Quick answer: To size a stair stringer, divide the total rise by about 7.5 inches for the number of risers, multiply the tread depth (10–11 in) by the treads for total run, then take the diagonal — the square root of (rise² + run²) — for the stringer length.
This stair stringer calculator finds the exact stringer length, riser and tread cuts, cut angle, and how many stringers you need for deck or interior stairs — with a code check for riser height.
A stair stringer is the sawtooth-cut board that supports the treads and risers. Getting it right is all about splitting your total rise into equal steps, then finding the diagonal length of the board and the notch cuts.
After you notch out the treads and risers, the remaining solid wood behind the cuts is called the throat, and it must be at least 3.5 inches to stay strong. A 2x12 (11.25 in actual) leaves plenty of throat; a 2x10 (9.25 in) only works for shallow steps. For almost all deck and interior stairs, use 2x12 stringers.
Stringer spacing depends on stair width and the decking or tread material:
Interior stairs are usually steeper (7.5 in rise, 10 in run) to save space, while deck stairs are more relaxed (6.5–7 in rise, 11 in run) because they're outdoors and often carry more traffic. Deck stringers are also pressure-treated and attached to the framing with a ledger or angle brackets rather than housed in a skirt.
Divide your total rise by about 7.5 inches to get the number of risers, then divide the total rise by that count for the exact riser height. Multiply the tread depth (usually 10 to 11 inches) by the number of treads for total run. The stringer length is the diagonal: the square root of (total rise squared plus total run squared).
A 2x12 is the standard stringer board because after cutting the notches you must leave at least 3.5 inches of solid wood (the throat). A 2x10 only works for very shallow cuts, so most builders use 2x12.
Space stringers no more than 16 inches apart for 5/4 decking or 12 inches for heavy loads. A standard 36-inch-wide stair needs at least 3 stringers; wider stairs need 4 or more.
It depends on total rise, not the board. A typical single 2x12 (about 12 feet) can cut a stringer for roughly 13 to 14 steps before you need a taller board or an intermediate landing.
The IRC limits residential stair risers to a maximum of 7.75 inches and requires a minimum tread depth of 10 inches. This calculator flags any riser height that exceeds code.
Subtract the thickness of one tread board from the bottom riser cut so the finished bottom step matches all the others. This is called 'dropping the stringer.'