Beam Span Calculator

A beam span is the horizontal distance a structural beam can safely cross between its support points (posts, walls, or columns). Getting the size right matters in both directions: an undersized beam can fail, and an oversized one wastes money and headroom.

Enter the ACTUAL dimensions, not the nominal size

This is the mistake that makes people over-span by a fifth. A “2×10” has not measured 2 by 10 inches since the 1960s. It is dressed to 1.5 × 9.25, and losing a quarter of the width and three quarters of an inch of depth costs more than it sounds: type 2 and 10 into the boxes below instead of 1.5 and 9.25 and the answer comes back about 20% longer than the timber can actually carry.

Built-up beams add together: three 2×10s side by side give 4.5 × 9.25, not 6 × 10.

Key Variables in Beam Sizing

The maximum safe span of a beam depends on several factors:

• Wood species and grade: Different species have different strength ratings (Fb, fiber stress in bending). Douglas Fir and Southern Yellow Pine are stronger than SPF (Spruce-Pine-Fir). Appearance grading (Select, #1, #2) also affects allowable stress.
• Beam dimensions: A larger cross-section (width × depth) dramatically increases span capacity. Doubling the depth quadruples the bending strength.
• Load: The beam must support its own weight plus live loads (people, furniture) and dead loads (floors, roofing, insulation). Typical floor live load is 40 psf (1.9 kPa); roof live load is 20–30 psf (1.0–1.4 kPa).
• Spacing: The tributary width, how much of the floor or roof load each beam carries, depends on how far apart the beams are spaced.

Span Table Reference (Douglas Fir #2, 40 psf Live + 10 psf Dead)

Metric equivalents: 1 foot = 0.3048 m; 1 psf = 47.88 Pa.

Those are published span-table figures. Run the dressed dimensions through the calculator on this page and it lands at or just below every one of them, by design, for the reason set out below.

Bending is rarely what stops you. Deflection is.

Most people size a beam on bending stress and are surprised when the span table comes out shorter than their arithmetic. That gap is deflection, and it governs almost every residential floor beam.

A beam can be nowhere near breaking and still bounce enough to crack tile, pop drywall seams, and make a floor feel cheap. Codes therefore cap how far a loaded beam may sag:

• L/360 under live load. A 16-foot span may drop no more than 16 × 12 ÷ 360 = 0.53 inches when people and furniture are on it. This is the floor limit and it is usually the binding one.
• L/240 under total load, live plus dead together.

Deflection depends on the moment of inertia I = bd³/12 and the modulus of elasticity E of the species, which is a completely different property from the bending strength Fb. Two species can share an Fb and sag differently.

This calculator runs all three checks (bending, L/360, L/240), reports which one wins, and then trims the answer by 10% for the factors it does not model: load duration, wet service, repetitive-member action, notches, holes, and the connection at each end. That trim is what keeps it at or under the published table on every row above rather than over it, which is the direction an error on a structural page has to fall.

Important Warning

This calculator provides estimates for preliminary planning only. All structural calculations for permitted construction must be verified by a licensed structural engineer. Local building codes, soil conditions, load paths, and connection details must be evaluated for each specific project.