Beam Span Calculator

This calculator sizes a built-up header or girder, the horizontal member that carries a bearing wall's load over a window, a door or an open span, straight from the prescriptive span tables in the International Residential Code (IRC). Pick the wall type, what the wall supports, the ground snow load and the building width, and it lists every lumber combination the code allows with its maximum span and the number of jack studs needed at each end.

The numbers come from 2021 IRC Table R602.7(1) for exterior bearing walls and Table R602.7(2) for interior bearing walls; the 2018 and 2024 editions print the same values. Each table covers No. 2 Douglas fir-larch, hem-fir, Southern pine and spruce-pine-fir together, so species does not change the answer. Building width is measured perpendicular to the ridge: a wider building puts more roof and floor on the wall, so spans shrink. The code allows interpolation between the 12, 24 and 36 foot columns, and the calculator does that, rounding down to the inch.

The tables assume the top of the member is braced by joists, rafters or blocking framing into it. When it is not, 2x8 through 2x12 members drop to 70% of the table span and 2x4 or 2x6 members need a design. Anything outside the tables, such as a building wider than 36 feet, snow above 70 psf, a point load from a post above, or a long basement girder carrying more than the listed floors, calls for an engineered beam sized by a designer or your lumber supplier, and your local building department has the final say.