Deck & Fence
Stair Calculator
Lay out a set of stairs from the total rise. This stair calculator works out the number of risers and their height, the treads, total run, stair angle and stringer length, checks them against the limits you set, and lists the tread and stringer material.
Result
Enter your measurements and press Calculate. What to buy, the working and the cost will appear here.
Layout tool, not a code check. This calculator works out stair geometry from the limits you enter. Riser and tread limits, headroom, handrails, guards and landings are set by your local building code.
How to use the stair calculator
- Measure the total rise from the lower finished floor to the upper finished floor (or deck surface to ground).
- Enter your maximum riser and tread depth. The defaults are common US residential values.
- Optionally enter the available run to check the stairs fit the space.
- Set the stair width and number of stringers, then press Calculate.
Stair formulas
- Risers = total rise ÷ maximum riser, rounded up.
- Riser height = total rise ÷ number of risers (every riser the same).
- Treads = risers − 1, because the upper floor is the last step.
- Total run = treads × tread depth.
- Stringer length = √(rise² + run²).
Worked example: basement stairs
A basement has 105 in from floor to floor and 12 ft of space for the stairs, with a 7 3/4 in maximum riser and 10 in treads:
- Risers: 105 in ÷ 7.75 in max, rounded up = 14
- Riser height: 105 in ÷ 14 = 7.5 in
- Total run: 13 treads × 10 in = 130 in
- Stringer: √(rise² + run²) = 13.93 ft
Risers: 14; Treads: 13; Total run: 10.83 ft; Stair angle: 38.9°; Stringer length: 13.93 ft. Open this example in the calculator.
Comfort rules of thumb
- Riser + tread ≈ 17–18 in (about 430–460 mm).
- 2 × riser + tread ≈ 24–25 in (about 610–635 mm), the classic stride rule.
- Angle around 30–37° feels comfortable for most people; steeper stairs are harder to climb safely.
- Every riser the same height: uneven steps are a trip hazard, which is why the rise is divided evenly.
Cutting stringers
Stringers are usually cut from 2 × 12 lumber, at least a foot longer than the calculated length to allow for the plumb cut at the top and the level cut at the bottom. The bottom riser is typically cut shorter by the thickness of one tread so every step finishes the same height. Use three stringers for a 36 in stair, more for wider stairs or thin treads.
Assumptions and limitations
- Straight stairs only; landings, winders and spiral stairs are not covered.
- Tread nosing overhang is not added to the tread depth.
- The defaults are common US values; your local code may differ.
Stair questions
How many steps do I need for 8 feet?
For a 96 in rise with a 7 3/4 in maximum riser: 96 ÷ 7.75 = 12.4, so 13 risers of about 7.38 in and 12 treads.
What is the maximum riser height?
Many US residential codes allow 7 3/4 in, with a minimum tread of 10 in. Commercial and other codes are stricter. Check locally.
How long should stair stringers be?
At least the diagonal: √(rise² + run²). For a 105 in rise and 130 in run that is about 167 in (13.9 ft), so buy 16 ft boards.
Formulas, unit conversions and default values are explained on our methodology page. Results are estimates; check manufacturer data and local codes before you buy or build.