Joist Span Calculator
Check how far a 2x6, 2x8, 2x10, or 2x12 floor or deck joist can span at 12, 16, or 24 inches on center — straight from the IRC span tables, with the smallest size that covers your span.
Joist, spacing, and span
Span is the clear horizontal distance between the faces of the supports (beam, bearing wall, or ledger) — not the overall length of the board.
Results
2x10 Douglas fir-larch No. 2 joists at 16 in on center can span up to 15 ft 5 in (IRC Table R502.3.1(2)) — your 14 ft span is within that limit.
Maximum allowable span
15 ft 5 in
Your 14 ft span is within the tabulated maximum, with 1 ft 5 in to spare.
Uniform-load tables don't cover concentrated loads like a bathtub, stone countertop, or hot tub — those need added joists or an engineer's check.
Source table: IRC Table R502.3.1(2)
Maximum span (metric): 4.70 m
Smallest listed size for 14 ft at 16" o.c.: 2x10
These are prescriptive-code values for planning. Engineered plans, the IRC edition your jurisdiction has adopted, and local amendments govern — confirm with your building department before you build.
How this works
This calculator doesn't compute a span from a formula — it looks one up. The IRC's joist span tables are the result of a full structural check (bending strength, shear, and deflection limited to L/360) for each species, grade, size, and spacing under a fixed load. For floors, values come from IRC Table R502.3.1(2) for residential living areas at 40 psf live load and 10 psf dead load. For decks, values come from IRC Table R507.6, which assumes No. 2 lumber in wet service at 40 psf live and 10 psf dead load, with an incising factor for the fir/hem-fir/SPF group. The calculator returns the tabulated maximum, compares it to your planned clear span, and finds the smallest listed size that clears your span at the spacing you chose. We transcribed only rows we could verify against a published copy of each table and left out anything we couldn't. Sources: IRC Table R502.3.1(2) as reproduced by the City of Republic, MO building department; IRC Table R507.6, Deck Joist Spans.
Floor joist span table (IRC R502.3.1(2))
Maximum clear spans from IRC Table R502.3.1(2), residential living areas, 40 psf live load, 10 psf dead load, L/Δ = 360. Rows are grouped by joist spacing. Values are feet-inches (15-5 = 15 ft 5 in).
| Spacing | Species / grade | 2x6 | 2x8 | 2x10 | 2x12 |
|---|---|---|---|---|---|
| 12" o.c. | Douglas fir-larch No. 1 | 10-11 | 14-5 | 18-5 | 22-0 |
| 12" o.c. | Douglas fir-larch No. 2 | 10-9 | 14-2 | 17-9 | 20-7 |
| 12" o.c. | Hem-fir No. 1 | 10-6 | 13-10 | 17-8 | 21-6 |
| 12" o.c. | Hem-fir No. 2 | 10-0 | 13-2 | 16-10 | 20-4 |
| 12" o.c. | Spruce-pine-fir No. 1 / No. 2 | 10-3 | 13-6 | 17-3 | 20-7 |
| 16" o.c. | Douglas fir-larch No. 1 | 9-11 | 13-1 | 16-5 | 19-1 |
| 16" o.c. | Douglas fir-larch No. 2 | 9-9 | 12-7 | 15-5 | 17-10 |
| 16" o.c. | Hem-fir No. 1 | 9-6 | 12-7 | 16-0 | 18-7 |
| 16" o.c. | Hem-fir No. 2 | 9-1 | 12-0 | 15-2 | 17-7 |
| 16" o.c. | Spruce-pine-fir No. 1 / No. 2 | 9-4 | 12-3 | 15-5 | 17-10 |
| 24" o.c. | Douglas fir-larch No. 1 | 8-8 | 11-0 | 13-5 | 15-7 |
| 24" o.c. | Douglas fir-larch No. 2 | 8-1 | 10-3 | 12-7 | 14-7 |
| 24" o.c. | Hem-fir No. 1 | 8-4 | 10-9 | 13-1 | 15-2 |
| 24" o.c. | Hem-fir No. 2 | 7-11 | 10-2 | 12-5 | 14-4 |
| 24" o.c. | Spruce-pine-fir No. 1 / No. 2 | 8-1 | 10-3 | 12-7 | 14-7 |
Deck joist span table (IRC R507.6)
Maximum clear spans from IRC Table R507.6, deck joists, No. 2 grade, wet service, 40 psf live load, 10 psf dead load, L/Δ = 360. Cantilever limits are not shown. 2x6 is omitted (see FAQ).
