Stair tread and riser board sizes
Tread and riser boards are sized from the stair layout: the tread covers one unit run plus the nosing that overhangs the riser below, and the riser board fills the gap between the top of one tread and the underside of the next. The IRC measures tread depth from nosing to nosing, so the board is deeper than the run by the nosing projection[1].
| Total rise | Risers | Riser | Tread board depth (10 in run) | Riser board |
|---|---|---|---|---|
| 6′ 0″ | 10 | 7-3/16″ | 11″ | 6-3/16″ |
| 7′ 0″ | 11 | 7-5/8″ | 11″ | 6-5/8″ |
| 8′ 0″ | 13 | 7-3/8″ | 11″ | 6-3/8″ |
| 8′ 9″ | 14 | 7-1/2″ | 11″ | 6-1/2″ |
| 9′ 0″ | 14 | 7-11/16″ | 11″ | 6-11/16″ |
| 9′ 6″ | 15 | 7-5/8″ | 11″ | 6-5/8″ |
Stair riser calculator: height and count
The riser height is the total rise divided by the number of risers, and the count is the rise divided by the maximum riser, rounded up: 7-3/4 in under the IRC (R311.7.5.1, R318.7.5.1 in 2024)[1][2]. Every riser in the flight must match within 3/8 in, so cut all riser boards from one measurement after the stringers are in. The full layout, including the code check, is in the stair calculator.
Board widths and tread material
- Solid treads: one board the full depth; 11-1/4 in (a 1x12 or 2x12, PS 20) covers a 10 in run with a 1-1/4 in nosing[3].
- Two-board treads on decks: two 5-1/2 in boards with a gap make about 11-1/4 in; see the deck stair calculator.
- Nosing rules: 3/4 to 1-1/4 in where the tread is under 11 in; details on the stair nosing page.
Common tread and riser mistakes
- Cutting risers to the full riser height. The tread sits on top, so the board is one tread thickness shorter.
- Treads cut to the run only. Add the nosing, or the treads stop flush with the risers.
- Cutting before the stringers are set. Measure each opening; a stair that is out by 1/8 in shows at the top step.
Frequently asked questions
How deep should a stair tread board be?
The unit run plus the nosing: a 10 in run with a 1 in nosing needs an 11 in tread board.
How tall is a stair riser board?
One riser height minus the tread thickness. With 7-1/2 in risers and 1 in treads, cut 6-1/2 in riser boards.
How many treads does a stair have?
One fewer than the risers, because the upper floor is the last step.
What does a stair riser calculator do?
It divides the total rise into equal risers no taller than the code maximum, 7-3/4 in under the IRC.