This stair calculator designs a staircase from the floor-to-floor height: how many risers and treads you need, the exact riser height, total run, stringer length — and whether the result sits inside common building-code comfort limits.
How Stair Design Works
Stairs are designed backwards from one fixed number — the total rise, finished floor to finished floor. You choose how many risers divide it (rounding up so no step is too tall), which fixes the exact riser height: total rise ÷ risers. Treads are always one fewer than risers — the upper floor itself is the last "tread." Multiply treads by your chosen tread depth for the total run, and Pythagoras gives the stringer. This calculator is part of our Construction Calculators collection.
Worked Example (Verified)
Floor-to-floor height 108 inches (a 9 ft ceiling plus floor structure), targeting 7.5″ risers:
- Risers: 108 ÷ 7.5 = 14.4 → round up to 15 risers
- Exact riser: 108 ÷ 15 = 7.2 inches — every step identical, which is the whole secret of safe stairs
- Treads: 14, at 11″ each → total run 154 inches (12 ft 10 in)
- Stringer: √(154² + 108²) ≈ 188 inches; stair angle ≈ 35.1°
- Comfort: 2 × 7.2 + 11 = 25.4″ — inside the classic band
The Rules That Make Stairs Feel Right
- Uniformity above all: codes typically allow only ⅜″ variation between risers in a flight. The brain learns the first step and expects every other — a single off step is a trip hazard.
- Blondel's formula: 2 × riser + tread ≈ 24–25.5″ matches the human stride; stairs in this band "feel" effortless.
- Common residential limits (US IRC): max riser 7¾″, min tread 10″, min headroom 6 ft 8 in, handrails on flights with 4+ risers. Always confirm your local code — it governs, not any calculator.
- Measure the total rise last: after finished flooring is chosen — a ¾″ floor change alters every riser if you measured to subfloor.
Related Tools
- Construction Calculators — the full collection
- Triangle Calculator — the stringer geometry
Frequently Asked Questions
How many steps do I need for a 9-foot ceiling?
Total rise is usually ~108″ (ceiling + joists/flooring). At a 7.5″ target: 15 risers of 7.2″ and 14 treads.
What is the ideal riser height and tread depth?
Around 7–7.5″ risers with 11″ treads is the comfortable residential classic (2R + T ≈ 25″). Codes cap risers at 7¾″ and require treads of at least 10″ in most US jurisdictions.
Why are there fewer treads than risers?
The upper floor surface serves as the final tread — you step up the last riser onto the floor itself, so treads = risers − 1.
How long should my stringer board be?
At least the hypotenuse shown (total run² + total rise², square-rooted), plus material to cut the ends — the example's 188″ means buying 16 ft stringer stock.
What angle should stairs be?
Comfortable stairs run 30–37°. Our example is 35.1°. Steeper than ~40° starts feeling like a ladder; shallower eats floor space.
Is this stair calculator free?
Yes — free, instant, no sign-up. Verify final designs against your local building code before cutting stringers.