Design code-compliant stairs from total rise height. Calculates number of steps, riser height, tread depth, stringer length, and staircase angle — with UK and US building code checks.
X is what % of Y: (X ÷ Y) × 100
X% of Y: (X ÷ 100) × Y
% change from A to B: ((B − A) ÷ A) × 100
Original value before X% added: Final ÷ (1 + X/100)
Original value before X% removed: Final ÷ (1 − X/100)
Worked examples: 45 is what % of 180? (45 ÷ 180) × 100 = 25%. 15% of 240? 0.15 × 240 = 36. Price rose from 80 to 96 — % increase: ((96−80) ÷ 80) × 100 = 20%. Price was 120 after 20% tax added — original pre-tax: 120 ÷ 1.20 = 100.
The ideal riser height is 175mm (7 inches) for residential stairs. UK Building Regulations allow 150–220mm; US codes typically allow up to 7¾ inches (197mm).
The comfort rule states: 2 × Riser + Tread = 600–650mm (or 24–26 inches). A good stair design balances riser height and tread depth so each step feels natural to climb.
Divide the total floor-to-floor height (rise) by your desired riser height, then round to the nearest whole number. Recalculate the actual riser: actual riser = total rise ÷ number of steps.
UK regulations require a minimum going (tread depth) of 220mm for private stairs. US codes typically require a minimum of 10 inches (254mm).
The stringer is the diagonal board supporting the stairs. Length = √(total rise² + total run²), where total run = number of treads × tread depth.