Stair Calculator
Work out the number of steps, the exact riser height, the total run and the stringer length for a staircase from its total rize, and check the comfort rule.
How many steps, how tall each is, and how much floor the stairs take.
How the stair calculator works
Divide the total rize by your target riser height and round to a whole number of risers; the actual riser is then the rize divided by that number, so every step is identical. Treads = risers − 1 (the top floor is the last "tread"). Total run = treads × tread depth, and the stringer is the hypotenuse of rize and run.
The comfort rule of thumb — twice the riser plus the tread between about 600 and 650 mm (24–25 in) — is what the verdict checks. Building codes set riser and tread limits (commonly a maximum riser of about 190–200 mm and a minimum tread of 240–250 mm); check yours.
Formula: risers = round(rise ÷ target riser) · riser = rise ÷ risers · run = (risers − 1) × tread
Worked examples
| Inputs | Riser height | Note |
|---|---|---|
| 2,700 mm rize, 175 mm target, 250 mm treads | 180 | 15 risers of 180 mm, 3.5 m run |
| 2,700 mm rize, 190 mm target, 240 mm treads | 192.9 | 14 risers, a steeper stair |
| 106 in rize, 7 in target, 10.5 in treads | 7.1 | 15 risers of 7.07 in |
FAQFrequently asked questions
Why are treads one fewer than risers?
The top landing or floor acts as the final tread, so a stair with 15 risers has 14 treads between the floors.
What is the comfort rule?
Twice the riser plus the tread should be about 600–650 mm (24–25 in) — a stride that feels natural. It comes from Blondel's 17th-century rule and still underlies most codes.
What riser and tread sizes do codes allow?
Typically risers of 150–190 mm (up to about 7¾ in in the US) and treads of at least 240–250 mm (10 in). Rules differ by country and building type — check the applicable code.
How much floor space do stairs take?
The total run plus landings: a 2.7 m rize at 250 mm treads needs 3.5 m of run, so a straight flight is over 4 m long including the top and bottom clearances.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Gypsum Association — gypsum board sizes — 4 × 8 and 4 × 12 ft sheets
- ADA Standards for Accessible Design — ramps (§405) — maximum running slope 1:12
- International Code Council — the International Residential Code
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.