Joist & 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.
| Spacing | Max span | Governs | Design load |
|---|---|---|---|
| Run the calculator to see the span table | |||
How to use the joist & rafter span calculator
Four quick steps give you a preliminary maximum span from a bending and deflection check — plus whether your target span passes and how it changes across spacings.
Pick member & size
Choose floor, deck, ceiling joist or rafter, then the lumber size and the species and grade you’ll actually buy.
Set loading & spacing
Confirm the live and dead loads, on-center spacing, and deflection limit. The member type presets typical values.
Check a target span
Enter the clear span you need to cover, and the tool flags pass or fail and how much capacity you’re using.
Read results & save a PDF
See the bending and deflection spans, which one governs, a span table by spacing, and a downloadable report.
How a span estimate is calculated
A member has to be both strong enough (bending) and stiff enough (deflection). The tool works out the span each check allows and reports the smaller. Here is the exact math.
Worked example — 2×10 DF-L No. 2 floor joist @ 16″ o.c., 40/10 psf, L/360
Section: S ≈ 21.4 in³, I ≈ 98.9 in⁴
Adjusted Fb′: 900 × 1.0 × 1.1 × 1.15 ≈ 1,139 psi
Load: (40 + 10) × 1.33 = 66.7 plf
Bending span ≈ 15′-7″ · Deflection span ≈ 16′-5″
Max ≈ 15′-7″ — bending governs at this spacing. (Estimate — verify against code tables.)
Span estimating charts
Handy lookups for lumber section properties, reference design values, typical loads and deflection limits, and how span changes with spacing. These feed the estimate — they are not a substitute for code span tables.
| Nominal | Actual depth | Section modulus S | Moment of inertia I |
|---|---|---|---|
| 2×6 | 5.5 in | 7.6 in³ | 20.8 in⁴ |
| 2×8 | 7.25 in | 13.1 in³ | 47.6 in⁴ |
| 2×10 | 9.25 in | 21.4 in³ | 98.9 in⁴ |
| 2×12 | 11.25 in | 31.6 in³ | 178.0 in⁴ |
All at 1.5″ actual width. S drives bending capacity; I drives stiffness — note how I roughly doubles with each size up.
| Species & grade | Bending Fb | Stiffness E |
|---|---|---|
| Douglas Fir-Larch No. 2 | 900 psi | 1.6M psi |
| Douglas Fir-Larch No. 1 | 1,000 psi | 1.7M psi |
| Hem-Fir No. 2 | 850 psi | 1.3M psi |
| SPF No. 2 | 875 psi | 1.4M psi |
| Southern Pine No. 2 | 1,050 psi | 1.6M psi |
| Southern Pine No. 1 | 1,250 psi | 1.7M psi |
Typical published reference values. Higher Fb spans farther in bending; higher E spans farther in deflection. Confirm against your lumber’s grade stamp.
| Member | Live load | Dead load | Deflection limit |
|---|---|---|---|
| Floor joist | 40 psf | 10 psf | L/360 (live) |
| Deck joist | 40 psf | 10 psf | L/360 (live) |
| Ceiling joist | 10 psf | 5 psf | L/240 (total) |
| Rafter | 30 psf | 15 psf | L/240 (total) |
Rafters use a load-duration factor (CD 1.15) for snow. Loads are typical starting points — use the values your code and design require.
| Spacing | Max span (est.) | Governs |
|---|---|---|
| 12″ o.c. | ~18′-0″ | Bending |
| 16″ o.c. | ~15′-7″ | Bending |
| 19.2″ o.c. | ~14′-3″ | Bending |
| 24″ o.c. | ~12′-9″ | Bending |
This calculator’s estimate for one configuration — closer spacing spans farther. Your code span table may differ; always verify.
Everything the calculator works out
One member spec gives you a full preliminary span picture — both governing checks, a pass/fail against your target, and how it all shifts with spacing.
Key figures behind every span check
A fast first pass on framing sizes
From a deck sanity check to preliminary sizing before the drawings, the same calculator gives a quick estimate — then you verify.
Planning a deck or floor and wants a ballpark on whether a joist size will reach before talking to the inspector.
- Pick the member and lumber size
- Enter your target span
- Take the result to your building dept.