| Spacing | Species / grade | 2x6 | 2x8 | 2x10 | 2x12 |
|---|---|---|---|---|---|
| 12" o.c. | Southern pine (No. 2, treated) | — | 13-1 | 16-2 | 18-0 |
| 12" o.c. | Douglas fir-larch / hem-fir / SPF (No. 2, treated) | — | 12-6 | 15-8 | 18-0 |
| 12" o.c. | Redwood / western cedars / ponderosa / red pine (No. 2) | — | 11-8 | 14-11 | 17-5 |
| 16" o.c. | Southern pine (No. 2, treated) | — | 11-10 | 14-0 | 16-6 |
| 16" o.c. | Douglas fir-larch / hem-fir / SPF (No. 2, treated) | — | 11-1 | 13-7 | 15-9 |
| 16" o.c. | Redwood / western cedars / ponderosa / red pine (No. 2) | — | 10-7 | 13-0 | 15-1 |
| 24" o.c. | Southern pine (No. 2, treated) | — | 9-8 | 11-5 | 13-6 |
| 24" o.c. | Douglas fir-larch / hem-fir / SPF (No. 2, treated) | — | 9-1 | 11-1 | 12-10 |
| 24" o.c. | Redwood / western cedars / ponderosa / red pine (No. 2) | — | 8-8 | 10-7 | 12-4 |
Worked example: a basement-to-living-room floor
You're framing a 14 ft wide room with joists running wall to wall, and the lumberyard stocks Douglas fir-larch No. 2. At 16 in on center, IRC Table R502.3.1(2) lists 12 ft 7 in for a 2x8 — too short for 14 ft — and 15 ft 5 in for a 2x10. The 2x10 works with 1 ft 5 in to spare. If you wanted to stay with 2x8s, dropping to 12 in on center raises the 2x8 limit to 14 ft 2 in, which clears 14 ft by just 2 in — legal, but a thin margin for a floor that may get a tile bathroom later. Most framers would pick the 2x10 at 16 in: fewer joists, a stiffer floor, and room for a heavier finish.
A second example: a treated-pine deck
A ground-level deck extends 12 ft from the house ledger to a beam. The yard sells No. 2 treated southern pine. Under IRC Table R507.6 at 16 in on center, a 2x8 spans 11 ft 10 in — 2 in short of 12 ft — so it fails. A 2x10 spans 14 ft 0 in and passes with 2 ft to spare. The alternative is 2x8 at 12 in on center, which spans 13 ft 1 in: it passes too, but needs a third more joists and hangers. If the lumber were cedar instead, the 2x10 at 16 in drops to 13 ft 0 in — still enough for 12 ft, but with half the margin.
Common mistakes
Measuring the board length instead of the clear span
A 16 ft joist resting on two beams does not have a 16 ft span. Span is the clear distance between supports; the board also includes bearing length at each end and any cantilever. Compare the clear span to the table, then add bearing and overhang to find the board length to buy.
Using the floor table for a deck
The floor table assumes dry interior service. Deck lumber is wet and, for fir, hem-fir, and SPF, incised — both reduce its rated strength. A DF-L No. 2 2x10 at 16 in on center spans 15 ft 5 in as a floor joist but only 13 ft 7 in as a deck joist under R507.6. Using the floor number outdoors oversizes the span by almost 2 ft.
