Stair calculator

This stair calculator solves a straight flight of stairs from one measurement. Enter the total rise — the vertical distance from the finished lower floor to the finished upper floor — and it gives you the number of risers, the height of each step, the tread depth, the total run, the stair angle, and the stringer length you need to buy. Every figure is held at full precision the whole way through and rounded once at the end.

Stair stringer calculator

The stringer is the diagonal across the whole flight. Its length is √(rise² + run²). For a 9 ft rise over 11 ft 8 in of run the stair stringer comes out at 14 ft 8-13/16 in, which means a 16 ft 2×12 with something to spare. Always buy the next length up: the notches eat into the board, and the throat left behind the deepest cut is what actually carries the load.

Rise and run calculator for stairs

You cannot pick a riser height directly because the risers have to divide the total rise exactly. The calculator divides the rise by your target riser, rounds to a whole number of steps, then divides back. That is why asking for 7-1/4 in on a 9 ft rise gives 7-3/16 in: fifteen risers is the count that fits, and 108 ÷ 15 = 7.2 in. Measure from finished floor to finished floor — a flooring layer you forget at 3/4 in becomes a 1/16 in error on every step.

Deck stair calculator

Deck stairs usually run to a fixed landing point, so the available run is the constraint rather than the rise. Switch to DECK mode and enter the available run: the calculator fits whole treads into that distance first and solves the risers to suit, instead of solving the run to fit the risers. Measure the deck rise from the top of the decking to the finished grade or the top of the landing pad, and remember that the pad settles while the deck does not.

Stair angle and comfortable stairs

A comfortable stair angle sits between 30° and 37° for a main flight. The rule of thumb is that one riser plus one tread lands near 17-18 in, and two risers plus one tread lands near 25 in. The stair angle calculator above shows the exact angle for your flight. If the angle climbs past 40° the treads start to feel tight, and below 25° you end up with a ramp more than a stair.

Residential stair code requirements

The IRC sets limits rather than prescribing dimensions. The key numbers that catch builders are the maximum riser height at 7-3/4 in (IRC R311.7.5.1), the minimum tread depth at 10 in (R311.7.5.2), and the rule that the tallest and shortest riser in a flight cannot differ by more than 3/8 in. Minimum headroom is 6 ft 8 in (R311.7.2), minimum clear width is 36 in (R311.7.1), and a landing must be at least 36 in deep (R311.7.6). Guards are required at 30 in above the floor below and stand 36 in tall (R312.1). The calculator warns when your flight approaches any of these limits. Local amendments override the IRC values, so check the code in force where you are working.

Winders, landings, and flights that do not fit

When a straight flight will not fit the space, the three moves are a deeper tread, a landing that splits the rise in two, or winders that turn the corner. A landing resets the riser count — each flight is solved on its own rise, and the landing itself must be at least as deep as the stair is wide. Winders are measured at the walk line, 12 in from the narrow end. These cases are handled in the app, where multiple flights and landings are solved together.