Wood · Stairs
Stair calculator
Risers = floor-to-floor height ÷ the riser you want, rounded up; then riser = height ÷ risers, and one tread fewer than risers. 2700 mm floor to floor with risers of at most 180 mm: 15 risers of 180 mm and 14 treads. A 260 mm going gives 2R + G = 620 mm, the comfortable step, a pitch of 34.7° and 3640 mm of total going. Pick the rule (England, the US or Spain) and the page checks every limit.
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How this is calculated
The limits checked: England (Approved Document K, private stair) rise at most 220 mm, going at least 220 mm, pitch at most 42°, 550 ≤ 2R + G ≤ 700 mm. US (IRC R311.7.5) riser at most 7¾ in, tread depth at least 10 in. Spain (CTE DB-SUA 1, 4.2.1) general use: riser 130–185 mm, going at least 280 mm, 540 ≤ 2R + G ≤ 700 mm; inside a dwelling: riser at most 200 mm, going at least 220 mm.
What this page does not do
- Straight flights only. No winders, no landings, no spiral stairs.
- No headroom or handrail check. Headroom (2 m in England and Spain, 6 ft 8 in in the US), handrails and guarding are in the same rules: check them on the drawing.
- No nosing allowance. The going is measured nosing to nosing, as the rules measure it.
Questions people ask
- How do you calculate the number of stairs?
- Divide the floor-to-floor height by the riser height you want and round up: 2700 mm ÷ 180 mm = 15 risers of exactly 180 mm. There is one tread fewer than risers, 14, because the top step is the upper floor.
- What is the 2R + G rule for stairs?
- Twice the riser plus the going should be about 600 to 640 mm, the length of a comfortable stride: 2 × 180 + 260 = 620 mm. England's Approved Document K requires 550 to 700 mm and Spain's CTE 540 to 700 mm for general use.
- What are the maximum riser and minimum tread in the US?
- Under the International Residential Code, R311.7.5: risers at most 7¾ inches and treads at least 10 inches deep, measured nosing to nosing.