Stair Calculator

How Riser and Tread Count Is Calculated

The one number you can't estimate is the total rise — the exact vertical distance from the bottom finished floor to the top finished floor. You can't just pick a riser height and expect it to divide evenly into that distance, so the standard layout method works backward: divide the total rise by your target riser height, round to the nearest whole number of risers, then divide the total rise by that whole number to get the actual riser height every step will use. This calculator does exactly that. Every riser in a flight must come out equal (within about 3/8 inch, per most residential codes), since uneven risers are a well-documented tripping hazard. The number of treads is always one less than the number of risers, because the top step is effectively the upper floor itself.

Blondel's Rule Keeps the Stair Comfortable

Once the actual riser height is fixed, the tread depth (run) shouldn't be chosen arbitrarily either. The 17th-century French architect François Blondel observed that a comfortable stride roughly follows 2 x riser height + run = 24 to 25 inches — a taller riser calls for a shallower tread and vice versa, mirroring a natural walking gait. This calculator checks your preferred run against that range and flags whether the combination lands in the comfortable zone, is merely acceptable, or feels cramped/awkward. Building codes such as the U.S. International Residential Code (IRC) separately cap maximum riser height around 7.75 in and require a minimum run of about 10 in — those are legal minimums, not comfort targets, and specific limits vary by jurisdiction, so treat the defaults here as a reasonable starting assumption rather than a universal rule.

Stringer Length Is the Hypotenuse

The diagonal stringer — the structural board the treads and risers are fastened to — is the hypotenuse of a right triangle formed by the total rise and total run, found with the Pythagorean theorem (stringer length = square root of rise squared plus run squared). The calculator also reports the stair angle (the arctangent of rise over run); most comfortable residential stairs fall between roughly 30 and 35 degrees from horizontal — steeper starts to feel like a ladder, shallower eats excessive floor space. If you're planning the room the stairway sits in, the square footage calculator can help account for the footprint the total run will consume, and the concrete calculator is useful if you're pouring a concrete stair or landing.