Floor Joist Calculator
Count floor joists and check the span. Enter the length of the wall the joists sit along (ft), the joist span (ft), spacing (12, 16, 19.2, or 24 in on center), and lumber size from 2x6 to 2x12 to get joist count, lumber totals, and an IRC span check.
Example: with Wall length joists sit along (ft) 20 · Joist span (ft) 12 · Spacing (on center) 16 in OC · Joist size 2x8 → Joists needed: 16 joists at 16 in on center.
- Joist lumber192 linear ft of 2x8 (plus 40 lf rim)
- Board feet256 board feet
- Span checkWithin limit — 2x8 at 16 in OC spans up to 12 ft 7 in (DF-L #2, 40/10 psf)
Computed by the calculator below using its default values. Change any input to see your own numbers.
Count = wall length ÷ spacing, rounded up, plus one to close the run. Span check uses the 2018 IRC floor joist table for Douglas fir-larch #2 at 40 psf live / 10 psf dead load.
Counting joists the way framers do
Joists are laid out from one end at a fixed on-center spacing, and the run always ends with a joist at the far wall — that is the +1 in the math. Divide the wall length in inches by the spacing, round up, add one. A 20 ft wall at 16 in OC works out to exactly 15 spaces, so 16 joists.
The count covers common joists only. You still need rim (band) joists along the two ends — about 2 × the wall length in linear feet — plus doubled joists under parallel partition walls and around stair or chimney openings, where headers and trimmers are doubled too.
Reading the span check honestly
The span check compares your joist span to the 2018 IRC residential floor table for Douglas fir-larch #2 at 40 psf live load, 10 psf dead load, and L/360 deflection — the standard living-space case. Southern pine, hem-fir, and SPF have their own columns, higher grades span farther, and bedrooms (30 psf) get more span while tile floors and heavy loads get less. Treat a pass here as "plausible", not "approved": your local code table is the authority.
How it’s calculated
Joist count = ceil(wall length × 12 ÷ spacing) + 1. Linear feet = count × span; board feet = linear feet × (nominal thickness × width ÷ 12): 1.0 for 2x6, 1.333 for 2x8, 1.667 for 2x10, 2.0 for 2x12. Span limits from 2018 IRC Table R502.3.1(2), Douglas fir-larch #2, 40 psf live / 10 psf dead, L/360 (e.g., 2x8 at 16 in OC: 12 ft 7 in).
Span figures apply to one species/grade and the standard residential load case only — verify against your local code and lumber grade stamp before framing.
Max floor joist spans at 16 in OC (DF-L #2, 40/10 psf, L/360)
| Joist size | Max span |
|---|---|
| 2x6 | 9 ft 9 in |
| 2x8 | 12 ft 7 in |
| 2x10 | 15 ft 5 in |
| 2x12 | 17 ft 10 in |
2018 IRC Table R502.3.1(2), Douglas fir-larch #2. Other species, grades, and loads differ — check the full table.
Common mistakes
- Measuring the joist span (the direction joists run) and using it as the layout length — the count comes from the perpendicular wall the joists sit along.
- Forgetting the +1 closing joist, which shorts the order by one on every run.
- Buying exact count with no extras: crowned, split, or wet boards get culled, and blocking eats offcuts. Add 2-3 sticks.
- Trusting one span table for any lumber: SPF #2 spans noticeably less than Douglas fir; the grade stamp on the wood decides which column applies.
Frequently asked questions
What is the formula for how many floor joists I need?
Count = wall length in inches ÷ on-center spacing, rounded up, + 1. For 20 ft at 16 in OC: 240 ÷ 16 = 15, +1 = 16 joists.
How far can a 2x8 floor joist span?
12 ft 7 in at 16 in OC for Douglas fir-larch #2 under 40 psf living-space loads per the 2018 IRC. At 24 in OC that drops to 10 ft 3 in, and weaker species drop further.
Is 16 or 24 inch spacing better?
16 in OC is the residential default — it suits standard subfloor and shortens deflection. 24 in OC saves joists but requires thicker subfloor and cuts allowable span about 18%, so it only pays on short spans.
What is a board foot?
A board foot is 144 cubic inches of nominal lumber — 1 ft of 1x12, or 0.75 ft of 2x8. Lumberyards quote framing packages in board feet, which is why the calculator reports it alongside linear feet.
Does a passing span check mean my floor design is approved?
No. It means the span fits one common table (DF-L #2, 40/10 psf). Different species, wet service, heavy finishes like tile, or point loads change the answer — confirm with your local code table or an engineer.