Check how far a floor or deck joist can span. Choose the lumber size, spacing, species and grade, and the design loads to get the maximum allowable span, what limits it (bending or deflection), and a complete span table for every size and spacing.
Last updated:
Joist use
psf
30 sleeping rooms, 40 living areas and decks (IRC)
psf
10 typical; 15–20 for tile or heavy finishes
Enter your values to see the result.
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How to use the Joist Span Calculator
Pick indoor floor or outdoor deck. Decks use wet-service and treated-lumber adjustments.
Choose the lumber size, spacing and species/grade stamped on your lumber.
Set the live and dead loads: 40 psf live and 10 psf dead covers most living areas and decks.
Choose a deflection limit, L/360 for most floors or stricter for tile and stone.
Read the maximum span and use the span table to compare other sizes and spacings.
How the joist span is calculated
A joist is treated as a simply supported beam carrying a uniform load. The load per foot of joist is w = (live load + dead load) × spacing ÷ 12. The allowable span is the smallest of:
Bending: L = √(8 · Fb′ · S ÷ w), with section modulus S = b·d²/6
Deflection: L = ∛(384 · E′ · I ÷ (5 · n · wlive)), with I = b·d³/12 and n = 360 for an L/360 limit
Shear: L = 4 · Fv′ · A ÷ (3 · w)
Fb′, E′ and Fv′ are the NDS reference design values for visually graded dimension lumber multiplied by the size factor, the repetitive member factor (1.15 for joists 24 in o.c. or closer) and, for decks, wet-service and incising factors. This is the same method behind the IRC and AWC span tables, so results match them closely; tables round down and some cap deck spans (for example at 18 ft).
Douglas Fir-Larch No.2, dry service, calculated with the method above:
Size
12 in o.c.
16 in o.c.
24 in o.c.
2×6
10′-8″
9′-8″
8′-2″
2×8
14′-1″
12′-9″
10′-5″
2×10
18′-0″
15′-7″
12′-8″
2×12
20′-10″
18′-1″
14′-9″
Reference design values used
Species and grade
Fb (psi)
E (million psi)
Fv (psi)
Douglas Fir-Larch No.1 / No.2
1,000 / 900
1.7 / 1.6
180
Hem-Fir No.1 / No.2
975 / 850
1.5 / 1.3
150
Spruce-Pine-Fir No.1/No.2
875
1.4
135
Southern Pine No.2 (2×6 / 2×8 / 2×10 / 2×12)
1,000 / 925 / 800 / 750
1.4
175
Western Cedars No.2
700
1.0
155
Size factors CF for Fb (not used for Southern Pine, whose values are size-specific): 2×6 1.3, 2×8 1.2, 2×10 1.1, 2×12 1.0.
Frequently asked questions
How far can a 2×10 floor joist span?
A Douglas Fir-Larch No.2 2×10 at 16 inches on center carries 40 psf live and 10 psf dead over a clear span of about 15 ft 7 in, limited by bending. At 24 inches on center the span drops to about 12 ft 8 in, and for 30 psf bedroom loads at 16 inches it rises to about 17 ft 5 in. Species, grade and loads change the result, so enter your own values.
What is the maximum span for a 2×8 deck joist?
For Southern Pine No.2 at 16 inches on center with 40 psf live and 10 psf dead load, the calculated span is about 11 ft 9 in, in line with the 11 ft 10 in listed in the IRC deck joist table (R507.6). Douglas Fir, Hem-Fir and SPF treated lumber is weaker after incising, so it spans about 11 ft 1 in at the same spacing.
How is joist span calculated?
The span is the smallest of three limits for a simply supported beam with a uniform load: bending, L = √(8·Fb′·S/w); deflection, L = ∛(384·E′·I/(5·n·w)) for an L/n limit; and shear. Fb′, E′ and Fv′ are the NDS reference design values for the species and grade multiplied by adjustment factors such as size, repetitive member, wet service and incising.
What live load should I use?
The IRC uses 30 psf for sleeping rooms and 40 psf for other living areas and decks. Use higher loads for heavy items such as hot tubs, water beds, aquariums or libraries, and add dead load for tile, stone or plaster ceilings. A hot tub can exceed 100 psf, which usually needs an engineer.
What does L/360 mean?
It is a deflection limit: under the live load, the joist may sag no more than its span divided by 360, about 1/2 inch on a 15 ft span. Stiffer limits such as L/480 or L/720 are recommended under tile and natural stone to prevent cracking.
Is span measured center to center?
Joist span tables give the clear span: the horizontal distance between the faces of the supports, not including the bearing length or any cantilever. Measure between the inside faces of the beams, ledger or walls.
Can I use this instead of the building code tables?
Use it to plan and compare. Always confirm with the span tables adopted by your local building department, the lumber grade stamp and, for unusual loads, a structural engineer. The calculator does not cover notches, holes, point loads, cantilevers or engineered lumber.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.