Understanding your result
The headline is the number of joists, with the spacing and the rim boards in the line beneath it. The stats give the rim boards, hangers, blocking and the total lumber; the table lists each with the floor, the spacing and the lumber length.
Joists are counted along the length of the floor across them: that length divided by the spacing, rounded up, plus one for the far end. Doubling the end joists, or the joists under a partition wall, adds two. Rim boards run along both ends of the joists, the full length of the floor, in the lumber length you buy, so a 16 ft floor in 14 ft lumber takes two pieces per side.
Hangers are two per joist when the joists hang from a beam or ledger. Blocking fills each bay between joists, one row for every 8 ft of span; the lumber is the spacing wide per block, close enough for an order. The linear feet total is joists and rims at the stock length. The panels that go on top are counted by the subfloor plywood calculator, the insulation between them by the underfloor insulation calculator, and if a new floor ends up at a different height from the next room, the floor height calculator works out the transition.
How we calculate this
The count is geometry and nothing else. The sizing rules it deliberately leaves out live in the International Residential Code, section R502, and its span tables; the American Wood Council publishes a free span calculator built on the same data. The subfloor prep guide explains why a stiff floor matters for tile and how deflection is judged, and the radiant heating guide covers floors that will be heated, where joist spacing decides how hydronic tubing is laid.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Spacing | 16 in on centre | The most common US floor framing; 12, 19.2 and 24 in are the other standard spacings |
| Rim boards | both ends of the joists | Standard platform framing |
| Blocking | one row per 8 ft of span | Common builder practice; IRC R502.7 sets the minimum lateral restraint |
| Bearing | 1 1/2 in on wood, 3 in on masonry | IRC R502.6 |
| Lumber length | the next 2 ft step above the span | Lumber is sold in 2 ft increments, 8 to 20 ft |
| Sizing | not done here | IRC Tables R502.3.1(1) and (2); American Wood Council span calculator |
Assumptions last reviewed October 8, 2026.
The calculator does not size joists, beams or headers, check deflection, frame stair or chimney openings, or account for point loads such as a bathtub, a kitchen island or a heavy safe. Openings need doubled headers and trimmers; large loads may need extra joists or a beam. All of that is design work for a qualified person and, in most places, a permit. The guide to installer rates covers what flooring work costs once the structure is sound.
Two worked examples
A 20 ft floor at 16 in centres, 14 ft span
Rims, one row of blocking, 16 ft lumber.
- Joists: 240 ÷ 16 = 15 spaces, plus 1 = 16 joists
- Rims: 2 × ceil(20 ÷ 16) = 4 boards
- Blocking: 1 row × 15 bays = 15 blocks, about 20 ft of lumber
- Lumber: (16 + 4) × 16 = 320 ft, plus blocking
Sixteen joists of 16 ft for a 14 ft span leaves a foot at each end for bearing and trimming. The depth of those joists is the question the span table answers, and it is the more expensive question.
A 12 ft floor at 12 in centres, hung from a ledger
10 ft span, doubled end joists, hangers, 10 ft lumber.
- Joists: 144 ÷ 12 = 12 spaces, plus 1 = 13, plus 2 doubled = 15 joists
- Hangers: 2 × 13 = 26
- Rims: 2 × ceil(12 ÷ 10) = 4; blocking 1 row × 12 bays = 12 blocks
Twelve-inch spacing is common under tile and stone, where a stiffer floor is wanted. It costs a third more joists than 16 in spacing for the same floor.
Where to find your inputs
Floor length across the joists. The dimension along which the joists are spaced, wall to wall.
Span. The clear distance each joist spans between its supports: walls, beams or a ledger.
Spacing. From the plans, or 16 in if you are matching typical framing.
Lumber length. The next standard length above the span plus bearing.
Common mistakes
- Counting joists along the span. They are spaced along the other dimension.
- Forgetting the last joist. Spaces plus one.
- Lumber the exact length of the span. Joists need bearing at both ends.
- Using this page to size joists. It counts; the span tables size.
- No hangers on a ledger. Joists framed into a ledger or beam need hangers and the specified nails.
- Working without a permit. Structural work usually needs one.
Questions people ask
- How many joists for a 16 foot floor at 16 inches on centre?
- The joists are counted along the length of the floor across them. 16 ft is 192 inches; 192 ÷ 16 = 12 spaces, so 13 joists including both ends. At 24 inches on centre it is 9 joists, and at 12 inches it is 17.
- What size joists do I need for a 12 foot span?
- It depends on the species, grade, spacing and load, so the answer comes from the span tables in the International Residential Code (Tables R502.3.1(1) and (2)) or the American Wood Council span calculator. For common lumber at 16 inches on centre and residential loads, 2 × 8 often reaches about 12 ft and 2 × 10 about 15 ft, but check the table for your lumber.
- Do joists need blocking?
- The IRC requires blocking at supports and lateral restraint for deep joists, and many builders add a row of blocking or bridging at mid-span on spans over about 8 ft to stiffen the floor and spread loads. The calculator adds one row per 8 ft of span when ticked.
- How long must joists bear on their supports?
- The IRC (R502.6) requires at least 1 1/2 inches of bearing on wood or metal and 3 inches on masonry or concrete, unless the joist is hung in a hanger. That is why the lumber is bought in the next length up from the span.
- Are joist hangers needed?
- When joists frame into a beam or a ledger board rather than sitting on top of a wall or beam, yes. Each joist then needs two hangers, one at each end, sized to the joist and nailed with the nails the hanger maker specifies.
- Do I need a permit to change floor joists?
- In most places, structural framing work needs a permit and an inspection. Ask your building department before adding, cutting, notching or replacing joists in an existing house.