Stair Stringer Calculator
Measure the total rise from the lower surface to the top surface, and choose a tread depth. The calculator picks the fewest equal risers that keep each riser at or under 7¾ in, then works out the run and the length of board you need to cut the stringer from.
The 7¾ in riser and 10 in tread figures are the residential limits in the International Residential Code. Your jurisdiction may differ, so confirm locally.
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Worked example
Inputs: total rise 42 in; tread depth 10 in.
- Minimum risers: 42 ÷ 7.75 = 5.42, round up to 6.
- Riser height: 42 ÷ 6 = 7.0 in.
- Treads: one fewer than risers, because the top step is the floor itself: 5.
- Total run: 5 × 10 in = 50 in.
- Stringer length: √(42² + 50²) = √4264 = 65.3 in, so plan on a board at least 6 ft long (2x12 is the usual stock).
Result: 6 number of risers.
Where the formula comes from
International Residential Code R311.7.5.1 (maximum riser height 7¾ in) and R311.7.5.2 (minimum tread depth 10 in). Stringer length is the Pythagorean theorem on the total rise and run. Local code may differ.
Estimates only. Results are planning figures, not engineering or code advice. Check your local building code and confirm quantities with your supplier.
Frequently asked questions
Why is the number of treads one less than risers?
The top riser lands on the upper floor or deck surface, which acts as the top step, so only the steps below it need a tread.
Where do I measure the total rise?
From the finished lower surface to the finished upper surface. If you will add decking or a landing pad, measure to those finished heights, not the rough framing.
Do all risers have to be the same?
Keep them uniform. Building codes limit how much riser height may vary within a flight, and uneven steps are a trip hazard. Treads should be uniform too.
Does the first or last riser need adjusting for tread thickness?
Yes, often. If treads sit on top of the stringer, the bottom riser is cut shorter by the tread thickness so every step ends up the same height. Mark your layout carefully before cutting.