| Size | Max span | Governs | at this spacing/load |
|---|
| Species & grade | Fb (psi) | Fv (psi) | E (psi) |
|---|---|---|---|
| SPF #2 | 875* | 135 | 1,400,000 |
| Hem-Fir #2 | 850* | 150 | 1,300,000 |
| Douglas Fir-Larch #2 | 900* | 180 | 1,600,000 |
| Southern Pine #2 | 975-1,250 | 175 | 1,600,000 |
| Douglas Fir-Larch #1 | 1,000* | 180 | 1,700,000 |
| SPF Select Structural | 1,250* | 135 | 1,500,000 |
A 2x10 floor joist in a typical species and grade (say SPF or Hem-Fir #2) spanning a 40 psf live / 10 psf dead residential floor runs roughly 15 to 16 feet at 16 inches on center, and less at 24 inches. The exact number depends on species, grade, spacing and load, and is usually governed by the L/360 live-load deflection limit rather than bending. This calculator solves all three checks for your inputs, but it is a preliminary estimate, not a substitute for an engineer's sizing.
For ordinary residential floor members it is almost always deflection: the L/360 live-load limit keeps floors from feeling bouncy well before the wood is anywhere near its bending capacity. Short, heavily loaded members can be governed by shear, and longer or lightly loaded ones by bending. This tool reports the maximum span from each check and tells you which one controls.
The IRC and IBC use L/360 for live-load deflection of floors and L/240 for total load. Ceilings with brittle finishes use L/240 live, and cantilevers and some roof members have their own limits. A stiffer target such as L/480 is common for tile floors and long spans where bounce is a concern. This calculator applies L/360 live and L/240 total by default.
It applies the same NDS reference design values and bending, shear and deflection formulas that underlie the IRC span tables, with load-duration, size and repetitive-member adjustment factors. But it does not check bearing, lateral stability, connections, notching, holes, vibration, concentrated or point loads, or unusual load paths, and reference values vary by supplier and region. Treat every result as preliminary and have the final framing sized and stamped by a licensed structural engineer.
Joists are repetitive horizontal members carrying floor or ceiling loads, spaced 12 to 24 inches apart and sharing load through the sheathing, which earns them a repetitive-member bending bonus. Rafters do the same for a sloped roof. A beam or girder is a single, larger (often built-up) member that collects the loads from many joists over a wider tributary width, so it gets no repetitive bonus and is sized on its own.