Stair calculator
Enter the total rise to get how many steps you need, the riser height, tread depth, total run, stair angle and stringer length. Risers and treads are checked against IRC 2021.
Your stair
15 @ 7-3/16"
Set a framing square to 7-3/16" on one arm and 10" on the other to mark the notches, and take one tread thickness off the bottom riser. The app checks the flight against the ruleset you pick.
stringer = √(rise² + run²)
RISE 9' 0" RUN 11' 8" STRINGER 14' 8-13/16"
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), the same sum the diagonal calculator does. 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.
How many stairs for an 8, 9 or 10 foot ceiling
The total rise is measured floor to floor, so it is the ceiling height plus the thickness of the floor above. With 2×10 joists and 3/4 in subfloor that adds 10 in, so an 8 ft ceiling is about 106 in of rise, a 9 ft ceiling about 118 in and a 10 ft ceiling about 130 in. Measure your own floor to floor before you cut: finished flooring on either level changes it.
Steps by rise, at a 7-1/4 in target riser and 10 in treads
| Total rise | Risers | Treads | Total run |
|---|---|---|---|
| 30 in, a low deck | 4 @ 7-1/2" | 3 | 2' 6" |
| 48 in, a 4 ft rise | 7 @ 6-7/8" | 6 | 5' 0" |
| 96 in, 8 ft floor to floor | 13 @ 7-3/8" | 12 | 10' 0" |
| 106 in, 8 ft ceiling | 15 @ 7-1/16" | 14 | 11' 8" |
| 118 in, 9 ft ceiling | 16 @ 7-3/8" | 15 | 12' 6" |
| 130 in, 10 ft ceiling | 18 @ 7-1/4" | 17 | 14' 2" |
Riser heights are to the nearest sixteenth. A flight always has one tread fewer than it has risers, because the top step is the floor.
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 a riser is over 7-3/4 in or a tread is under 10 in. Headroom, width, landings and guards are measured on site. Local amendments override the IRC values, so check the code in force where you are working.
When a straight flight does not fit
If the run is too long for the space, the choices are a steeper stair within the code limits, a landing that splits the rise into two flights, or winders that turn the corner. This calculator solves one straight flight. For a flight split by a landing, solve each part on its own rise.
Questions about stairs
How do you calculate stair stringers?
Take the total rise and the total run, and the stringer is √(rise² + run²). Mark the notches with a framing square set to the riser height on one arm and the tread depth on the other, then take one tread thickness off the bottom riser so the finished steps come out equal.
What is the maximum riser height?
IRC 2021 sets 7-3/4 in for dwelling stairs in R311.7.5.1. Commercial work under the IBC is tighter at 7 in. Local amendments override both, and your building official has the last word.
What is the 7-11 rule for stairs?
Shorthand for a 7 in riser and an 11 in tread. It sits inside the residential limits of a 7-3/4 in maximum riser and a 10 in minimum tread, and it climbs at 32.4712°, which is a comfortable stair to walk.
How many steps do you need for a 42 inch rise?
Six risers at exactly 7 in, with five treads. 42 ÷ 6 = 7, which is one of the rare rises that divides clean. At a 10 in tread that is 4 ft 2 in of run and a 4 ft 5-1/8 in stringer.
How many stairs do I need for a 9 foot ceiling?
About 16 risers and 15 treads. A 9 ft ceiling with a 2×10 floor above is about 118 in floor to floor, and 118 ÷ 16 is 7-3/8 in per riser. At 10 in treads the flight runs 12 ft 6 in. Measure your actual floor to floor, because the floor thickness changes the count.
How long is a 2×12 for a 5 step stringer?
Five risers at 7 in over four treads at 10 in is a 35 in rise and a 40 in run, so the diagonal is 53-1/8 in. A 6 ft board covers it with waste for the cuts.
Do the risers all have to be the same height?
Within 3/8 in. The tallest and the shortest riser in one flight cannot differ by more than that, which is why the riser count is solved before the height rather than the other way round.
How do you work out stairs for a deck?
A deck stair usually runs to a fixed landing point, so the run is the constraint. Switch to DECK mode and give it the available run: it fits the treads into that distance first and solves the risers to suit, instead of solving the run to fit the risers.
What is a comfortable stair angle?
Between 30° and 37° for a main flight. The rule of thumb behind it is that one riser plus one tread lands near 17-18 in, and two risers plus one tread lands near 25 in.
What does IRC say about stair tread depth?
IRC 2021 R311.7.5.2 sets a minimum tread depth of 10 in for dwelling stairs. The tread is measured from nosing to nosing, not from the riser face. Treads under 10 in trigger a warning.
