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www.homeexpertly.comJoist & Rafter Span Calculator — Max Allowable Span Estimate
Estimate how far a wood floor joist, deck joist, ceiling joist, or rafter can span. Pick the member, lumber size, species/grade, spacing, and loads — and see the maximum allowable span from a bending + deflection check, whether your target span passes, and a span table across spacings.
Preliminary estimate only — not engineered design. Results come from simplified bending and deflection formulas with typical reference values and are not a substitute for local code span tables (IRC) or a licensed engineer. Do not use for permitting or construction. Always verify size, species, grade, spacing, loads, span, and connections with your building department and a professional.
1
Member & size
Floor
joist
Deck
joist
Ceiling
joist
Rafter
roof
2×6
5.5"
2×8
7.25"
2×10
9.25"
2×12
11.25"
2
Loading & spacing
psf
psf
3
Check a span (optional)
ft
Leave 0 to skip the pass/fail checkMax allowable clear span (est.)
—
Set your member and click Calculate
Bending
—
limited span
Deflection
—
limited span
Governed by
—
criterion
Calculation breakdown
Actual size—
Section modulus S—
Moment of inertia I—
Adjusted Fb′—
Modulus E—
Design load—
Bending span—
Deflection span—
Result summary
Bending span
—
Deflection span
—
Max allowable
—
Design load
—
Deflection limit
—
Capacity used
—
Max span by spacing
| Spacing | Max span | Governs | Design load |
|---|---|---|---|
| Run the calculator to see the span table | |||
Visualizations
Max allowable span by spacing (ft)
Beam & header spans: a girder or door/window header carries tributary load from the floors, walls, and roof above, so its span depends on that load path — not just its own size. This tool sizes repetitive joists & rafters. For a built-up beam or header, have the tributary width and loads run by your building department or engineer, or use your local prescriptive header table (e.g., IRC Table R602.7).