Joist & Rafter Span Calculator
Find how far a 2×6 to 2×12 joist or rafter can span. Set the species, grade, spacing and loads; spans are worked out the way the IRC and AWC tables are, with a span table for your loads.
How to Use
- Choose what the lumber is: floor joists, ceiling joists or rafters. Typical loads fill in.
- Pick the species and grade, the size and the spacing.
- Adjust the live (or snow) load and the dead load if your code or design calls for different values, and the deflection limit.
- Read the maximum span and what limits it: bending, deflection or shear.
- Enter your actual span to see how much of the allowable you are using, and read the table for every size and spacing.
Worked Example
Douglas fir-larch No. 2 2×10 floor joists at 16 in centres, 40 psf live and 10 psf dead load, L/360. Each joist carries 16 in of floor: (40 + 10) × 16 ÷ 12 = 66.7 lb per foot, or 5.56 lb per inch. A 2×10 is 1½ × 9¼ in: section modulus S = 21.39 in³, moment of inertia I = 98.93 in⁴.
Bending: the allowable stress is 900 psi × 1.1 (size factor) × 1.15 (repetitive members) = 1,139 psi, so the longest span is √(8 × 1,139 × 21.39 ÷ 5.56) = 187.3 in = 15 ft 7 in. Deflection: under the live load alone (4.44 lb/in), with E = 1,600,000 psi and a limit of span ÷ 360, the span can reach ∛(384 × 1,600,000 × 98.93 ÷ (5 × 4.44 × 360)) = 196.6 in, 16 ft 5 in. Shear allows far more. Bending governs: 15 ft 7 in, the figure in IRC Table R502.3.1(2).
The common mistake: measuring the span on the wrong line. Span is the clear distance between the faces of the supports, not the length of the joist and not centre to centre of the walls. A joist cut 16 ft long on 3½ in plates has a clear span of about 15 ft 5 in.
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Formulas
Where Span Tables Come From
The span tables in the International Residential Code and the American Wood Council’s Span Tables for Joists and Rafters are not measured: they are calculated, exactly as here, from the design values in the National Design Specification (NDS) for Wood Construction. Those values come from grading rules and from testing thousands of full-size boards; each species group and grade has its own allowable bending stress, shear stress and stiffness, and Southern pine has values for each width.
Floor joists are usually limited by deflection or bending, depending on the load; long spans and light loads tend to be governed by stiffness. That is why engineered I-joists and floor trusses, which are much stiffer for their weight, have taken over long floor spans.
About This Calculator
This calculator works out the maximum span of a sawn-lumber floor joist, ceiling joist or rafter from its species, grade, size and spacing and the loads you set, using the same method as the IRC and AWC tables and current NDS design values. It shows which limit governs, checks your own span, and builds a span table for every size and spacing at your loads. Spans for rafters are horizontal.
Everything runs in your browser; nothing is sent anywhere. It assumes simply supported members, braced by sheathing, in dry service. For permits, follow the span tables your local code adopts, and have notched, cantilevered or point-loaded members checked by a professional.
Related calculators: Beam Load Calculator, Rafter Calculator, and Deck Calculator.
Frequently Asked Questions
How far can a 2×10 floor joist span?
For a living area (40 psf live, 10 psf dead, L/360) at 16 in centres: about 15 ft 7 in in No. 2 Douglas fir-larch, 15 ft 5 in in Spruce-pine-fir, 15 ft 2 in in Hem-fir and 14 ft 0 in in No. 2 Southern pine, whose 2×10 bending value is lower. At 12 in centres the Douglas fir joists reach 18 ft.
How are span tables calculated?
Each span is the shortest of three limits. Bending: the moment wL²/8 must not exceed the allowable stress times the section modulus. Deflection: the sag under live load, 5wL⁴/384EI, must stay under the limit, usually span ÷ 360 for floors. Shear: the reaction must not exceed the allowable shear. The allowable stresses come from the NDS design values for the species and grade, with factors for size, repeated members and load duration.
Why are my results a few inches different from an older span table?
Design values change. Douglas fir-larch No. 2 rose from 875 to 900 psi in bending in 2012, and Southern pine values were lowered in 2013, so tables printed before then can differ by a few inches. This calculator uses the current values; for permits, use the table your local code adopts.
What loads should I use?
US residential codes use 40 psf live for living areas, 30 psf for sleeping rooms and attics with storage, 10 psf (or 20 with limited storage) for ceiling joists, and 10 to 20 psf dead load. Rafters carry the roof snow load or 20 psf roof live load, whichever is larger. Decks are designed for 40 psf live in most places.
What does L/360 mean?
The largest sag allowed is the span divided by 360: 0.5 in on a 15 ft joist. Floors use L/360 so they do not feel bouncy and finishes do not crack; rafters without a ceiling below use L/180, and with a ceiling attached L/240. Tile and stone floors are often designed to L/480 or stiffer.
How do I use the Joist & Rafter Span Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Floor joists
DF-L No. 2 2×10 at 16 in, 40/10 psf: 15 ft 7 in, limited by bending.
Bedroom floor
The same joists at 30 psf live: 17 ft 5 in, still limited by bending.
Southern pine
SP No. 2 2×12 at 16 in, 40/10 psf: 16 ft 6 in.
Ceiling joists
SPF No. 2 2×6 at 16 in with 10 psf live, 5 psf dead, L/240: 16 ft 11 in, limited by deflection.
Rafters
DF-L No. 2 2×8 at 16 in, 30 psf snow, 10 psf dead, L/180: 15 ft 4 in measured horizontally.
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