Stair Layout Calculator
Total rise = finished floor (or grade) to finished floor above.
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Stair Calculator
Quick answer: risers = total rise ÷ max riser height, rounded up; treads = risers − 1; total run = treads × tread depth. A 108 in rise with a 7.75 in maximum riser needs 14 risers of 7.71 in, 13 treads, and a 130 in run at 10 in treads, on stringers about 169 in long.
Lay out a straight run of stairs for a deck, porch, basement, or second floor: riser count and height, treads, total run, stringer length, angle, and quick checks against the IRC limits.
Stair layout
How to calculate stair risers, treads, and stringers
Stairs start from one measurement: the total rise, from the finished surface at the bottom to the finished surface at the top. Every riser in the flight must be the same height (within 3/8 in under the IRC), so the rise is divided into equal steps first, and the run follows from the tread depth.
Risers = ⌈Rise ÷ max riser⌉ · Treads = Risers − 1
Run = Treads × T · Stringer = √(Rise² + Run²)
- Measure the total rise at the stair location with finished flooring (or the final landing pad) in mind. Measure with a level and straightedge if the ground slopes.
- Divide by the maximum riser — 7.75 in under the IRC — and round up to a whole number of risers.
- Find the actual riser height: total rise ÷ number of risers. Round to the nearest 1/16 in for layout.
- Count treads: one fewer than risers when the top floor or deck acts as the last step.
- Total run = treads × tread depth. Check it fits the space and leaves a landing at the bottom.
- Stringer length = √(rise² + run²). Buy stock 1–2 ft longer to allow for the top and bottom cuts.
Worked example
A deck sits 40 in above the patio. The builder wants treads made from two 5.5 in boards with a 1/4 in gap (11.25 in deep) and a 36 in wide stair with stringers no more than 16 in apart.
- Risers: 40 ÷ 7.75 = 5.16 → 6 risers of 40 ÷ 6 = 6.67 in (about 6 11/16 in)
- Treads: 6 − 1 = 5 treads; run = 5 × 11.25 = 56.25 in
- Stringer: √(40² + 56.25²) = √4,764 = 69.0 in (5 ft 9 in) — cut from 8 ft 2×12s
- Angle: atan(6.67 ÷ 11.25) = 30.7°; 2R + T = 13.33 + 11.25 = 24.6 in (comfortable)
- Stringers: (36 − 1.5) ÷ 16 = 2.2 → 3 spaces → 4 stringers
Risers and stringer length by total rise
Using a 7.75 in maximum riser and 10 in treads. Riser heights rounded to the nearest 1/16 in.
| Total rise | Risers | Riser height | Treads | Total run | Stringer length |
|---|---|---|---|---|---|
| 24 in | 4 | 6 in | 3 | 30 in | 38.4 in |
| 30 in | 4 | 7 1/2 in | 3 | 30 in | 42.4 in |
| 36 in | 5 | 7 3/16 in | 4 | 40 in | 53.8 in |
| 48 in | 7 | 6 7/8 in | 6 | 60 in | 76.8 in |
| 96 in (8 ft) | 13 | 7 3/8 in | 12 | 120 in | 153.7 in |
| 108 in (9 ft) | 14 | 7 11/16 in | 13 | 130 in | 169.0 in |
| 120 in (10 ft) | 16 | 7 1/2 in | 15 | 150 in | 192.1 in |
Stair building tips and common mistakes
- Measure to finished surfaces. If the top floor gets 3/4 in hardwood and the bottom stays bare concrete, the first and last risers end up uneven. Account for flooring before you cut.
- Drop the stringer for the bottom tread. When treads sit on top of the stringer, cut the bottom of the stringer down by one tread thickness, or the first riser comes out taller than the rest.
- Mind the 3/8 in rule. The IRC allows no more than 3/8 in difference between the tallest and shortest riser (and the deepest and shallowest tread) in a flight. Uneven steps are a leading cause of trips.
- Nosing and open risers. Treads under 11 in deep need a 3/4–1 1/4 in nosing on stairs with solid risers. Open risers are allowed in houses, but openings more than 30 in above the floor may not pass a 4 in sphere — check local code.
- Leave headroom and landings. Keep at least 6 ft 8 in of clear headroom measured from the line of the nosings, at least a 36 in landing at the top and bottom, and a landing within any flight that rises more than 151 in.
- Size the stringers. Notched 2×12 stringers leave roughly 5 in of wood below the notches at typical riser/tread sizes; long spans or heavy treads may need a mid-span support. Composite treads often require stringers 12 in apart — read the product guide.
Stair calculator FAQ
How do I calculate the number of stairs I need?
Divide the total rise in inches by the maximum riser height and round up. For a 108 in floor-to-floor rise with a 7.75 in maximum riser, 108 ÷ 7.75 = 13.9, so you need 14 risers of 7.71 in (about 7 11/16 in). The number of treads is one less than the risers: 13.
What is the maximum stair riser height by code?
The International Residential Code (IRC R311.7.5.1) limits riser height to 7 3/4 inches, and the largest riser in a flight may not exceed the smallest by more than 3/8 inch. Local codes and commercial (IBC) stairs can be stricter, often 7 inches maximum.
What is the minimum tread depth for stairs?
The IRC (R311.7.5.2) requires a tread depth of at least 10 inches, measured nosing to nosing. Where the tread depth is less than 11 inches, stairs with solid risers need a nosing projection of 3/4 to 1 1/4 inches.
How long does a stair stringer need to be?
Stringer length is the hypotenuse of the total rise and total run: the square root of rise squared plus run squared. A 40 in rise with 6 risers and 5 treads of 11.25 in has a 56.25 in run, so the stringer is about 69 in (5 ft 9 in). Buy a board at least a foot longer for the top and bottom cuts.
What is the 2R + T stair rule?
Two risers plus one tread should total about 24 to 25 inches for a comfortable stride. A 7 in riser with an 11 in tread gives 25 in, and a 7.5 in riser with a 10 in tread also gives 25 in. It is a comfort guideline, not a code requirement.
How we built this calculator: results use the standard volume, area, and coverage formulas shown on this page, with typical U.S. material densities, bag yields, and waste factors that you can override. Written and checked by the YardsCalculator editorial team. Last reviewed: October 2026.