Floor Joist Span Calculator — ft, in to joist size

Floor Joist Sizing Notes

This page does not generate an allowable structural span. Enter the allowable span from the exact locally adopted prescriptive table, engineered design, or manufacturer documentation that governs the selected member. Use it to pick a nominal size before pricing lumber — feed the chosen size into the board feet lumber calculator for material cost, or into the stud wall calculator for the bearing walls below.

How to use

Enter the clear span face-to-face between supports (not the room dimension — subtract bearing). Pick spacing (16" o.c. is the residential default), species, grade, and the live/dead loads for the room. The page highlights whether the clear span is within the allowable span you entered; it does not select a member. For floor systems carrying a deck or balcony, the deck calculator applies the 40–60 psf live load and ledger detailing.

Formulas

Bending: maximum moment M = wL²/8 is set equal to F_b·S where S = bd²/6, giving L_bend = √(8·F_b·S / w). Deflection: Δ = 5wL⁴/(384·E·I) with I = bd³/12, limited to L/360 under live load, giving L_defl = ∛(384·E·I / (5·w·360)). The governing span is the smaller of the two. Actual surfaced dimensions are used (2×10 = 1.5" × 9.25").

Loads & deflection (IRC)

Living areas: 40 psf live + 10–20 psf dead. Sleeping rooms: 30 psf live. Dead load covers subfloor (2–4 psf), ceiling below (5–8 psf), and finishes. IRC requires L/360 for live load and L/240 for total load — a 14 ft span allows 14×12/360 = 0.47" live deflection. Tile and stone finishes want L/480 to keep grout from cracking. Verify local amendments. For sheathing over the joists, size panels with the plywood & OSB calculator; deck guardrails use the railing calculator.

On install day

Three things estimators miss on joist orders. Bearing length — IRC §R502.6 requires 1½″ on wood/metal and 3″ on masonry; long unsupported runs need a built-up beam or LVL, not a deeper joist. Bridging or blocking — IRC §R502.7 requires mid-span blocking for joists deeper than 2×10 over 8 ft; without it, a passing span calc still bounces in service. Stock length and waste — 2×10 and 2×12 are typically sold in 2-ft increments up to 20 ft; add 10–15% framing waste plus rim and band joists when you order. Typical lumber price in 2025 USA is roughly $1.20–$2.00/lf for #2 SPF 2×10 and $1.80–$2.80/lf for 2×12. Price the takeoff with the board feet calculator and check fence-post or deck-post requirements with the fence calculator.

FAQ

Is this an engineered design? No — it is a prescriptive estimate for typical simple spans. Multi-span, cantilevered, or notched joists, and openings/headers, require an engineer.

Why does my span fail at 24" o.c. but pass at 16"? Wider spacing increases the tributary load per joist, so deflection governs sooner. Tighten spacing or go up a size.

Which grade should I assume? #2 is the most common stock grade and a safe default; Select Structural is rarely worth the premium for residential floors.