How to use the beam span calculator
- Choose the support case: roof and ceiling only, or roof, ceiling and one center-bearing floor.
- Enter building width perpendicular to the ridge (12 to 36 ft; the table may be interpolated) and the opening width.
- Pick a built-up size (two, three or four 2× members). The page shows the maximum span, whether the opening fits, the smallest size that works, and the tabulated jack-stud count.
Interior floor joists are on the floor joist span calculator. Nominal and actual lumber sizes are on the lumber size calculator.
Table R602.7(1) and its footnotes
Spans come from 2021 IRC Table R602.7(1), as reprinted in the 2021 Seattle Residential Code, Chapter 6. The table uses the same numbers for No. 2 Douglas Fir-Larch, Hem-Fir, Southern Pine and Spruce-Pine-Fir. This page uses the 30 psf ground-snow column; footnote e says to use that column when ground snow is less than 30 psf and roof live load is 20 psf or less.
Footnote c allows interpolation between the 12, 24 and 36 ft width columns. Footnote f says that if the top of a 2×8, 2×10 or 2×12 header is not laterally braced, multiply the tabulated span by 0.70 — this calculator does not apply that reduction automatically.
A worked example: 6-foot opening, 24-foot building width
For a roof-and-ceiling header in a 24-foot-wide building, two 2×8s may span 5′-9″, which is short of a 6-foot opening. Two 2×10s may span 6′-10″ and work, with two jack studs at each end. At a 30-foot width, the same two 2×10s interpolate to 6′-3½″.
Sources & assumptions
How this calculator gets its numbers. Full method: our methodology.
- Maximum header and girder spans are 2021 IRC Table R602.7(1) for exterior bearing walls, as reprinted in the 2021 Seattle Residential Code, Chapter 6. The table uses the same spans for No. 2 grade Douglas Fir-Larch, Hem-Fir, Southern Pine and Spruce-Pine-Fir.
- This page implements two support cases — roof and ceiling only, and roof, ceiling and one center-bearing floor — in the 30 psf ground-snow column. Footnote e says to use that column when ground snow is less than 30 psf and roof live load is ≤ 20 psf.
- Building width may be interpolated between the 12, 24 and 36 ft columns (footnote c). Widths outside 12–36 ft are rejected. Jack-stud counts at interpolated widths are rounded up.
- Footnote f (0.70 span reduction when the top of a 2×8, 2×10 or 2×12 header is not laterally braced) is disclosed and is not applied automatically.
- This is a table lookup, not a stamped design. Other support cases and Tables R602.7(2) and R602.7(3) are not included.
Sources
- City of Seattle, SDCI, 2021 Seattle Residential Code, Chapter 6 — Wall Construction. Public reprint of 2021 IRC Table R602.7(1), including footnotes a–f used in the assumptions above.
Last reviewed . The adopted code tables and a design professional win.
Frequently asked questions
What size header do I need for a 6-foot opening?
For roof and ceiling only, 24 ft building width and the 30 psf snow column, two 2×8s span 5′-9″ and are short; two 2×10s span 6′-10″ and work. A center-bearing floor above shortens those spans.
Can I interpolate between building widths?
Yes. IRC Table R602.7(1) footnote c allows interpolation between the 12, 24 and 36 ft columns. This page will not extrapolate outside 12–36 ft.
Does this cover interior beams or built-up girders under a floor only?
No. The subset implemented here is exterior bearing-wall headers from Table R602.7(1) for two support cases. Other rows and Table R602.7(2)/(3) are not in this calculator.
Is this a stamped design?
No. It is a code-table lookup. Unusual loads, missing lateral bracing, or openings outside the table need a design professional. Local code governs.