Stair Calculator
Steps, riser height, run and stringer lengthRuns in your browser
- Within the IRC residential limits (riser ≤ 7¾ in, tread ≥ 10 in).
- Comfortable: two risers plus one tread is 24–26 in.
Stringer length is the diagonal from the bottom floor to the top floor; buy lumber at least a foot longer for the cuts. Add 1 in nosing to treads if your code requires it.
Material estimates only. Round up, buy a little extra, and check with your supplier. Structural work (footings, framing, roofs) must follow local building codes; consult a licensed contractor where required.
How it works
Formulas
Risers = total rise ÷ max riser height, rounded upRiser height = total rise ÷ risers; treads = risers − 1Total run = treads × tread depthStringer length = √(total rise² + total run²)Worked example
Input: Total rise 108 in, max riser 7¾ in, 10 in treads, 36 in wide.
- 108 ÷ 7.75 = 13.9 → 14 risers of 108 ÷ 14 = 7.71 in (about 7 11/16 in).
- 13 treads × 10 in = 130 in (10 ft 10 in) of run.
- √(108² + 130²) = 169 in ≈ 14 ft 1 in of stringer at 39.7°.
- 36 in wide → 3 stringers.
Result: 14 risers of 7.71 in, 13 treads, 14 ft 1 in stringers.
Reading the result
- Two risers plus one tread of about 24–26 in usually feels comfortable; the example gives 25.4 in.
Limits
- Straight stairs only; landings, winders and handrail/guard rules are not covered.
- Check local code and the tread maker's span rating.
Sources
- International Residential Code R311.7 (stairways)
How to use
- 1Measure the total rise from finished floor to finished floor.
- 2Keep the 7¾ in maximum riser (IRC) or enter a lower one for gentler stairs.
- 3Enter the tread depth (10 in minimum) and the stair width.
- 4Read the number of risers and treads, the exact riser height, total run and stringer length.
FAQ
How do I calculate the number of stairs?
Divide the total rise by the maximum riser height and round up. For a 108 in (9 ft) rise: 108 ÷ 7.75 = 13.9 → 14 risers of 7.71 in each.
Why are there fewer treads than risers?
The top 'step' is the upper floor itself, so a staircase with 14 risers has 13 treads.
What are the code limits for residential stairs?
The International Residential Code (R311.7.5) allows risers up to 7¾ in and requires treads of at least 10 in, with at most 3/8 in variation between steps. Local codes can differ.
How many stringers do I need?
At least two, spaced no more than about 18 in apart for 2× wood treads (often 12 in for composite). A 36 in wide stair needs 3.
Can I use metric units?
Yes. Switch to Metric and enter meters and centimeters; results show cubic meters, square meters or tonnes alongside US units.