Every stair is a trade between how tall each step is and how deep. Short risers on deep treads are easy on the knees but take a lot of floor; tall risers on shallow treads save space and feel like a ladder. The IRC sets the outer limits, and an old architect’s rule finds the comfortable middle.
The three numbers the inspector checks first
The IRC caps a riser at 7-3/4", wants every tread at least 10" deep measured nosing to nosing, and allows no more than 3/8" between the tallest and shortest riser (or the deepest and shallowest tread) in a flight. That last rule is the one that catches hand-built stairs, almost always at the bottom step.
Section numbers here are the 2021 IRC (R311.7). The 2024 edition keeps the same limits and moves the stair sections to R318. Your local adoption and amendments, and your inspector, have the last word.
How the IRC measures
Risers are measured vertically between the leading edges of neighbouring treads; treads horizontally from nosing to nosing. Where the tread is under 11" and the risers are closed, each nosing must project 3/4" to 1-1/4". Open risers are allowed, but where the stair is more than 30" above the floor below, the opening must stop a 4" sphere.
The stair must be at least 36" wide, have 6'-8" of headroom measured plumb from the nosing line, and a single flight may not rise more than 151" between landings.
Blondel’s rule: 2R + T
In 1675 the French architect François Blondel noticed that a comfortable stair matches a normal stride: twice the rise plus the tread comes to about 63–65 cm, roughly 25". Modern guidance uses a band of 24" to 26".
The classic 7-11 stair scores 25" dead on. A code-minimum 7-3/4" × 10" stair scores 25-1/2", inside the band but steep at 37.8°. A 6" × 12" garden stair scores 24" and feels lazy and long.
comfortable when 24" ≤ 2 × rise + tread ≤ 26" also quoted: rise + tread ≈ 17"–18", rise × tread ≈ 70–75
Choosing the riser count
Start with the fewest legal risers (total rise ÷ 7-3/4", rounded up), then try one more. On a 108" basement stair the minimum is 14 risers at 7-11/16"; one more brings it to 7-3/16" and adds a tread of run. If the floor plan allows the extra run, take the gentler stair.
Once the riser is fixed, pick the tread that puts 2R + T near 25". The rise-and-run chart below gives that tread for every common riser.
Stair angle
The stair angle is atan(rise ÷ run). Most people find 30° to 35° comfortable; the IRC limits allow up to about 37.8°. Steeper than that is ship-ladder territory, permitted only in narrow cases.