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Stair Calculator

The stair calculator helps you design comfortable stairs: it finds the stair stringer length, as well as the rise and run of individual steps.

Clear
Risers15each 7.2 in — 7 3/16 in
Treads14always one fewer than the risers
Actual riser height7.2 inwithin the usual 7¾ in maximum
Total run12.833 ft12 ft 10 in of floor space
Stringer length15.675 ft15 ft 8.1 in along the diagonal, before cutting
Stair angle33.207°in the comfortable 30–37° range
2 × rise + run25.4 inthe classic rule wants 24–25 in
Rise + run18.2 ina second rule of thumb wants 17–18 in
Tread depth including nosing12 in
If you used one more riser6.75 in eachshallower steps over 13.75 ft of run
If you used one fewer7.7143 in eachsteeper, but shorter
Tread material42 sq ftat 3 ft wide
Headroom checkwithin codemeasured plumb from the tread nosing, codes want 6 ft 8 in

The method

steps = total rise ÷ target riser, then divide evenly

Divide the rise, do not assume it

You cannot pick a riser height and hope it fits: the total rise is fixed by the building, so you choose the number of risers and let the arithmetic give you the height. A 108-inch rise at a 7-inch target gives 15.43 risers, so use 15 at 7.2 inches each — every one identical.

That last point is the one that matters most. Codes allow no more than about ⅜ inch of variation between risers in a flight, because people climb stairs by feel and an odd step is exactly where they trip. Split the rise evenly and cut the bottom riser short by the tread thickness so the finished flight comes out even.

Treads are always one fewer than risers, because the upper floor serves as the last tread.