Stair calculator

Divide the total rise (floor to floor, including finished floors) by a target riser of about 7 in and round to the nearest whole number — then divide back to get the exact riser height. All risers must be equal within 3/8 in: an uneven step is the classic trip hazard.

US residential code (IRC) allows risers up to 7¾ in and treads at least 10 in. The comfortable sweet spot is a 7 in riser with an 11 in tread, which also satisfies the 2×riser + tread ≈ 24–25 in comfort rule.

The stringer is the hypotenuse: square root of (total rise² + total run²). A 9 ft rise with a 12 ft run needs a stringer of 15 ft — buy stock one size longer to allow for the cut angles at top and bottom.

Comfortable indoor stairs run 30–38°; above 42° they feel like a ladder and most codes stop allowing them as the main stair. Headroom over any tread must be at least 6 ft 8 in.