How to Calculate Stairs: Stringers, Steps & Treads
Quick answer: divide total rise by a ~7-in riser to get the step count, then treads follow. A 10-ft rise needs about 17 risers and 16 treads, running roughly 12.5 ft. Here's the full method.
- Riser and tread basics
- Counting steps
- Stringer length
- Worked example
- Code limits
- Common mistakes
Riser and tread basics
A stair is made of risers (the vertical part) and treads (the horizontal part you step on). Building code commonly allows risers up to about 7–7.75 in and treads about 10–11 in deep. Use these to size a comfortable, code-friendly stair.
The two numbers work together: a stair feels right when riser + tread lands in a comfortable range (roughly 17–18 in combined). A tall riser with a shallow tread feels steep; a short riser with a deep tread eats too much floor space.
Counting steps
Total rise (floor to floor) ÷ riser height = number of risers, rounded up. The number of treads is always one less than the number of risers — the top riser lands at the upper floor, not on a tread.
Risers = total rise ÷ 7 in (or your chosen rise), treads = risers − 1. Measure the actual floor-to-floor rise with a tape, not a guess — even a 1-inch error over 17 risers throws every step off slightly.
Stringer length
The stringer (the angled board supporting the steps) spans the diagonal. Its length ≈ √(total run² + total rise²) in feet, where total run = treads × tread depth. For a 12.5-ft run and 10-ft rise, the stringer is about 16 ft.
Buy stringer stock longer than your calculated diagonal so you have room to lay out and trim the notches — the layout process wastes a few inches at each end.
Worked example
- Total rise = 120 in (10 ft)
- Risers = 120 ÷ 7 = 17.1 → 17 risers (each ~7.06 in)
- Treads = 17 − 1 = 16 treads
- At 10-in treads, total run = 16 × 10 = 160 in = 13.3 ft
Keep all risers the same height — divide the total rise by the riser count and cut to that exact number, rather than free-handing each step.
Code limits
Local code often caps riser height at 7.75 in and sets minimum tread depth around 10 in, with a minimum width. Check your code before cutting — stair dimensions are safety-critical and inspected.
Common residential limits: risers ≤ 7.75 in, treads ≥ 10 in, width ≥ 36 in for most homes. Handrail rules vary, so confirm with your local building department before you build.
Common mistakes
- Miscounting treads. It's risers minus one, not equal.
- Uneven risers. Keep every riser the same height — a variation trips people.
- Ignoring code. Risers over ~7.75 in fail inspection in most areas.
- Forgetting the stringer angle. Measure the diagonal, not the vertical rise, for the stringer board.
- Rounding inconsistently. Round risers consistently, then recalc the exact riser height from the total rise.
FAQ
Divide the total rise in inches by your riser height (about 7 inches). Round up for the number of risers; treads are one less.
A 10-foot (120 in) rise needs about 17 risers at ~7 inches each, which means 16 treads.
A stair stringer spans the diagonal: length ≈ square root of (total run squared + total rise squared). For a 12.5-ft run and 10-ft rise, that's about 16 feet.
Most residential codes allow risers up to about 7.75 inches, with treads at least 10 inches deep. Check local code, as stairs are inspected.