Stair Stringer Calculator

The calculator divides your total rise — the floor-to-floor height — by your target riser to find the number of steps, rounding to the nearest whole riser. Dividing the rise by that count gives the exact riser height, so every step comes out identical; the IRC allows no more than 3/8 in of variation between the tallest and shortest riser in a flight.

A stair always has one fewer tread than risers, because the top landing acts as the last step. Total run is the tread count times the tread run, and the cut angle — the angle you set on the saw or mark with a framing square — is atan(riser ÷ tread run). The stringer length is the hypotenuse of total rise and total run, which is why the calculator suggests the next standard board length up.