How this stair calculator works
Stairs are laid out backward from one number: the total rise, finished floor to finished floor. The calculator divides it into the largest set of equal risers near your target height, keeps them inside the IRC limit, and everything else follows:
riser = rise ÷ risers · run = treads × tread depth · stringer = √(rise² + run²)
Worked example: stairs for an 8 ft floor
An 8 ft ceiling with 2×10 joists and subfloor gives a total rise near 106 in. Divided by a 7 in target: 15 risers of 7 1/16 in, 14 treads at 10 in for a run of 11′-8″, and a stringer of 14′-7 5/8″ — cut from a 16 ft 2×12 at 37.1°.
Stair rise and run: the code numbers
- Riser height: 7¾ in maximum (IRC R311.7.5.1), and no more than 3/8 in variation across the finished stair.
- Tread depth: 10 in minimum, nose to nose; treads under 11 in need a ¾–1¼ in nosing.
- Width & headroom: 36 in clear width and 6 ft 8 in of headroom over the nosing line, minimum.
- Landings: a landing at least as deep as the stair is wide at top and bottom — a door can't swing over the top step.
These are the national IRC numbers — local amendments can be tighter, and commercial (IBC) stairs use 7 in / 11 in. The badges on the calculator flag the two limits that kill inspections most often.
Stair layouts by total rise
| Total rise | Risers | Riser height | Total run | Stringer → board |
|---|---|---|---|---|
| 96 in | 14 | 6 7/8″ | 10′-10″ | 13′-5 5/8″ → 14 ft |
| 100 in | 14 | 7 1/8″ | 10′-10″ | 13′-8″ → 14 ft |
| 104 in | 15 | 6 15/16″ | 11′-8″ | 14′-6 3/8″ → 16 ft |
| 106 in | 15 | 7 1/16″ | 11′-8″ | 14′-7 5/8″ → 16 ft |
| 112 in | 16 | 7″ | 12′-6″ | 15′-7 3/16″ → 16 ft |
| 120 in | 17 | 7 1/16″ | 13′-4″ | 16′-8″ → 18 ft |
How to cut a stair stringer
- Lay out with a framing square: clamp stair gauges at the riser and tread numbers, step the square along the board, and mark every step before cutting.
- Drop the stringer: trim the bottom cut by one tread thickness, or the first step ends up tall and the top step short — the classic failed-inspection mistake.
- Cut one, copy the rest: test-fit the first stringer, then trace it. A 36 in stair wants three stringers, 16 in on center.
- Overcut nothing: finish inside corners with a handsaw or jigsaw — circular-saw overcuts weaken the stringer exactly where it carries load.
Stair calculator FAQ
How many steps do I need for an 8 ft ceiling?
Usually 15 risers. An 8 ft ceiling plus joists and subfloor gives a total rise around 106 in, and 106 ÷ 15 = a comfortable 7 1/16 in riser with 14 treads. Always work from the measured floor-to-floor rise, not the ceiling height.
What is the maximum stair riser height?
The IRC (R311.7.5.1) caps risers at 7¾ in, and the difference between the tallest and shortest riser on a finished stair can be no more than 3/8 in. Commercial stairs under IBC are stricter — 7 in max riser, 11 in min tread.
What is the minimum tread depth?
10 in under the IRC, measured nose to nose — and treads under 11 in must have a nosing of ¾ to 1¼ in. 10½–11 in treads feel noticeably safer walking down; use the chips above to compare the run they cost you.
How long a 2×12 do I need for a stair stringer?
The stringer for the default 106 in rise runs 14 ft 7 5/8 in point to point, so you buy a 16 ft 2×12 — the calculator names the shortest standard board for your layout. Pick straight, tight-knot stock; the stringer is the structure.
What is a comfortable stair angle?
Residential stairs land between about 30° and 38°; the default layout is 37.1°. Shallower needs more run, steeper gets harder to descend — the old carpenter check is riser + tread ≈ 17–18 in, which 7 1/16 + 10 satisfies.
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