How Many Floor Joists Do I Need? Spacing, Rim Joists, Sheets
Floor joists are counted from the length of the floor and the spacing. A floor 16 ft long, with joists 16 in on center, needs 13 joists and two rim joists. The floor joist calculator counts them for your floor.
Joists for each length
| Length of floor | 12 in | 16 in | 19.2 in | 24 in |
|---|---|---|---|---|
| 8 ft | 9 | 7 | 6 | 5 |
| 10 ft | 11 | 9 | 8 | 6 |
| 12 ft | 13 | 10 | 9 | 7 |
| 16 ft | 17 | 13 | 11 | 9 |
| 20 ft | 21 | 16 | 14 | 11 |
| 24 ft | 25 | 19 | 16 | 13 |
Stock length and sheets for each span
For a floor 16 ft long, with 10% extra for the subfloor:
| Span | Stock length | Subfloor sheets of 4 × 8 ft |
|---|---|---|
| 8 ft | 8 ft | 5 |
| 10 ft | 10 ft | 6 |
| 12 ft | 12 ft | 7 |
| 14 ft | 14 ft | 8 |
| 16 ft | 16 ft | 9 |
The parts of the floor frame
- Joists. The boards that carry the floor, set on edge across the span.
- Rim joists. The two boards that close the ends, as long as the floor.
- Blocking. Short pieces between the joists that keep them from twisting.
- Hangers. Metal brackets that hold the joist to a beam or a ledger.
Before you order
- Get the joist size from the span table in your code or from your plans, since it depends on the span, the spacing and the grade of lumber.
- Add doubled joists under walls that run the same way, and headers and trimmers around stairs and chimneys.
- Crown the joists, with the high edge up, and glue and screw the subfloor.
- Do not notch or drill joists except where the code allows.
See also the deck materials calculator and the stud calculator. These are estimates: check your local building code.
Frequently asked questions
How many joists do I need?
The length of the floor in inches divided by the spacing, rounded up, plus one: 13 joists for a floor 16 ft long at 16 in on center.
How far apart should floor joists be?
Usually 16 inches on center. Some floors use 12, 19.2 or 24 inches, depending on the joist size, the span and the subfloor.
What size joist do I need?
It depends on the span, the spacing, the species and grade of the lumber and the load. Use the span tables in your local code or ask the building department. This page counts joists but does not size them.
What is a rim joist?
The board that closes the ends of the floor frame. It holds the ends of the joists upright and gives a surface for the wall above.
How many subfloor sheets do I need?
7 sheets of 4 × 8 ft for a floor 16 × 12 ft, with 10% extra.
More guides
Tools & materials for this project
Affiliate links As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.