Stair Calculator

How the stair calculation works

Start with the total rise: the vertical distance from the finished lower floor to the finished upper floor (not floor-to-ceiling — measure to the top of the finished floor above). The calculator divides total rise by your target riser height and rounds to a whole number of risers, because every riser in a flight must be identical. It then works backwards: actual riser height = total rise ÷ number of risers. There is always one fewer tread than risers — the upper floor itself is the last "step" — so total run = (risers − 1) × tread depth. The stringer length is the diagonal: √(total rise² + total run²); treat it as a minimum board length and confirm against your layout method before cutting.

Comfort is checked two classic ways. Blondel's rule says 2 × riser height + tread depth should land between 24″ and 25″ — that matches the stride of an average adult. The riser + going rule says riser + tread should be about 17″–18″. Residential codes in most of the U.S. also cap riser height at 7¾″ and require tread depth of at least 10″, and a stair angle between about 30° and 37° feels the most natural. Always confirm the numbers against your local building code before you build.