How long a board do I buy?

Longer than the stringer length here. The diagonal is the minimum; notching for the treads and risers and squaring the ends eats into it, so add a foot or two and cut to fit. A 2×12 is the usual stock.

How many stringers does the stair need?

At least three for a typical 36-inch-wide residential stair — two outers and one center — and more for wider stairs or heavier decking. Spacing and count are structural; follow your plan or code.

How do I mark the notches?

Set a framing square to the riser height on one leg and the tread run on the other, then step it down the board once per tread. Each step is one notch — the calculator's notch count is how many times you move the square. Then “drop” the stringer: trim the bottom by one tread thickness so the first step finishes the same height as the rest once the treads are on.

Stair Stringer Calculator

A stringer is the notched board that carries the treads. Enter the step count, riser height, and tread run and this gives the board length to cut and the number of notches to mark.

Stringer length

167.1in

Total rise: 105 in
Total run: 130 in
Tread notches: 13 notches

Stringer layout estimate only. Stringer size, count, spacing, and attachment are structural and must follow stamped plans or local building code. Riser/tread limits (IRC: 7-3/4-in max riser, 10-in min tread, uniform within 3/8 in) are code-governed — verify before cutting.

How these numbers are worked out

The formula

Total rise = risers × riser height. Treads (and notches) = risers − 1. Total run = treads × tread run. Stringer length = √(total rise² + total run²) — the hypotenuse the stringer follows. Each notch removes one riser-height vertical and one tread-run horizontal step.

Constants and sources

Total rise = risers × riser; total run = (risers − 1) × tread; stringer = √(rise² + run²); notches = risers − 1. Layout geometry — not a structural design.

How it was checked

Last reviewed June 16, 2026. Before this calculator was published, its formula, constants and worked example were checked in a separate, AI-assisted review that redid the arithmetic and tested the factual claims; the site owner then approved it for publishing. No engineer or tradesperson reviews these pages. Where a figure depends on the product (a density, a coverage rate, a bag size), it is either an input you can change above or a fixed planning value stated on this page, so check it against your product's label or your supplier.

Spot a wrong number or an outdated figure? Send a correction.

Worked example

14 risers at 7.5 in and a 10-in tread: total rise = 105 in; 13 treads × 10 = 130-in run; stringer = √(105² + 130²) ≈ 167 in (13 ft 11 in). Cut from a 2×12 and order it a foot or two longer to allow for the notches.

FAQs

How long a board do I buy?: Longer than the stringer length here. The diagonal is the minimum; notching for the treads and risers and squaring the ends eats into it, so add a foot or two and cut to fit. A 2×12 is the usual stock.
How many stringers does the stair need?: At least three for a typical 36-inch-wide residential stair — two outers and one center — and more for wider stairs or heavier decking. Spacing and count are structural; follow your plan or code.
How do I mark the notches?: Set a framing square to the riser height on one leg and the tread run on the other, then step it down the board once per tread. Each step is one notch — the calculator's notch count is how many times you move the square. Then “drop” the stringer: trim the bottom by one tread thickness so the first step finishes the same height as the rest once the treads are on.