Clad
Stair calculator: rise, run and stringer length
Stairs come down to a few numbers: the total rise, the riser height, the tread depth and the stringer length. Here is how to work each one out, with an example you can check on your own tape.
How do you calculate stair rise and run?
Measure the total rise: from the finished floor at the bottom to the finished floor at the top. Divide it by a riser height near 7 inches and round to a whole number of risers. Then divide the total rise by that number to get the exact riser height. The number of treads is one fewer than the number of risers, because the top floor is the last step. The total run is the number of treads times the tread depth.
Example. Total rise 105 in (8 ft 9 in). 105 ÷ 7 = 15 risers, so each riser is exactly 7 in. With 14 treads of 10 in, the total run is 140 in (11 ft 8 in).
What is the maximum riser height and minimum tread depth?
The US International Residential Code (IRC) allows a riser up to 7 3/4 in and requires a tread at least 10 in deep, and the risers in one flight may differ by no more than 3/8 in. A common comfort check is that two risers plus one tread come to 24 to 25 in (2 × 7 + 10 = 24). Your local code is the one that counts, so check it before you cut.
How do you calculate stair stringer length?
The stringer is the long side of a right triangle: its legs are the total rise and the total run. Length = √(rise² + run²), and the angle is arctan(rise ÷ run). It is the length of the line along the notch points, so buy stringer stock longer than the result.
Example. Rise 105 in, run 140 in: √(105² + 140²) = 175 in = 14 ft 7 in. The angle is arctan(105 ÷ 140) = 36.9°. The first riser is cut shorter than the others by the thickness of the tread, so every step is the same height once the treads are on.
How do you work out deck stairs?
Measure the total rise from the top of the deck surface down to the ground (or to the top of the landing pad), then follow the same steps. A deck stair calculator needs the finished surface at both ends, because a tread adds thickness on top of the stringer. Set the stringers on a firm landing, not on bare soil.
Stair calculator with a landing
A landing splits the climb into two flights. Work out each flight with its own rise and run, then add the landing between them. Under the IRC a landing is at least as wide as the stairs and at least 36 in deep in the direction of travel.
Stair tread calculator: how many treads?
Treads = risers − 1. With 15 risers you need 14 treads, and the stringer has 14 notches for them.
Stairs in Clad
Clad's stairs tool takes the total rise and the tread depth, gives the riser height, the number of risers and treads, the total run, the stringer length and the angle, and shows a side view. It lists the options for one riser more or fewer, so you can pick the closest to your ideal riser. L and U stairs with a landing are included.
See what Clad does. Clad is coming soon to the iPhone.
More guides
- Roof pitch and rafters
- Feet and inches calculator
- Concrete calculator
- Cut list calculator
- Deck baluster spacing
How this page was made
Written with the help of an AI assistant (Claude, by Anthropic). Every worked example was computed again by a program and checked against its formula. Building codes differ by place: check yours before you build.