Floor Joist Span Calculator

How the Span Is Calculated

AWC’s floor joist tables are built on stiffness: for each size and spacing they list the span at which the joist deflects span ÷ 360 under the live load, for E values from 0.8 to 2.4 million psi, and below each column the bending design value (Fb) the lumber must have to carry the live plus dead load at that span. The calculator looks up your species, grade and size in AWC’s 2024 Design Values (normal-duration Fb for repetitive members and E), reads the span at that E, and checks Fb. If the lumber’s Fb is lower than required, bending governs and the span drops to the column whose required Fb matches, using the straight-line interpolation AWC permits. Spans are rounded down to the inch.

Worked example: Douglas Fir-Larch No. 2 2x10 has E = 1,600,000 psi and Fb = 1,140 psi. Table F-2 (40 psf live, 10 psf dead) at 16 inches gives 16′-5″ at E = 1.6, but that needs Fb = 1,255 psi. An Fb of 1,140 falls between the 1.3 column (1,093 psi, 15′-3″) and the 1.4 column (1,148 psi, 15′-8″), 85% of the way, so the span is 15′-3″ + 0.85 × 5″ = 15′-7″.

Rules and Limits of the Tables

AWC’s tables assume at least three joists spaced no more than 24 inches on center, fully supported, with sheathing nailed to the top edge, lumber at 19% moisture or less, and spans measured face to face of the supports. The Fb values are for repetitive members at 24 inches or less. Table F-1 (30 psf live) covers sleeping rooms and attic floors, F-2 (40 psf) other living areas and decks, and F-5 (40 psf live, 20 psf dead) floors with tile or stone up to 10 psf or 1.5 inches of lightweight concrete. Spans stop at 26 feet. Bearing (compression perpendicular to grain) also has to be checked: AWC’s tables give the required value by span and bearing length.

Building Codes and These Tables

Most US jurisdictions adopt the International Residential Code, whose Table R502.3.1(1) and (2) list floor joist spans by species and grade for the same 30 and 40 psf loads. Code tables are tied to the design values in effect when that edition was written, so a locally adopted IRC edition can show different spans from AWC’s 2024 values. Your local code and building official have the final word.

Have It Checked by a Building Official or Engineer

A span table answers one question about a simple, uniformly loaded joist. Ask your building department or a structural engineer before you build, and always when a joist carries a wall, a tub, a piano, a beam or a point load; when it is notched, drilled or cantilevered; when it supports another floor; on decks attached to a house; or when lumber is wet, damaged or not grade-stamped. Many jurisdictions require a permit and inspection for new floor framing.

What This Calculator Does Not Cover

It does not size rafters, ceiling joists, beams, headers, girders or engineered lumber (I-joists, LVL), and it does not check bearing, vibration, notches, holes, connections or point loads. Decks have additional rules for wet service, ledgers and footings; follow your local code and the deck guidance your building department uses.

Common Mistakes

Measuring the span center to center. Tabulated spans are the clear distance between supports.

Assuming any 2x10 is the same. Species and grade change E and Fb; a No. 2 spans less than Select Structural.

Using the 30 psf table for living rooms. It is for sleeping rooms and attic floors.

Forgetting tile. Tile or stone adds dead load; use the 20 psf dead load table.

Ignoring the grade stamp. Use the species and grade printed on the lumber.

Frequently Asked Questions

How to Use This Calculator

1. Choose the floor load — sleeping rooms, living areas or tile.
2. Pick species and grade — from the lumber’s grade stamp.
3. Choose size and spacing — 2x6 to 2x12, 12 to 24 inches on center.
4. Read the allowable span — and have the design checked.