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Stair Stringer Layout Calculator

Lay out a cut stair stringer from total rise: number of risers and treads, exact riser height and tread run within code (max 7¾ inch rise, min 10 inch run, 2R + T rule), stringer length and lumber size, throat depth check for a 2×12, the number of stringers by tread width, and the top-step drop for finished flooring.

Total rise (finished floor to finished floor / landing)

Desired tread run (nosing to nosing)

Code maximum riser

Stair width

Tread material thickness

Finished floor thickness at the top (if not yet installed)

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How it works

Risers = ceil(total rise ÷ max riser); riser height = total rise ÷ risers; treads = risers − 1; total run = treads × run; stringer = √(rise² + run²); throat = 11¼″ − R·T ÷ √(R² + T²); comfort 2R + T ≈ 24–25″

Stair layout starts from the one number you cannot change — the total rise — divided into the fewest equal risers that stay under the code maximum (7¾″ in the IRC), because every riser must match within 3/8″ and an odd step is a trip hazard. Treads are one fewer than risers, and the run per tread is chosen for comfort using the old carpenter's rule that twice the rise plus the run should be about 24–25 inches. The stringer is the hypotenuse; cutting the sawtooth out of a 2×12 leaves only a narrow "throat" of wood, so steep or deep stairs need a 2×14 or LVL, and the bottom of every stringer is dropped by one tread thickness so the finished first step is not taller than the rest.

Worked example

A 44″ rise needs 6 risers (44 ÷ 7.75 = 5.7, round up) of 7.333″ (7 5/16″) and 5 treads; at a 10½″ run the total run is 52½″, the angle 34.9°, the stringer 68.5″ (buy 8 ft 2×12s) and 2R + T = 25.2 — comfortable. The 2×12 throat is 5.2″, acceptable; a 42″ wide stair takes four stringers, and the bottom riser is cut at 5.83″ for 1½″ treads.

Frequently asked questions

What are the code limits?

IRC R311.7: riser ≤ 7¾″, tread ≥ 10″ (nosing to nosing), variation ≤ 3/8″, headroom ≥ 6′-8″, width ≥ 36″, nosing ¾–1¼″ where treads are under 11″. Commercial (IBC) is 7″ max rise and 11″ min tread.

How do I handle a landing or winders?

Treat each flight separately with its own rise, keep the riser height identical in both, and give the landing at least the stair width in the direction of travel (36″ min).

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