MAX JOIST SPAN
Floor joist span chart — IRC Table R502.3.1
Maximum clear span for No. 2 grade floor joists at 10 psf dead load and an L/360 deflection limit. 30 psf live load covers sleeping rooms and attics reached by a fixed stair; 40 psf is general living area. Simple span between bearing points — no cantilever, no concentrated loads.
| Species | Joist | 16" o.c. / 30 psf | 16" o.c. / 40 psf | 24" o.c. / 30 psf | 24" o.c. / 40 psf |
|---|---|---|---|---|---|
| Doug Fir-Larch | 2×6 | 10'-9" | 9'-9" | 9'-3" | 8'-3" |
| Doug Fir-Larch | 2×8 | 14'-2" | 12'-9" | 11'-8" | 10'-5" |
| Doug Fir-Larch | 2×10 | 17'-5" | 15'-7" | 14'-3" | 12'-9" |
| Doug Fir-Larch | 2×12 | 20'-3" | 18'-1" | 16'-6" | 14'-9" |
| Spruce-Pine-Fir | 2×6 | 10'-3" | 9'-4" | 8'-11" | 8'-1" |
| Spruce-Pine-Fir | 2×8 | 13'-6" | 12'-3" | 11'-6" | 10'-3" |
| Spruce-Pine-Fir | 2×10 | 17'-2" | 15'-5" | 14'-1" | 12'-7" |
| Spruce-Pine-Fir | 2×12 | 19'-11" | 17'-10" | 16'-3" | 14'-7" |
| Southern Pine | 2×6 | 10'-3" | 9'-4" | 8'-6" | 7'-7" |
| Southern Pine | 2×8 | 13'-3" | 11'-10" | 10'-10" | 9'-8" |
| Southern Pine | 2×10 | 15'-8" | 14'-0" | 12'-10" | 11'-5" |
| Southern Pine | 2×12 | 18'-6" | 16'-6" | 15'-1" | 13'-6" |
IRC 2021 Tables R502.3.1(1) and R502.3.1(2), No. 2 grade, 10 psf dead load. Identical in the 2024 edition.
Heavier finishes put you in the 20 psf dead load column, which is shorter. Run your actual case above.
About this calculator
This floor joist span calculator returns the maximum allowable simple span for residential floor joists, read directly from IRC Table R502.3.1(1) for sleeping areas at 30 psf live load and Table R502.3.1(2) for living areas at 40 psf, dead load 10 psf columns. These two tables are identical in the 2021 and 2024 editions. The calculator covers four joist sizes (2×6 through 2×12), two standard spacings (16 and 24 in o.c.), both residential live loads, and three common framing species (Doug Fir-Larch, Spruce-Pine-Fir, Southern Pine) at #2 grade with an L/360 deflection limit. Spans assume a simple span between bearing walls, no cantilever, and no concentrated loads. ESTIMATE ONLY — verify with an engineer or your local building official before framing.
How to use this calculator
Pick the joist size (2×6 through 2×12), the on-center spacing (16 inches is residential standard, 24 inches saves lumber but reduces span), the live load (40 psf for general living areas including kitchens and dining rooms, 30 psf for sleeping rooms only), and the lumber species available in your region (Doug Fir-Larch on the West Coast, Spruce-Pine-Fir in the Northeast and Midwest, Southern Pine in the Southeast).
The calculator returns the maximum allowable simple-span length read from IRC Table R502.3.1(1) (30 psf sleeping areas) or Table R502.3.1(2) (40 psf living areas), dead load 10 psf column. Spans assume #2 grade lumber and an L/360 deflection limit — the standard residential configuration. Both tables are identical in the 2021 and 2024 editions.
Worked example
For 2×10 SPF #2 joists at 16" o.c. supporting a 40 psf live load (general living area):
Max span: 15'-5" per IRC Table R502.3.1(2), dead load 10 psf column.
Same joists at 24" o.c.: max span drops to 12'-7" — an 18% reduction, not the 10-15% a rule of thumb suggests.
Switching to Doug Fir-Larch at 16" o.c., 40 psf: 15'-7", only 2 inches more than SPF. The species spread at #2 grade is smaller than most people expect.
For a sleeping-room-only floor (30 psf live load, Table R502.3.1(1)), 2×10 SPF at 16" o.c. spans 17'-2" — 1 ft 9 in longer than the same joist under living-area load.
For wider rooms: 2×12 SPF at 16" o.c. and 40 psf reaches 17'-10". Beyond that, engineered I-joists or LVLs.
Common mistakes & waste factors
Mixing up joist size and span. "12-ft joists" might mean joists that ARE 12 ft long, or joists that span 12 ft. The table gives max span — your actual joist length should match that.
Forgetting the 30 vs 40 psf distinction. Using sleeping-room loads (30 psf) for a kitchen or dining room is a code violation. When in doubt, use 40 psf.
Picking the wrong species. The calculator handles three common species. Engineered I-joists, LVLs, and dimensional lumber from less common species (Hem-Fir, Western Cedar) need their own tables — don't substitute.
Treating max span as the actual span. The table value is the MAXIMUM. A floor that's 14 ft wide with 2×10 joists at 16" o.c. is fine even though the 2×10 could span 15'-7" — you don't need to push to the limit.
Rules of thumb
2×10 SPF #2 at 16" o.c.: 15'-5" at 40 psf, 17'-2" at 30 psf.
Going up one joist size adds roughly 2.5-3 ft of span. Going from 16" to 24" o.c. costs about 18%.
At #2 grade the species order is Doug Fir-Larch, then Spruce-Pine-Fir, then Southern Pine — SP is the weakest of the three in the current IRC tables, not the middle. DFL beats SP by about 11% at 2×10; DFL and SPF are within 2%.
L/360 deflection limit means a 16-ft span can deflect at most 16 × 12 ÷ 360 = 0.53 inch under live load.
Every value here is the 10 psf dead load column. Heavier finishes push you to the 20 psf column, which is shorter.
For spans beyond about 18 ft in dimensional lumber, switch to engineered I-joists or LVLs.
Common questions
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