Is this a code-approved span table?

No. It is a mechanics calculator using simple beam formulas. Code-approved span tables include species, grade, duration factors, repetitive-member effects, bearing, notching, holes, vibration, and other rules that are not fully captured here.

What load should I enter for a floor joist?

A common residential starting point is 40 psf live load plus about 10 psf dead load, but actual design loads depend on the building use, finishes, partitions, tile, storage, and local code.

Why does joist depth matter so much?

The moment of inertia of a rectangular joist is b d³ / 12, so depth is cubed. A deeper joist is much stiffer than a shallow joist with similar material.

What deflection limit should I use?

L/360 is common for many floor checks, but brittle finishes such as tile or stone may require stricter limits. Always check the governing code or product installation instructions.

Can I use this for engineered I-joists?

Not directly. Engineered I-joists need manufacturer-published stiffness, shear, moment, web-hole, bearing, and vibration data. Use this page for simple rectangular sawn-lumber style sections only.

Engineering

Span Calculator for Joists

Estimate a joist span from bending stress and deflection limits using simple beam formulas for uniformly loaded, simply supported joists.

This page is a mechanics-based preliminary span check. It is not a substitute for building-code span tables, product-specific engineered lumber data, or a structural engineer. For code-oriented wood span selection, compare against the American Wood Council span resources.

Actual joist width b (in)

Actual joist depth d (in)

Spacing (in on-center)

Total floor load (psf)

Allowable bending stress Fb (psi)

Modulus of elasticity E (psi)

Deflection limit L/

Known span to check (ft, optional)

Allowable span estimate

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Bending-controlled span—

Deflection-controlled span—

Section properties—

Known span check—

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How this joist span calculator works

The calculator treats each joist as a simply supported beam carrying a uniform line load from the floor area assigned to that joist. It checks two common service conditions: bending stress and vertical deflection. The smaller span is reported as the preliminary allowable span.

For actual residential framing, use code-recognized resources such as the American Wood Council Span Options Calculator for Wood Joists and Rafters, manufacturer data, and local building-code requirements.

Worked example

2×10 actual size: b = 1.5 in, d = 9.25 in Spacing = 16 in o.c. Total load = 50 psf Line load = 50 × 16 / 12 = 66.67 plf I = 98.93 in⁴ S = 21.39 in³

If the deflection span is smaller than the bending span, the floor may feel bouncy before the joist reaches its stress limit. This is why stronger lumber alone does not always solve serviceability problems.

Common joist span mistakes

Using nominal lumber dimensions instead of actual dimensions is a major mistake. A nominal 2×10 is usually about 1.5 inches by 9.25 inches, not 2 inches by 10 inches. Another common mistake is checking live load only when total load is needed for some service checks.

Common questions

No. It is a mechanics calculator using simple beam formulas. Code-approved span tables include species, grade, duration factors, repetitive-member effects, bearing, notching, holes, vibration, and other rules that are not fully captured here.
A common residential starting point is 40 psf live load plus about 10 psf dead load, but actual design loads depend on the building use, finishes, partitions, tile, storage, and local code.
The moment of inertia of a rectangular joist is b d³ / 12, so depth is cubed. A deeper joist is much stiffer than a shallow joist with similar material.
L/360 is common for many floor checks, but brittle finishes such as tile or stone may require stricter limits. Always check the governing code or product installation instructions.
Not directly. Engineered I-joists need manufacturer-published stiffness, shear, moment, web-hole, bearing, and vibration data. Use this page for simple rectangular sawn-lumber style sections only.