Stair Calculator
Enter the total height from floor to floor and your preferred riser and tread sizes. You'll get the step count, exact riser height, total run and stringer length — with code checks.
Enter the total rise and your riser and tread sizes.
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How to use the stair calculator
- Measure the total rise from the finished floor at the bottom to the finished floor (or deck surface) at the top. Include flooring you haven't installed yet.
- Set the maximum riser height. The calculator divides the rise into equal risers no taller than this.
- Set the tread depth — the horizontal distance from one riser to the next, not counting any nosing overhang.
- Read the layout and the checks against the International Residential Code (IRC).
How the stair math works
Worked example: 108 in (9 ft) floor to floor
- 108 ÷ 7.5 = 14.4 → 15 risers of 108 ÷ 15 = 7.2 in (about 7 3/16 in)
- 14 treads × 10.5 in = 147 in = 12.25 ft of total run
- Stringer: √(108² + 147²) = 182.4 in ≈ 15.2 ft along the pitch line — buy 16 ft boards
- Comfort: 2 × 7.2 + 10.5 = 24.9 in ✓
There's always one fewer tread than risers because the top floor itself acts as the last step. Forgetting this is one of the most common layout mistakes.
Residential code limits (IRC)
| Dimension | IRC requirement |
|---|---|
| Riser height | 7¾ in maximum |
| Tread depth | 10 in minimum (measured nosing to nosing) |
| Variation between risers or treads | ⅜ in maximum within a flight |
| Headroom | 6 ft 8 in minimum |
| Width | 36 in minimum clear width |
Many local codes adopt the IRC with amendments, and commercial stairs follow different rules. Check with your building department — and see our guide to stair dimensions for nosings, handrails and landings.
Why equal risers matter
Your feet learn the rhythm of a stair after two or three steps. A single riser that's taller or shorter than the rest — often the top or bottom one, where flooring was added later — is a classic cause of trips and falls. That's why the code limits the variation to ⅜ inch and why the calculator divides the rise exactly rather than using a round number.
Frequently asked questions
How many steps for an 8 ft rise?
96 in ÷ 7.5 in = 12.8, so 13 risers of about 7⅜ in, with 12 treads. At 10½ in treads that's a total run of 10.5 ft.
How long should a stair stringer be?
At least the diagonal of the total rise and run, plus extra at the ends for the top connection and bottom cut. The calculator gives the pitch-line length; round up to the next standard board length.
How many stringers do I need?
A common rule is to space stringers no more than about 16 to 18 inches apart, so a 36 in wide stair uses three. Thinner or composite treads may need closer spacing — check the tread manufacturer and your local code.
What is the most comfortable stair?
Stairs where two risers plus one tread add up to about 24 to 25 inches, such as a 7 in riser with an 11 in tread, match a natural stride.
Sources
- International Residential Code, Section R311.7 — stairway dimensions.