Stair Calculator

1. Measure the total rise: from the top of the finished deck or floor straight down to where the bottom step will land. If the ground slopes, measure at the landing spot, not at the deck.
2. Count the risers: divide the rise by the tallest step you want (7-3/4 in is the IRC limit) and round up. Then divide the rise by that whole number for the exact riser height. Every riser in a flight has to be within 3/8 in of the others.
3. Count the treads: one fewer than the risers, because the deck or floor is the top step.
4. Total run: treads times the run per step. That is how far the stairs reach out from the top.
5. Stringer length: the square root of rise squared plus run squared, then about one step extra for the cuts at each end, rounded up to a stock length.

• Risers 7-3/4 in at most; treads at least 10 in deep, measured nosing to nosing.
• A nosing of 3/4 to 1-1/4 in when treads are less than 11 in deep.
• At least 36 in wide, and 6 ft 8 in of headroom above the nosings.
• A handrail on at least one side once there are 4 or more risers, 34 to 38 in above the nosings.
• A landing at the top and bottom, at least as wide as the stairs and 36 in deep, and a landing partway up if the rise passes 151 in.

These come from the 2021 International Residential Code. Many towns amend it, so your building department has the final word.

The AWC's DCA 6 deck guide calls for 2x12 stringers. Cut (notched) stringers can span about 6 ft measured across the ground; past that, set a 4x4 post on a footing under them, add a landing, or use solid stringers with treads on cleats or hangers, which can span up to 13 ft 3 in. With only cut stringers, use at least 3, no more than 18 in apart.

Each notch removes wood. The calculator shows how much is left under the notches, the "throat"; DCA 6 analyzed cut stringers with about 5 in left, so aim for at least 5 in. Taller risers and deeper treads leave less.

Clamp stair gauges to a framing square at the riser height on the tongue and the run on the blade, and mark each step along the 2x12. Cut the top so it sits flush under the deck surface, and cut the bottom shorter by one tread thickness so the first step comes out the same height as the rest. Cut one, test it in place, then use it as the pattern.

Stringers are long: a 30 in rise fits on 8 ft boards, but a full story of stairs needs 14 to 16 ft 2x12s, which stick far out of a short bed. The truck check flags the overhang, and the deck calculator plans the deck the stairs come down from.

• Measure the total rise from the finished deck or floor to the finished ground or pad where the bottom step lands, not to the dirt you will dig out. (International Code Council)
• Every riser in a flight must be within 3/8 in of the others. Divide the total rise evenly; do not shorten the last step. (International Code Council)
• Deck stairs need 2x12 stringers. Cut stringers can only span about 6 ft; longer runs need a post under them or solid stringers. (American Wood Council)
• Cut the first stringer, test it in place, then trace it for the rest. Shave the bottom by one tread thickness so the first step comes out the same height.

• Measuring the rise to the dirt instead of to the finished pad or landing.
• Forgetting to drop the stringer by one tread thickness, which makes the bottom step tall and the top step short.
• Making the last step a different height to make it work. Code allows only 3/8 in difference.
• Cutting the notches too deep, leaving a thin throat that cracks.
• Setting stringers on bare soil. They need a footing or a concrete pad.