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Construction & Home
Stair Calculator — Rise, Run, Risers, Treads & Stringer Length
Lay out a code-compliant staircase from one measurement: total rise
Building a staircase comes down to one starting number: the total rise — the vertical distance from the lower finished floor to the upper finished floor. Everything else (how many steps, how tall each step is, how deep each tread is, how long to cut the stringers) falls out of that single measurement. Get the layout right and the stairs feel natural; get it wrong and every step feels off, fails inspection, or becomes a trip hazard.
The method is the same one carpenters have used for generations. Number of risers = total rise ÷ a target riser height (about 7.5 in), rounded to the nearest whole number. You can't have a fractional step, so you round and then back-solve the exact riser height: riser height = total rise ÷ number of risers. A staircase always has one more riser than it has treads, because the top "step" is the upper floor itself — so treads = risers − 1.
Worked example: a deck 45 inches off the ground. 45 ÷ 7.5 = 6 risers. Riser height = 45 ÷ 6 = 7.5 in (right in the sweet spot). Treads = 6 − 1 = 5 treads. At a 10.5 in tread depth, total run = 5 × 10.5 = 52.5 in. The stringer is the hypotenuse: stringer = √(rise² + run²) = √(45² + 52.5²) = 69.2 in — round up and buy a board at least that long.
The International Residential Code (IRC) sets the safety limits this calculator checks for you:
• Maximum riser height: 7.75 in. Steps taller than this are a fall risk and fail inspection. • Minimum tread depth: 10 in (measured nose-to-nose, not counting overhang). Most builders use 10.5–11 in for comfort. • Maximum variation between any two risers or treads: 0.375 in (3/8 in). Uneven steps are the #1 stair-fall cause.
A comfortable stair also follows a rule of thumb: 2 × riser + tread ≈ 24–25 in, or riser + tread ≈ 17–18 in. With a 7.5 in riser and a 10.5 in tread, 2(7.5) + 10.5 = 25.5 — close, and within a usable range.
This calculator takes your total rise, an optional target riser height and tread depth, and returns the full layout: riser count, exact riser height, tread count, total run, and the minimum stringer length to cut — then flags anything that falls outside IRC limits so you catch it before you cut a single board.
📖 See also: How to Calculate a Tip (and Split the Bill)
Calculator
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📰 Formula
• Number of risers = round(total rise ÷ target riser height) • Riser height = total rise ÷ number of risers • Number of treads = number of risers − 1 • Total run = number of treads × tread depth • Stringer length = √(total rise² + total run²) IRC code constants: • Maximum riser height: 7.75 in • Minimum tread depth: 10 in (nose-to-nose) • Max variation between risers or treads: 0.375 in (3/8 in) • Comfortable target riser: 7–7.75 in; comfortable tread: 10–11 in
📰 Formula
• Number of risers = round(total rise ÷ target riser height) • Riser height = total rise ÷ number of risers • Number of treads = number of risers − 1 • Total run = number of treads × tread depth • Stringer length = √(total rise² + total run²) IRC code constants: • Maximum riser height: 7.75 in • Minimum tread depth: 10 in (nose-to-nose) • Max variation between risers or treads: 0.375 in (3/8 in) • Comfortable target riser: 7–7.75 in; comfortable tread: 10–11 in
🧪 Worked examples
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Example 2
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Example 3
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Example 4
⚠️ Common mistakes
- Measuring total rise to the subfloor instead of the finished floor — the finish flooring on each level changes the rise.
- Allowing a riser height over 7.75 in, which fails IRC and feels steep and unsafe.
- Using a tread depth under 10 in (nose-to-nose), the IRC minimum — overhang doesn't count toward depth.
- Forgetting that there's always one more riser than treads (the top floor is the final riser).
- Rounding the riser count the wrong way, leaving one step a different height than the rest.
💡 Tips
- Always divide the actual rise by the chosen riser count to get the exact, even riser height — never just assume 7.5 in.
- Aim for a 7–7.5 in riser with a 10.5–11 in tread for the most comfortable, code-friendly stair.
- Buy stringer stock longer than the calculated stringer length so you have room for the cuts and the top/bottom adjustments.
- Keep every riser and every tread within 3/8 in of each other — uneven steps are the leading cause of stair falls.
- Add the tread thickness when you mark stringers so the finished step heights stay equal top to bottom.
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❓ Frequently asked questions
How do I calculate the number of stairs I need?
Divide the total rise (finished floor to finished floor) by a target riser height of about 7.5 in, then round to the nearest whole number — that's your number of risers. The number of treads is always one less. For a 45 in rise: 45 ÷ 7.5 = 6 risers and 5 treads.
What is the ideal riser height and tread depth?
A comfortable stair has a riser of 7 to 7.75 in and a tread depth of 10 to 11 in. The IRC caps riser height at 7.75 in and sets a 10 in minimum tread depth measured nose-to-nose. Most builders settle on roughly 7.5 in risers with 10.5 in treads.
How do I find the stringer length?
The stringer is the diagonal of a right triangle whose legs are the total rise and the total run. Use stringer = √(rise² + run²). For a 45 in rise and 52.5 in run, that's √(45² + 52.5²) = 69.2 in, so buy a board longer than that.
What is total run on a staircase?
Total run is the horizontal distance the staircase covers, equal to the number of treads times the tread depth. With 5 treads at 10.5 in each, the total run is 52.5 in. It tells you how much floor space the stairs will take up.
Why is there one more riser than tread?
The top tread is the upper floor itself, so you never build a separate tread for it — but you still climb a riser to reach it. That's why treads always equal risers minus one. A 6-riser stair has 5 treads.
What does the IRC say about stair dimensions?
The International Residential Code limits riser height to a maximum of 7.75 in and requires a minimum tread depth of 10 in. It also requires that risers and treads not vary more than 3/8 in from each other within a flight, to keep the steps even and safe.
How do I measure total rise correctly?
Measure the vertical distance from the lower finished floor to the upper finished floor, including any finish flooring on both levels. Measuring to the subfloor leaves out the floor covering and throws off every riser. Use a level and a straight board if the floors are far apart.
What if my riser height comes out too tall?
If dividing gives a riser over 7.75 in, add one more riser and recalculate. A 32 in rise ÷ 4 = 8.0 in (over code), but ÷ 5 = 6.4 in, which is compliant. Adding a riser always lowers the per-step height.
How much horizontal space do stairs need?
That's the total run. Multiply the number of treads by the tread depth. A typical 8 ft (96 in) rise produces about 12–13 treads, so at 10.5 in each you need roughly 126–137 in (about 10.5–11.5 ft) of floor length plus landing space.