Stair Comfort Rule (2R + T)
Also known as stair rule of thumb · comfortable stair · 2R plus T
Worked example: 7 in riser, 11 in tread → 25 in (0.635 m) — press Try an example to run it live, then adjust anything.
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Stair Comfort Rule (2R + T) explained
Twice the riser plus one tread should land near 25 inches. The rule dates to François Blondel, who in the 1670s reasoned that a person climbing lifts a foot twice as far as they advance it, so the two dimensions ought to trade off at a fixed rate. Metric practice uses 630 mm, close enough to 25 in that the two conventions rarely disagree in the field. A 7 in riser wants an 11 in tread; a 175 mm riser wants a 280 mm going.
Treat it as a comfort check rather than a law. Codes govern the riser and the tread separately, with minimum tread depths around 10 in or 250 mm, and a stair can satisfy every code number while still feeling wrong underfoot. If your 2R + T lands under about 24 in the stair feels cramped and mincing; over about 26 in and you find yourself stretching for each step. Both are the sort of thing you notice on the third trip carrying drywall.
Stair Comfort Rule (2R + T) formula
- = Stride constant (m)
- = Riser height (m)
- = Tread depth (m)
Missing one of these? Work it out first, then come back
- Riser height — Stair Riser Count from Total Rise, Area of a Triangle
- Tread depth — Stair Total Run from Risers and Tread Depth, Hydrostatic Pressure (P = ρgh)