Stair Calculator
Enter the total rise — floor to floor, not floor to ceiling — and get the riser count, the exact riser height, the total run, the stringer length and the angle, checked against the usual residential limits.
How to use this calculator
- Measure the total rise properly. Finished floor to finished floor, including the flooring you are going to lay on both levels. Getting this wrong by half an inch spreads across every riser and is the most common stair mistake.
- Let the tool divide the rise. You do not choose the riser height directly — you choose a target, and the rise divided by a whole number of risers gives the actual height. That is why the result is 7.2 in rather than a round number.
- Check the warnings. If the riser comes out over 7-3/4 in, add a riser; the tool shows what the next option looks like.
- Use the stringer length to buy material. It is the minimum board length before any cuts or a top hanger detail — buy the next size up.
The formula
risers = round( total rise ÷ target riser height )
actual riser = total rise ÷ risers
treads = risers − 1 (when the upper floor forms the last step)
total run = treads × tread depth
stringer length = √( rise² + run² )
angle = arctan( rise ÷ run )
Stringer length is Pythagoras on the whole flight — the same relationship as a roof's slope factor, applied to stairs.
| Check | Usual residential limit |
|---|---|
| Maximum riser height | 7-3/4 in |
| Minimum tread depth | 10 in |
| Variation between risers in a flight | 3/8 in |
| Minimum headroom | 6 ft 8 in |
| Minimum width above handrail | 36 in |
| Comfort rule | 2 × riser + tread = 24–25 in |
Worked example
A 9 ft floor-to-floor rise — 108 in — with a 7.5 in target riser and 10 in treads:
- Risers: 108 ÷ 7.5 = 14.4 → round to 14 risers
- Actual riser: 108 ÷ 14 = 7.714 in (7 in + 11/16)
- Treads: 14 − 1 = 13
- Total run: 13 × 10 = 130 in = 10 ft 10 in
- Stringer: √(108² + 130²) = 169 in, or 14 ft 1 in
- Angle: arctan(108 ÷ 130) = 39.7°
- 2R + T = 15.43 + 10 = 25.4 in — just outside the comfort range
At 7.714 in the riser only just clears the 7-3/4 in limit, and the comfort figure says the tread is slightly shallow for that riser. Fifteen risers would give 7.2 in each and a 11 ft 8 in run — better underfoot, but it needs another 10 in of floor space. That trade-off, run against comfort, is the entire design problem of a staircase.
Common rises
| Total rise | Risers | Each | Run at 10 in |
|---|---|---|---|
| 96 in (8 ft) | 13 | 7.385 in | 10 ft 0 in |
| 102 in | 14 | 7.286 in | 10 ft 10 in |
| 108 in (9 ft) | 15 | 7.200 in | 11 ft 8 in |
| 112 in | 15 | 7.467 in | 11 ft 8 in |
| 120 in (10 ft) | 16 | 7.500 in | 12 ft 6 in |
Frequently asked questions
How do I calculate the number of stairs I need?
Divide the total rise, finished floor to finished floor, by a comfortable target riser height of about 7 to 7.5 inches, then round to a whole number. Dividing the total rise by that whole number gives the exact riser height, which is rarely a round figure.
What is a comfortable stair rise and run?
A riser of 7 to 7.5 inches with a tread of 10 to 11 inches suits most people. The usual check is that twice the riser plus the tread lands between 24 and 25 inches — steep, shallow-tread stairs and shallow, deep-tread stairs both feel wrong underfoot.
What is the maximum riser height?
Most residential code sets 7-3/4 inches as the maximum riser and 10 inches as the minimum tread depth, and requires all risers in a flight to be within 3/8 inch of each other. Confirm the figures that apply in your jurisdiction.
How long a board do I need for a stair stringer?
The straight-line length is the square root of rise squared plus run squared. For a 108 inch rise and a 130 inch run that is 169 inches, or 14 feet 1 inch, so you would buy a 16 foot board to allow for the cuts.
Why is the number of treads one less than the number of risers?
Because the upper floor acts as the last tread. A flight with 14 risers steps up 14 times but only has 13 tread boards, with the fourteenth step landing on the floor above.