How to use the stair calculator
Measure the total rise: the vertical distance from the finished surface at the bottom to the finished surface at the top. That one measurement drives everything else. Enter the tread depth you'd like and the calculator works out how many risers you need, how tall each one is, how far the stairs will reach out from the top, and how long the stringers have to be.
The diagram shows the stringer layout to scale: each notch is one riser and one tread, and the top riser meets the upper floor. Check the total run against the space you have before you buy lumber; if it doesn't fit, a shorter tread won't help much below 10 inches, and you may need a landing or a turn.
The formulas
risers = total rise ÷ maximum riser, rounded up
riser height = total rise ÷ risers
treads = risers − 1
total run = treads × tread depth
stringer length = √(total rise² + total run²)
Worked example: 105 inch total rise, 10 inch treads
- Risers: 105 ÷ 7.75 = 13.5, round up to 14 risers.
- Riser height: 105 ÷ 14 = 7½ in exactly.
- Treads: 14 − 1 = 13 treads.
- Total run: 13 × 10 = 130 in (10 ft 10 in).
- Stringer: √(105² + 130²) = √27,925 ≈ 167.1 in (13 ft 11 in), so a 16 ft 2×12.
- Comfort check: 2 × 7.5 + 10 = 25 in, right at the top of the comfortable range.
The IRC stair rules at a glance
| Item | IRC requirement |
|---|---|
| Riser height | 7¾ in maximum |
| Tread depth | 10 in minimum, nosing to nosing |
| Variation in a flight | ⅜ in maximum between the largest and smallest riser or tread |
| Nosing | ¾ to 1¼ in, required when treads are under 11 in with solid risers |
| Headroom | 6 ft 8 in minimum, measured from the line of the nosings |
| Width | 36 in minimum clear above the handrail |
| Handrail | 34 to 38 in above the nosings, on at least one side when there are 4 or more risers |
This summarises IRC 2021 R311.7. Many places adopt the IRC with local changes, and decks and exterior stairs may have extra rules. Your building department has the final word.
Laying out a stringer
- Set a framing square on the 2×12 with the riser height on one leg and the tread depth on the other, using stair gauges to lock them.
- Mark each notch, sliding the square along the board, until you have the right number of risers.
- Drop the bottom. Cut the bottom riser shorter by one tread thickness. Otherwise the first step ends up taller than the rest once treads go on. The calculator shows this figure.
- Cut with a circular saw, and finish the inside corners of each notch with a handsaw so you don't weaken the stringer by over-cutting.
- Use the first stringer as a template for the others.
Common mistakes
- Measuring to the subfloor instead of the finished floor. Adding ¾ in of hardwood at the top makes the top riser short and breaks the ⅜ in rule.
- Not checking the run. A 9 ft rise needs about 11 ft of floor space. Measure before you start.
- Ignoring headroom. 6 ft 8 in is measured above every nosing, all the way up.
Questions people ask
How do I calculate the number of stairs I need?
Measure the total rise from the lower floor to the upper floor surface in inches and divide by 7.75, the maximum riser height in the IRC. Round up to a whole number. That's your number of risers; you'll have one fewer tread, because the top floor acts as the last step.
What is the maximum riser height for stairs?
The International Residential Code (R311.7.5.1) allows a maximum riser height of 7¾ inches, and the tallest and shortest riser in a flight can't differ by more than ⅜ inch. Some local codes are stricter, so check yours.
What is the minimum tread depth?
10 inches under the IRC, measured nosing to nosing. If the tread is less than 11 inches deep and the risers are solid, each tread needs a nosing that projects ¾ to 1¼ inches.
How long should a stair stringer be?
Use Pythagoras: stringer length = √(total rise² + total run²). For a 105 inch rise with 13 treads of 10 inches (130 inch run), that's √(105² + 130²) ≈ 167 inches, about 13 ft 11 in, so you'd cut it from a 16 ft 2×12.
How many stringers do I need?
Three for a typical 36 inch wide stair is common practice: one at each side and one in the middle. Wider stairs need more, and the spacing depends on your tread material, so check the tread maker's span or your local code.
What is the most comfortable stair rise and run?
A long-standing rule is that two risers plus one tread should come to about 24 to 25 inches. A 7 inch riser with an 11 inch tread (25) feels easy; a 7¾ inch riser with a 10 inch tread (25.5) is legal but steep.
Sources and assumptions
- 2021 International Residential Code, Section R311.7 Stairways (riser, tread, nosing, headroom, width, handrail)
- In the 2024 IRC these stair provisions moved to Section R318 with the same dimensions. Your local code may differ: check before you build.
- The 2R + T comfort rule (24-25 in) is a traditional design guideline, not a code requirement.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.