Assuming the lumber at the store is No. 1
Most framing lumber sold at home centers is No. 2 or 'No. 2 and better'. Check the grade stamp on the board before using a No. 1 row, and if a piece is stamped No. 3 or Stud grade, it doesn't belong in a joist span from this calculator at all.
Ignoring heavy finishes and point loads
The 10 psf dead load column doesn't cover a mortar-bed tile floor, and no uniform-load table covers a tub, island, or hot tub. Plan for doubled joists or added support under those loads, and treat a thin margin between your span and the table maximum as a warning, not a pass.
Mixing joist spacing and decking rating
A span that works at 24 in on center is no use if your subfloor or decking is only rated for 16 in. Check the panel or decking span rating first, then pick joist size for that spacing.
Treating an online calculator as the approved plan
This calculator reproduces published prescriptive tables so you can plan and budget. Your permit reviewer checks against the code edition your jurisdiction adopted, with any local amendments. Where plans are engineered, the engineer's sizes govern.
When these span tables stop applying
- Sleeping rooms and attics use different tables with lower live loads (30 psf and less), which allow longer spans. This calculator uses the 40 psf table, which is conservative for a bedroom but not the only legal option there.
- Dead loads over 10 psf — mortar-bed tile, stone, gypcrete, or heavy built-ins — need the 20 psf dead-load column of R502.3.1(2) or an engineered design; this calculator doesn't show that column.
- Engineered I-joists, open-web trusses, and LVL/LSL rim or joists are sized from the manufacturer's own span tables, not the IRC lumber tables.
- Snow loads: R507.6 deck spans are built on 40 psf live load. Where ground snow load exceeds that basis, the deck table's own footnotes and your local code set different limits.
- Cantilevers: deck joists may cantilever past a beam only within the limits in R507.6's cantilever columns (not shown here), and floor cantilevers carrying a wall or roof have their own tables.
- Notches, holes, and bored penetrations for plumbing or ductwork reduce a joist's capacity; the code limits their size and location separately from the span table.
- Species and grades not listed — including southern pine floor joists, Select Structural and No. 3 grades, and 2x6 deck joists — are omitted on purpose because we couldn't verify a current value. Use the code table your jurisdiction adopted or the American Wood Council span tables for them.
FAQ
How far can a 2x10 floor joist span?
It depends on species, grade, and spacing. Under IRC Table R502.3.1(2) for a living-area floor (40 psf live, 10 psf dead), a Douglas fir-larch No. 2 2x10 spans up to 15 ft 5 in at 16 in on center, 17 ft 9 in at 12 in on center, and 12 ft 7 in at 24 in on center. Hem-fir No. 2 at 16 in drops to 15 ft 2 in; spruce-pine-fir No. 1/No. 2 matches DF-L No. 2 at 15 ft 5 in.
How far can a 2x8 deck joist span?
Under IRC Table R507.6, a No. 2 treated southern pine 2x8 spans up to 11 ft 10 in at 16 in on center; the Douglas fir-larch / hem-fir / spruce-pine-fir group spans 11 ft 1 in; and redwood or western cedar spans 10 ft 7 in. At 12 in on center those rise to 13 ft 1 in, 12 ft 6 in, and 11 ft 8 in.
Why are deck joist spans shorter than floor joist spans for the same lumber?
Both tables use a 40 psf live load, but the deck table assumes No. 2 lumber in wet service — outdoor wood carries a moisture reduction on its design values — and the fir/hem-fir/SPF deck group also includes an incising factor for the cuts made so preservative can penetrate. Those reductions lower the allowable bending stress and stiffness, so the same 2x10 is rated for a shorter span on a deck than inside a dry house.
What is 'span' — the board length or the distance between supports?
