Floor Joist Span Calculator — ft, in to joist size
How to use this calculator
Enter the clear span face-to-face between bearing walls or beams (subtract bearing — not the room dimension), spacing (16″ o.c. is the residential default), species, and the live/dead loads. The first size that passes both bending and L/360 deflection is the recommended joist. Use it to spec lumber, then price it with the board feet calculator.
Live load: 40 psf for living areas (IRC default), 30 psf for sleeping rooms, 50 psf for assembly. Dead load: 10 psf light (subfloor only), 15 psf typical (subfloor + ceiling + finishes), 20 psf heavy (tile/stone). Deflection is checked at L/360 for live load — IRC requires it, and a stiffer L/480 keeps tile and stone from cracking.
| Joist Size | Actual (in) | Max Span | Deflection | Status |
|---|
Saved Calculations
| Time | Span | Spacing | Species | Load | Recommended | Status |
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Floor Joist Sizing Notes
Spans here are computed from simple-span bending and L/360 live-load deflection using NDS reference values for Spruce-Pine-Fir, Douglas Fir-Larch, Southern Pine, and Hem-Fir, then cross-checked against the prescriptive joist tables in IRC R502.3.1. 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 first row that passes both checks is highlighted as the recommended size. 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 Fb·S where S = bd²/6, giving Lbend = √(8·Fb·S / w). Deflection: Δ = 5wL⁴/(384·E·I) with I = bd³/12, limited to L/360 under live load, giving Ldefl = ∛(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.
