How the calculator works
Measure the total rise from the finished floor at the bottom to the finished floor at the top, or from the ground to the deck surface. Enter it with the riser height you are aiming for and the tread depth.
- Risers: total rise ÷ target riser, rounded to a whole number. If that puts each riser over the IRC maximum of 7 ¾ inches, the calculator adds one.
- Riser height: total rise ÷ risers. Every riser in a flight must be the same height to within 3/8 inch.
- Treads: risers − 1, because the top step is the upper floor.
- Total run: treads × tread depth.
- Stringer length: √(total rise² + total run²), the diagonal the stringer covers.
- Angle: the arctangent of riser ÷ tread.
The calculator also shows 2 × riser + tread, an old carpenters' rule of thumb: stairs where it adds up to 24 to 25 inches feel natural to climb.
Stair chart: risers, run and stringers
With a 7 ½-inch target riser and 10 ½-inch treads. Stringer stock leaves about a foot for the end cuts.
| Total rise | Risers | Riser height | Treads | Total run | Stringer length | Stringer board |
|---|---|---|---|---|---|---|
| 18 in (1 ft 6 in) | 3 | 6 in | 2 | 1 ft 9 in | 2 ft 3 11/16 in | 8 ft |
| 24 in (2 ft) | 4 | 6 in | 3 | 2 ft 7 ½ in | 3 ft 3 ⅝ in | 8 ft |
| 30 in (2 ft 6 in) | 4 | 7 ½ in | 3 | 2 ft 7 ½ in | 3 ft 7 ½ in | 8 ft |
| 36 in (3 ft) | 5 | 7 3/16 in | 4 | 3 ft 6 in | 4 ft 7 5/16 in | 8 ft |
| 42 in (3 ft 6 in) | 6 | 7 in | 5 | 4 ft 4 ½ in | 5 ft 7 ¼ in | 8 ft |
| 48 in (4 ft) | 7 | 6 ⅞ in | 6 | 5 ft 3 in | 6 ft 7 3/16 in | 8 ft |
| 60 in (5 ft) | 8 | 7 ½ in | 7 | 6 ft 1 ½ in | 7 ft 10 ⅞ in | 10 ft |
| 72 in (6 ft) | 10 | 7 3/16 in | 9 | 7 ft 10 ½ in | 9 ft 10 13/16 in | 12 ft |
| 96 in (8 ft) | 13 | 7 ⅜ in | 12 | 10 ft 6 in | 13 ft 2 ⅜ in | 16 ft |
| 102 in (8 ft 6 in) | 14 | 7 5/16 in | 13 | 11 ft 4 ½ in | 14 ft 2 ⅜ in | 16 ft |
| 108 in (9 ft) | 14 | 7 11/16 in | 13 | 11 ft 4 ½ in | 14 ft 6 1/16 in | 16 ft |
| 114 in (9 ft 6 in) | 15 | 7 ⅝ in | 14 | 12 ft 3 in | 15 ft 6 in | 18 ft |
| 120 in (10 ft) | 16 | 7 ½ in | 15 | 13 ft 1 ½ in | 16 ft 6 in | 18 ft |
IRC stair rules for houses
| Rule | Limit (IRC R311.7) |
|---|---|
| Riser height | 7 ¾ in maximum |
| Tread depth | 10 in minimum, nosing to nosing |
| Variation within a flight | 3/8 in maximum between the tallest and shortest riser, or the deepest and shallowest tread |
| Nosing | ¾ to 1 ¼ in, needed when treads are under 11 in deep |
| Stair width | 36 in minimum |
| Headroom | 6 ft 8 in minimum |
| Handrail | 34 to 38 in above the nosings, on flights with 4 or more risers |
Example: steps down from a deck 40 ½ inches high
You want deck stairs with comfortable 11 ¼-inch treads, the width of two 2×6 boards.
- Risers: 40.5 ÷ 7.5 = 5.4, which rounds to 5. But 40.5 ÷ 5 = 8.1 inches is over the 7 ¾-inch limit, so use 6 risers.
- Riser height: 40.5 ÷ 6 = 6 ¾ inches.
- Treads: 6 − 1 = 5.
- Total run: 5 × 11.25 = 56 ¼ inches, or 4 ft 8 ¼ in. Plan a landing pad that far out from the deck, plus room to step off.
- Stringer: √(40.5² + 56.25²) = 69.3 inches, about 5 ft 9 5/16 in. An 8-foot 2×12 is enough for each stringer.
- Check: 2 × 6.75 + 11.25 = 24.75 inches, right in the comfortable range, at an angle of 31°.
Tips
- Measure the rise at the stairs, not at the house. Ground slopes. Lay a level board out from the deck to where the bottom step lands and measure down from it.
- Use finished floor heights. If the bottom or top floor is getting tile or hardwood, include it, or the first or last step will be off.
- Drop the stringer by one tread thickness. After cutting, trim the bottom of the stringer by the thickness of one tread so the first step is not taller than the rest.
- Lay out with stair gauges. Clamp brass stair gauges to a framing square at the riser and tread marks and trace each notch.
- Leave at least 5 inches of wood below the notches. On a 2×12 stringer that keeps it strong; for long flights add a middle stringer and support.
Frequently asked questions
How do I calculate the number of stairs I need?
Divide the total rise, floor to floor, by the riser height you want and round to a whole number; that is the number of risers. If the result puts each riser over 7 ¾ inches, add one. A 108-inch rise at a 7 ½-inch target is 14.4, so 14 risers of 7 11/16 inches. There is one fewer tread than risers.
What is the code for stair riser height and tread depth?
The International Residential Code, which most US towns adopt for houses, allows risers up to 7 ¾ inches and needs treads at least 10 inches deep, nosing to nosing. Risers in one flight may not differ by more than 3/8 inch, and stairs need at least 6 feet 8 inches of headroom and 36 inches of width. Check your local code, which can be stricter.
How long does a stair stringer need to be?
The stringer runs along the diagonal, the square root of total rise² + total run². Stairs with a 30-inch rise and three 10-inch treads have a diagonal of about 42 ½ inches. Buy a board at least a foot longer for the top and bottom cuts; for most deck stairs an 8- or 10-foot 2×12 is enough.
How many steps do I need for a 3-foot deck?
A 36-inch rise takes 5 risers of 7 3/16 inches and 4 treads. With 10 ½-inch treads the stairs run 42 inches out from the deck, and each stringer is about 4 feet 7 inches along the diagonal.
What is a comfortable stair angle?
Between about 30 and 37 degrees. Most house stairs land around 35 to 37 degrees with 7 ½-inch risers and 10-inch treads. Shallower stairs with 6- to 7-inch risers and 11- to 12-inch treads are easier for children and older people, but take more floor space.
Why is there one fewer tread than risers?
The top of the stairs is the upper floor or the deck itself, so the last riser steps you straight onto it. A flight with 14 risers has 13 treads. That is also why total run is the number of treads, not risers, times the tread depth.
Last updated October 20, 2026.