Span is the clear horizontal distance between the supports the joist bears on — face of beam to face of beam, or ledger to beam — not the length of the board. A joist that rests 1 1/2 in onto a beam at each end is longer than its span. Measure between supports when you compare against a span table, and buy lumber long enough to include the bearing at both ends plus any cantilever.
What does 40 psf live load and 10 psf dead load mean?
Live load is the weight of people and furniture the floor must carry — the IRC sets 40 psf for living areas other than sleeping rooms, and 40 psf for decks. Dead load is the permanent weight of the structure itself: joists, subfloor, and finished flooring. The 10 psf column fits typical wood subfloor with carpet, vinyl, or wood flooring; heavy finishes such as tile over a mortar bed push the dead load toward the table's 20 psf column, which allows shorter spans and isn't shown in this calculator.
What does L/360 mean in the span tables?
It's the deflection limit: under full live load the joist may sag no more than its span divided by 360. On a 15 ft (180 in) span that's half an inch. It's a stiffness limit that keeps a floor from feeling bouncy and protects finishes. Tile floors usually want a stiffer floor than L/360, which is one more reason tile installations often use closer spacing or a deeper joist than the table minimum.
Can I use a span table for a floor with a bathtub, piano, or kitchen island?
Not on its own. The tables assume a uniformly distributed load. Concentrated loads — a cast-iron tub full of water, a stone kitchen island, a waterbed, a safe, a large aquarium — need extra support, usually doubled joists or blocking under the load, and sometimes a beam. The IRC commentary notes the tables allow minor concentrated loads like non-bearing partitions near a support, not heavy point loads mid-span.
Can a deck joist span table be used for a hot tub?
No. A filled hot tub loads a deck far beyond the 40 psf the R507.6 table is built around — often more than 100 psf across its footprint. Hot tubs on decks need framing designed for that load, and usually an engineer or the tub manufacturer's framing specification. Many installers set hot tubs on a slab instead.
Why isn't southern pine listed for floor joists?
Southern pine design values were revised in 2013, and the IRC's floor-joist spans for southern pine changed between editions as a result. Rather than publish a set of numbers that may not match the edition your jurisdiction adopted, this page omits southern pine floor spans. Check the IRC edition your building department enforces, or the American Wood Council span tables, for current southern pine floor values. Southern pine is included for decks, where two independent copies of R507.6 agreed.
Why can't I get a 2x6 deck joist span?
Published copies of IRC Table R507.6 we checked disagreed on the 2x6 values, while every 2x8 through 2x12 value matched. We only publish spans we could verify, so 2x6 deck spans are omitted rather than guessed. In practice 2x6 deck joists are limited to short spans under 10 ft anyway, and most deck framers start at 2x8.
Is 16 inches or 24 inches on center better for floor joists?
Sixteen inches is the common default because it gives longer spans for a given size and suits 3/4 in subfloor. Twenty-four inches saves joists but shortens the allowable span noticeably — a DF-L No. 2 2x10 drops from 15 ft 5 in at 16 in to 12 ft 7 in at 24 in — and needs subfloor rated for 24 in spacing. For decks, spacing also has to suit the decking: many composite boards require 16 in or 12 in spacing, especially on diagonal layouts.
Does a longer span need a bigger joist or tighter spacing?
Either works, and the calculator shows both options. Going up one size (2x8 to 2x10) typically adds about 3 ft of span at 16 in on center; tightening spacing from 16 in to 12 in adds roughly 1–2 ft. The third option is to cut the span in half with a beam or bearing wall mid-span, which lets a much smaller joist work.
Do I need a permit or engineer if my span is within the table?
Span tables let you size ordinary joists without an engineer under the prescriptive code, but most jurisdictions still require a permit for new floors and decks, and your building department's adopted code edition and local amendments govern. Engineered plans always override these tables. Anything outside the table's assumptions — heavy loads, snow loads above the table basis, engineered lumber, unusual species — needs an engineer or manufacturer's span data.
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