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Stair Calculator

Stairs are decided by one number: the total rise, finished floor to finished floor. Divide it by a target riser height (~7.5 in), round to a whole number of risers, and divide again — that exact riser height is what every step must be. A 9 ft rise works out to 14 risers at exactly 7.71 in, 13 treads and a run of about 11 ft 11 in. Enter your rise for your staircase.

Stair Calculator

Total rise, floor to floor (in)

Target riser height (in)

Tread depth (in)

Stair width (in)

Stair profile: every riser identical, treads one fewer than risers, and the stringer is the hypotenuse of rise and run.

How to use it

Measure the total rise: finished floor to finished floor, in inches. Include flooring thickness on both levels — this is the number everyone gets wrong.
Keep the target riser at 7.5 in unless your code or space says otherwise (US residential max is 7¾ in).
Set tread depth: 11 in is comfortable; 10 in is the residential minimum.
Read off: riser count, exact riser height, treads, total run, stringer length, angle — and the comfort check (2 × riser + tread should land near 24–26 in).

How the stair math works

Every riser in a flight must be identical — a variation of more than 3/8 in between the tallest and shortest riser is a code failure and a trip hazard. That is why the method is divide, round, divide again: total rise ÷ 7.5 gives a decimal; rounding to the nearest whole riser and dividing the rise by that count gives the exact height you cut, whether it is 7.50 in or 7.71 in. Treads are always one fewer than risers, because the upper floor is the last 'tread'.

Total run is treads × tread depth, and the stringer is the hypotenuse: √(rise² + run²). The comfort rule — Blondel's formula, 2 × riser + tread ≈ 24–26 in — predicts whether the stair will feel right. Steep, short stairs fail it; if your run does not fit the space, the honest fixes are a landing or a winder, never shrinking treads below 10 in or pushing risers past 7¾ in.

Worked example: a 108 in (9 ft) floor-to-floor rise

Risers: 108 ÷ 7.5 = 14.4 → 14 risers. Exact riser height: 108 ÷ 14 = 7.71 in (under the 7¾ in max — legal). Treads: 14 − 1 = 13 treads at 11 in = 143 in total run (11 ft 11 in). Stringer: √(108² + 143²) = √32,113 ≈ 179 in (about 14 ft 11 in) — buy 16 ft stringer stock. Angle: atan(108 ÷ 143) ≈ 37°. Comfort: 2 × 7.71 + 11 = 26.4 in — a touch steep; dropping the tread target is not the fix, adding a riser is (15 risers → 7.2 in risers, 154 in run, comfort 25.4 in). The calculator shows both numbers so you can choose.

The numbers that decide a staircase

Item	What to know
Riser height	Max 7¾ in (US residential IRC). All risers within 3/8 in of each other. Lower (7–7.5 in) feels easier, especially for kids and older knees.
Tread depth	Min 10 in (IRC), 11 in is the comfortable standard. Measured nosing to nosing.
Total run	Treads × depth. This is the floor space the stair eats — check it against the room before you cut anything.
Stringer length	The hypotenuse, plus allowance for cuts — buy the next stock length up. Wide stairs (over 36 in) need a third, centre stringer.

Stair mistakes that cost money

Measuring floor to subfloor, or forgetting the finished floor thickness on either level. A ¾ in error spread over 14 risers still leaves the first or last step visibly wrong.
Cutting every riser at the target 7.5 in instead of the exact divided height. The error accumulates and the last step absorbs all of it.
Making one riser shorter to fit a landing or a door swing. Unequal risers are how people fall — redesign the flight instead.
Ignoring headroom. You need 6 ft 8 in minimum clearance above the tread line; long runs under a floor opening fail this quietly.

Stair Calculator — common questions

How many steps do I need for a 9 ft ceiling?

Floor-to-floor is what matters — typically 108–120 in once joists and flooring are counted. At 108 in: 14 risers of 7.71 in and 13 treads. Measure your actual floor-to-floor height; ceiling height alone is not enough.

What is the ideal riser and tread size?

Around 7–7.5 in riser with an 11 in tread. Check with Blondel's formula: 2 × riser + tread should be 24–26 in. Code limits are 7¾ in max riser and 10 in min tread in US residential work.

How long is a stair stringer for 14 risers?

Stringer length = √(total rise² + total run²). For 108 in rise and 143 in run that is about 179 in — buy 16 ft stock and cut to fit.

Why is one step always a different height?

Because someone divided once and never divided again. Round the riser count to a whole number, then divide the total rise by that count and cut every riser to the exact result.

Sources and review. Limits follow the US residential code (IRC): max riser 7¾ in, min tread 10 in, riser variation ≤3/8 in, headroom ≥6 ft 8 in. Comfort check is Blondel's formula (2R + T ≈ 24–26 in). Always confirm with your local building department. Guide published November 2026. Spotted a wrong number? Send it via the contact page — corrections are checked against published data and fixed.

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