A staircase is a division problem wearing a hard hat. The building gives you one fixed number — the total rise, floor to finished floor — and your job is to split it into equal steps that human legs find comfortable and predictable. This calculator does the splitting: how many risers you need, the exact height each one ends up, how many treads that means, how far the flight runs along the floor, and the shortest stringer board that can carry it.
The arithmetic of a flight
Rounding up is the whole trick: you cannot build a fraction of a step, so you take the next whole number of risers and let each one come out slightly shorter than your target. Every riser in the flight is then identical — and identical is what makes stairs safe, because climbers' legs learn the rhythm by the second step and trip on the one step that differs. The last “tread” is the upper floor itself, which is why treads always number one fewer than risers. The stringer length is simply the hypotenuse of the flight's right triangle: the square root of rise² + run².
The 7-11 rule and the comfort formula
Generations of builders have aimed at a riser near 7 inches and a tread near 11 inches — the “7-11 rule”. The related comfort check used by designers is 2 × riser + tread ≈ 24–25 inches, roughly an adult's natural stride length on the flat. A flight that satisfies it feels effortless; push the riser toward 8 inches with a 9-inch tread and the same formula drops under 25 while the stair feels steep and mean, because it is the combination your legs judge, not either number alone. This page shows the comfort figure for your numbers so you can nudge the target riser or tread depth until it lands in range.
Codes, headroom and honesty
Building codes set hard limits — commonly a maximum riser height and a minimum tread depth for homes, with tighter rules for commercial buildings — but the numbers vary by jurisdiction and change between code editions, so check your local code before you cut anything. Headroom matters too: most codes want about 6 ft 8 in of clear height above the stair slope, measured at the nosing line. This calculator plans geometry, not compliance; treat its output as the layout to take to your inspector or stair builder, not as approval.
Worked example
The defaults are a typical floor-to-floor height of 108 in (9 ft). At a 7.5 in target: 108 ÷ 7.5 = 14.4, rounded up to 15 risers of exactly 7.2 in each, 14 treads, a run of 140 in (11 ft 8 in) at 10 in per tread, and a stringer at least 176.8 in long. The comfort check reads 2 × 7.2 + 10 = 24.4 in — comfortably in range.
How to use this tool
- Measure the total rise from finished floor to finished floor, in inches.
- Enter a target riser height — 7 to 7.5 in is the comfortable domestic range.
- Enter your tread depth (the going) — 10 in is the default here; 11 in is roomier.
- Read the riser count and the actual riser height; that actual figure is what every step must be built to.
- Check the total run fits your space, the comfort figure sits near 24–25 in, and your local code allows the numbers.
Buying stringers and planning landings
The stringer length shown is the straight-line minimum along the slope; the board you buy must be longer, because the top and bottom cuts, the seat cuts for each tread, and any tail past the last riser all consume length — builders commonly add at least a foot and choose the next stock length up. If the total run does not fit the room, the honest fixes are an L-shaped flight with a landing (which splits the risers into two runs) or a steeper ship's-ladder style stair where codes allow it — never simply shrinking every tread until the flight squeezes in, because shallow treads are where falls happen.