Riser options
Estimate only. "Meets" means it meets the IRC 2021 figures we check, not that the stair is approved or safe. Local codes differ (many areas use the 2024 IRC or local amendments): check with your building department, and use a licensed builder or engineer for anything structural.
Stringer cut layout
Tools for laying out stringers
- Stair gauges clamp to a framing square so every step marks the same.
- Framing squares for marking rise and run.
- Pre-cut stair stringers at Home Depot, if one fits your rise.
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How the stair maths works
Divide the total rise by 7-3/4" and round up to get the fewest risers allowed. Divide the rise by that count for the riser height, so every step is the same. With the floor or deck as the top step, there is one fewer tread than risers, so total run is treads × tread run. The angle is the arctangent of riser ÷ tread.
Stringer length here is (risers − 1) × the diagonal of one step, √(riser² + tread²): measured along the inside corners, from the foot of the first riser to the back of the top tread. It leaves out the top riser and the bottom cut, so buy a board at least risers × step diagonal long.
Exact layout: a riser such as 7.714" doesn't land on a tape mark. Stepping off 7-11/16" fourteen times loses 3/8", and that all ends up in the last step. The exact layout mixes risers 1/16" apart so the flight adds up to the real rise.
Throat is the solid wood left under the notches: board depth minus riser × tread ÷ step diagonal. AWC's DCA 6 deck guide is the usual reference for deck stair stringers: it's commonly cited for a 5" minimum throat, cut stringers no more than 18" apart, and a 6' maximum horizontal span between supports. Read the current edition for the details.
The uneven first step
The most common stringer mistake is a bottom step that doesn't match the others. Two things cause it. First, every tread sits on top of the stringer, so the bottom riser must be cut shorter by one tread thickness. Second, using a fixed riser such as 7-3/4" and putting the leftover at the bottom: on a 108" rise that gives 13 risers of 7-3/4" and one of 7-1/4". That 1/2" difference breaks the IRC's 3/8" limit. Open "More" above to check a fixed riser.
Questions
How many steps do I need for a 9 foot (108") rise?
At least 14 risers: 108 ÷ 14 = 7.714", about 7-11/16". That's 13 treads when the floor or deck is the top step. 15 risers at 7.20" gives a gentler stair with 14 treads.
What is the maximum stair riser height?
The 2021 IRC (R311.7.5.1) sets 7-3/4" as the maximum riser, with no more than 3/8" between the tallest and shortest in a flight. Treads must be at least 10" deep (R311.7.5.2). Many places adopt local changes, so check with your building department.
Why is the bottom step of a stringer cut shorter?
Each tread adds its thickness to the step below it, except at the floor. Dropping the stringer by one tread thickness keeps the first step the same height as the rest.
How long a board do I need for stair stringers?
Risers × step diagonal, rounded up to a stock length. For 14 risers at 7-11/16" and 10" treads that's about 14' 9", so buy 16' 2x12s.
Building a deck? The deck stairs calculator adds ground slope, the landing and a tread board list.
Safety
This calculator does geometry and checks a few IRC 2021 figures. It isn't a structural design or a code approval. Stairs usually need a permit and inspection. Check your local code, and consult a licensed builder or engineer about stringer size, spans and attachment.
Figures checked: riser ≤ 7-3/4" and ≤ 3/8" variation, tread ≥ 10", nosing 3/4"–1-1/4" on treads under 11", width ≥ 36", flight rise ≤ 151", handrail at 4+ risers, guards over 30", headroom 6' 8" (IRC 2021 R311.7, R312).