Comparing sizes and spacings quickly to plan a layout, then confirming against the code span table.
- Read the span table by spacing
- See bending vs deflection govern
- Match species to your lumber yard
Roughing in member sizes early in a design before an engineer runs the final numbers.
- Test loads and deflection limits
- Compare species and grades
- Hand off for engineered design
7 things to know about spans
Spans are where framing turns structural, so the stakes are real. These points keep the estimate honest and point you to the right final source.
Joist & rafter span calculator FAQ
The span, load, and deflection questions people ask most before sizing joists and rafters.
As a rough estimate, a 2×10 No. 2 Douglas Fir-Larch floor joist at 16″ o.c. carrying 40 psf live and 10 psf dead with an L/360 limit spans on the order of 15 to 16 feet — with bending usually the governing check at that spacing.
The exact number depends on species and grade, spacing, load, and the deflection limit. Closer spacing spans farther; a lower-stiffness species spans less.
This is a preliminary estimate — always confirm against your code span tables (IRC) and a licensed professional.
Two independent checks, and the shorter answer wins.
Bending checks whether it’s strong enough: allowable moment = Fb′ × section modulus S. Deflection checks whether it’s stiff enough: sag is limited to a fraction of the span (like L/360) and depends on stiffness E and moment of inertia I.
The maximum span is the smaller of the two. On long members deflection often governs; on short, heavily loaded ones, bending often does.
Bending is about strength — whether the member over-stresses or breaks. It compares bending stress from the load against the wood’s adjusted allowable value, Fb′.
Deflection is about stiffness — how much it sags. Even a member that won’t break can feel bouncy or crack finishes if it flexes too much, so codes cap deflection at a fraction of the span (L/360, L/240, L/480).
A member has to pass both; the calculator reports each span and uses the smaller.
Yes. Spacing sets how much floor or roof each member carries — the tributary load. Wider spacing puts more load on each joist, so it spans less; closer spacing lets each one span farther.
Common spacings are 12, 16, 19.2, and 24″ o.c. Going from 24″ to 16″ meaningfully increases the allowable span for the same lumber.
If a size just misses your target, tightening the spacing or moving up one lumber size are the two usual fixes.
L/360 means the member may sag no more than its span divided by 360. Over a 12-foot (144″) span, that’s 144 ÷ 360 = 0.4″ of allowable deflection.
L/360 under live load is standard for floors, where bounce is noticeable. L/240 (looser) is common for roofs and ceilings, and L/480 (tighter) is used for especially stiff floors, such as under tile.
A smaller denominator allows more sag.
No. This is a preliminary planning estimate from simplified bending and deflection formulas with typical published values — it is not engineered design and must not be used for permitting or construction.
Real code span tables (such as the IRC) and a licensed engineer account for things this tool does not: shear, bearing, vibration, connections, and load combinations.
Use it to explore options quickly, then confirm every value with your building department and a professional.
Not with this tool. It sizes repetitive joists and rafters, which each carry a strip of load set by their spacing.
A beam, girder, or door/window header collects the tributary load from the floors, walls, and roof above, so its size depends on that whole load path — not just its own dimensions.
Size headers and beams from a prescriptive header table (for example IRC Table R602.7) or have the tributary widths and loads run by your building department or a licensed engineer.
Use the species and grade you can actually buy locally, then confirm its design values. No. 2 is the common framing grade; No. 1 is stronger and stiffer.
Douglas Fir-Larch and Southern Pine have higher bending and stiffness values, so they span farther; Hem-Fir and SPF are lower.
The bending value Fb and stiffness E come from the exact species and grade, so match the calculator to your lumber and verify against the grade stamp and code before relying on any span.
Structural Notice — Preliminary Estimate Only: This calculator estimates a maximum span from simplified simple-span bending and deflection formulas using typical published reference values, adjusted only for load duration, size, and repetitive-member use. It does not check shear, bearing, vibration, connections, notching, or load combinations, and it does not cover beams, girders, or headers. It is not engineered design and must not be used for permitting or construction. Always verify member size, species, grade, spacing, loads, span, and connections against your local building code span tables (IRC) and a licensed design professional before building. For preliminary planning only.