How to use the joist span calculator
Choose floor, ceiling or deck joists, then the load case, species, grade, size and spacing. The calculator reads the maximum allowable span straight from the 2021 IRC tables. Enter your actual span and it tells you whether that joist works and which is the smallest size that would. The diagram compares every size at your spacing against your span.
Floor tables cover Douglas fir-larch, Hem-fir, Southern pine and Spruce-pine-fir in grades from Select Structural to No. 3. Deck joists use IRC Table R507.6, which assumes No. 2 grade lumber with a wet service factor, as treated deck framing is.
How the tables work
find the row: species and grade at your spacing
find the column: dead load and joist size
maximum span = that cell (feet-inches); no interpolation between spacings
The tables are based on bending strength and a deflection limit: L/360 for floors (span divided by 360 is the most the floor may sag under live load), L/240 for ceilings. For other species, grades or loads, the IRC refers to the American Wood Council's span tables.
Worked example: a living room floor, No. 2 Douglas fir-larch 2 × 10 at 16 in
- Table R502.3.1(2) (40 psf live, 10 psf dead), 16 in spacing, Douglas fir-larch No. 2.
- 2 × 10 column: 15 ft 7 in.
- For a 14 ft span, that 2 × 10 works with 1 ft 7 in to spare; a 2 × 8 (12 ft 9 in) would not.
Things the tables don't cover
- Concentrated loads: hot tubs, pianos, water beds, aquariums and bearing walls above need engineering.
- Notches and holes: R502.8.1 limits them (no notches in the middle third of the span; holes no larger than ⅓ of the depth). A badly placed cut can halve a joist's strength.
- Cantilevers: deck cantilevers have their own limits in Table R507.6, and floor cantilevers in R502.3.3.
- Engineered lumber (I-joists, LVL) uses the manufacturer's span tables, not these.
- Bouncy floors: L/360 is a code minimum. Tile floors and long spans often feel better one size up or at closer spacing.
Questions people ask
How far can a 2x10 joist span?
Under IRC Table R502.3.1(2) (living areas, 40 psf live, 10 psf dead), a No. 2 Douglas fir-larch 2×10 at 16 in on center spans 15 ft 7 in. The same joist at 12 in spacing spans 18 ft 0 in, and at 24 in, 12 ft 9 in.
How far can a 2x8 deck joist span?
IRC Table R507.6 gives a No. 2 Southern pine 2×8 at 16 in on center 11 ft 10 in for a 40 psf live load; Douglas fir-larch, Hem-fir or Spruce-pine-fir 11 ft 1 in. Heavier snow loads shorten the span.
What is the difference between 30 psf and 40 psf floor joist tables?
The IRC uses 30 psf live load for sleeping areas and attics reached by a fixed stairway (Table R502.3.1(1)), and 40 psf for all other living areas (Table R502.3.1(2)). The 40 psf table gives shorter spans.
When do I use the 20 psf dead load column?
The 10 psf dead load column covers ordinary floors. Heavier finishes such as tile or stone over a mortar bed, or plaster ceilings below, can push dead load toward 20 psf, which shortens the allowable span for many grades.
How is joist span measured?
Clear span between the faces of the supports. For deck joists the IRC measures from the face of the ledger or beam to the face of the next support, with cantilevers limited separately in Table R507.6.
Sources and assumptions
- 2021 International Residential Code, Tables R502.3.1(1) and R502.3.1(2): floor joist spans (read from codes.iccsafe.org)
- 2021 IRC Table R507.6: maximum deck joist spans (No. 2 grade, wet service, 10 psf dead load)
- 2021 IRC Tables R802.5.1(1) and R802.5.1(2): ceiling joist spans for uninhabitable attics
- Values are copied exactly from the tables and checked with a checksum in our tests. No interpolation between spacings is done. Footnote: Hem-fir SS 2×12 at 24 in with 20 psf dead load (16-10) requires 2 in of end bearing.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.