Stairs are one calculation you don’t eyeball — an uneven riser is a trip hazard and a failed inspection. Enter the total rise and get every layout number for the stringer.
Stair layout
Number of risers—
Number of treads—
Total run—
Stringer length (min board)—
IRC riser check (≤ 7¾ in)—
Layout tip: drop the bottom of the stringer by one tread thickness so the first step isn’t tall — this calculator’s riser height assumes you’ll do that. Riser variation within a flight should stay under 3/8 in.
How stair layout math works
Divide the total rise by the code maximum riser (7¾ in under the IRC), round up to get the riser count, then divide back to get the exact, equal riser height. Treads are always one fewer than risers, and the stringer is just the hypotenuse of rise and run.
risers = ceil(rise ÷ 7.75) · riser height = rise ÷ risers · stringer = √(rise² + run²)
Worked example
A deck 44 in off grade: 44 ÷ 7.75 = 5.7 → 6 risers at 7.33 in each, 5 treads at 10.5 in for a 52.5 in total run, and a stringer of 5.63 ft — cut from an 8-ft 2×12. Six equal 7.33-in steps feel right underfoot; five at 7¾ plus one oddball at the bottom is how ankles get rolled.
The details inspectors actually check
Max 7¾-in risers, min 10-in treads (with nosing rules), and no more than 3/8-in variation between the tallest and shortest riser in the flight. The classic field mistake is forgetting to drop the stringer by one tread thickness at the bottom — skip it and your first step is a leg-breaker taller than all the rest.
Frequently asked questions
What is the maximum riser height by code?
7¾ inches under the IRC for residential stairs, with treads at least 10 inches deep. Commercial (IBC) is stricter — typically 7-inch max risers and 11-inch treads. Always confirm your local amendments.
Why do I round the number of risers up, not down?
Rounding down would make each riser taller than the code max. Rounding up gives you one more, slightly shorter step — always legal, and more comfortable to climb.
What size lumber do I need for stringers?
2×12 is the standard, because notching for treads and risers removes a lot of meat — the remaining throat should stay at least 5 inches. Stringers typically sit 16 inches on center, so a 36-inch wide stair gets three.