Stair Stringer Calculator
Measure your total rise — the vertical distance from the lower finished floor to the upper finished floor — and this calculator works out how many risers you need, the exact riser height, the total run, and how long a board your stringers require.
Many US codes limit risers to 7.75 inches maximum and treads to 10 inches minimum, with no more than 3/8" variation between risers — but these are common limits, not universal ones. Requirements differ by state, municipality and stair type (deck vs. interior). Verify with your local building department before cutting, and remember stairs usually need a permit.
How this is calculated
Number of risers = total rise ÷ max riser height, rounded up. Rounding up is what keeps every riser at or under the limit.
Actual riser height = total rise ÷ number of risers. Treads = risers − 1 (the top landing acts as the last step). Total run = treads × tread run. Stringer length = √(total rise² + total run²) — the diagonal of the stair triangle.
If the top of the stair ends flush with the deck or floor rather than one step below it, your tread count equals your riser count; adjust accordingly.
Common questions
How do I measure total rise?
Measure vertically from the finished surface at the bottom of the stairs to the finished surface at the top. If flooring isn't installed yet, account for its thickness at both ends — an error here changes every riser.
Why does the calculator round the riser count up?
Dividing the rise into more steps makes each riser shorter. Rounding up guarantees no riser exceeds the max height you set. Fewer, taller risers would break that limit.
How long a board do I need?
Longer than the calculated stringer length — the diagonal is the theoretical minimum and you lose material to the plumb and level cuts at each end. For 2×12 stringers, buy the next standard length up and check the board is straight.
Can I use this for deck stairs?
The math is the same. Deck stairs often land on a pad or grade that isn't level with the interior floor, so measure the real rise at the spot where the stairs will land.