steel pipe — but that minimum is not a load rating; an adjustable steel column’s real capacity comes from the manufacturer, and a wood post’s from a span-rated table or an engineer. When post spacing needs to be wider than the table allows — common in an open-concept basement — the prescriptive path is exhausted and the girder needs to be engineered: an LVL, LSL, glulam, or steel beam sized by the manufacturer or a structural engineer.
Frequently Asked Questions
How far can a floor beam or girder span?
It depends on the beam size, ply, the building width, and what it carries. Per IRC Table R602.7(2) for an interior girder supporting one floor, a 2-ply 2×10 spans about 6'-6" in a 24-ft-wide house, a 3-ply 2×10 about 8'-1", and a 4-ply 2×12 up to 11'-0". A wider building or a second floor above shortens those spans. Enter your numbers and this calculator returns the smallest built-up girder that spans the post/pier distance you want, plus how many posts you need across the total run.
What size beam do I need for a basement?
A basement center beam is almost always an 'interior girder' in IRC terms — no roof or snow load, just the floor(s) above. Table R602.7(2) covers it: at a 24-ft building width supporting one floor, a 2-ply 2×8 reaches about 5'-5", a 2-ply 2×10 about 6'-6", and a 3-ply 2×12 about 9'-6". If the beam is part of an exterior/perimeter foundation wall that also carries roof load, use the exterior/perimeter mode instead — that's a different table.
Is a girder the same as a beam?
In framing, yes — “girder” and “beam” describe the same built-up member; IRC R502.5 even titles the table “Girder Spans and Header Spans.” The only real distinction the code draws is what the member does: a header spans an opening in a wall and lands on jack and king studs, while a girder runs through open floor framing and lands on posts, piers, or a foundation wall. This calculator sizes the girder application; use the Header Size Calculator for an opening in a wall.
How far apart should posts be under a girder?
As far apart as the girder's tabulated span allows for your building width and load — no farther. This calculator lets you either enter the post spacing you want and finds the smallest girder that reaches it, or pick a girder size and see its maximum span, then lays out posts evenly across your total run so no bay exceeds that span. Wider post spacing always needs a bigger (or more-ply) girder; past the largest tabulated size, an engineered LVL or steel beam is required.
What size post do I need to support a beam?
IRC R407.3 sets a code MINIMUM of 4×4 nominal wood or 3-inch Schedule 40 steel pipe — but that's a floor, not a load rating. Whether a 4×4 (or a larger post) is actually adequate depends on the tributary load it collects, which is a separate calculation from a span-rated post table or an engineer. An adjustable steel (Lally) column's real capacity is whatever the manufacturer rates it for at that height and diameter. This calculator states the code minimum and flags that load-based sizing is a separate step, rather than guessing a number.
How much bearing does a beam need on a post or foundation wall?
IRC R502.6 sets the minimum: 1½ inches of bearing on wood or metal, or 3 inches on masonry or concrete — or an approved joist hanger rated for the reaction in place of direct bearing. If the girder bears on masonry or concrete without direct contact, a 2-inch-nominal sill plate providing at least 48 square inches of bearing area satisfies the requirement instead.
Do I need hardware connecting the beam to the post, or does it just sit on top?
You need a positive connection, not just gravity contact. IRC R502.9 requires a connection that resists both uplift and lateral displacement wherever post-and-beam construction supports floor framing — typically a post cap or an equivalent approved connector at each bearing point, plus the post's own base restrained against lateral movement at the bottom per R407.3.