Laying out a stair stringer starts with one measurement: the total rise, the vertical distance from the lower finished surface to the upper one. From that, you divide into equal risers no taller than code allows, set a tread depth, and the run and stringer length follow. This stair stringer calculator does the math and checks the result against the residential limits in the International Residential Code (IRC Section R311.7.5): a maximum riser height of 7¾ inches and a minimum tread depth of 10 inches.
The calculator assumes a common straight stair where the top step is the upper floor or deck, so there is one fewer tread than risers. For deck stairs, pair it with the deck board calculator for the treads, and the concrete slab calculator for a landing pad at the bottom.
How to use the stair stringer calculator
- Measure the total rise from the finished surface at the bottom (floor, patio or grade landing) to the finished surface at the top. On sloping ground, measure where the bottom step will land.
- Leave the maximum riser at 7¾ in unless your local code or design calls for a lower step.
- Enter the tread depth. The IRC minimum is 10 inches; 10–11 inches is common for deck and basement stairs.
- Read the number of risers and the exact riser height, then the number of treads, the total run and the stringer length.
- Check the code line. If anything fails, add a riser (lower riser height) or increase the tread depth.
Formula
The comfort check 2R + T (twice the riser plus the tread) is a traditional rule of thumb; many designers aim for roughly 24–25 inches. It is not a code requirement.
Worked example
A deck with a total rise of 105 inches (8 ft 9 in) and 10 inch treads.
- Risers: 105 ÷ 7.75 = 13.55, rounded up to 14 risers.
- Riser height: 105 ÷ 14 = 7.5 in — under the 7¾ in maximum.
- Treads: 14 − 1 = 13. Total run: 13 × 10 = 130 in (10 ft 10 in).
- Stringer: √(105² + 130²) = √27,925 = 167.1 in (13 ft 11.1 in), so cut from 16 ft stock to allow for the top and bottom cuts.
- Angle: arctan(7.5 ÷ 10) = 36.9°. Comfort check: 2 × 7.5 + 10 = 25 in.
IRC stair rules to know
- Riser height: 7¾ in maximum (R311.7.5.1). Within a flight, the largest riser may not exceed the smallest by more than ⅜ in.
- Tread depth: 10 in minimum, measured horizontally between the leading edges of adjacent treads (R311.7.5.2), with the same ⅜ in tolerance.
- Nosing: where treads are less than 11 in deep, a nosing between ¾ and 1¼ in is generally required on solid-riser stairs.
- Headroom: at least 6 ft 8 in measured vertically from the sloped line of the tread nosings.
- Width: at least 36 in clear above the handrail height.
- Handrails are generally required on stairs with four or more risers; guards are required where open sides are more than 30 in above the floor or grade below.
Local amendments can change these numbers. Always confirm with your building department before cutting stringers.
Marking the stringer
Set a framing square on the stringer with stair gauges at the riser height on the tongue and tread depth on the blade, and step it along the board for each tread. Then drop the stringer: cut the bottom of the stringer shorter by the thickness of one tread, so the first step is not taller than the rest once treads are installed. Most deck stringers are cut from 2×12 lumber; leave enough solid wood (the "throat") below the notches, and follow your local code or DCA 6 for stringer spacing and spans.
Caveats and assumptions
- Measure the total rise to finished surfaces. Flooring or deck boards added later change the bottom and top steps.
- The stringer length is the theoretical diagonal; buy longer stock to allow for the plumb and level cuts.
- Commercial stairs follow the IBC, which has stricter limits (for example 7 in maximum risers and 11 in minimum treads).
- This calculator handles straight flights. Landings, winders and spiral stairs have additional rules.
Frequently asked questions
How many steps do I need for 8 feet?
For a 96 inch rise, 96 ÷ 7.75 = 12.4, so 13 risers of 7.38 inches and 12 treads.
What is the maximum stair riser height?
Under the IRC, 7¾ inches for residential stairs, with no more than ⅜ inch difference between the largest and smallest riser in a flight.
What is the minimum tread depth?
10 inches under the IRC, measured from nosing to nosing. 11 inches is the minimum under the IBC for commercial stairs.
How long is a stair stringer?
Use the Pythagorean theorem: the square root of total rise² plus total run². For 105 in rise and 130 in run, the stringer is about 167 in, so 16 ft stock works.
Why is there one fewer tread than risers?
On a typical stair the top "step" is the upper floor or deck itself, so the last riser leads onto it rather than onto another tread.
Sources
- ICC — 2021 International Residential Code, Section R311.7.5 Stair treads and risers — riser height, tread depth and nosing rules
- American Wood Council — DCA 6 Deck Construction Guide — deck stair stringer spans and spacing
- InterNACHI — Inspecting Stair Stringers — stringer cutting and throat depth
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.