- Total run
- 4,250 mm
- Stringer length
- 5,202 mm
- 2R + G
- 583.3 mm
- Concrete, one flight
- 1.12 m³
- ✓ Riser height166.7 mm · ≤ 190 mm
- ✓ Tread (going)250 mm · ≥ 250 mm
- ✓ Risers in the longest flight9 · ≤ 12
Minimum headroom 2,200 mm — enter the clear headroom to check it. Run and stringer are for the treads only; landings are not included in the run or the concrete. Local amendments can be stricter.
How the Stair Calculator works
Enter the floor-to-floor height and the tallest riser you will accept. The calculator takes the fewest risers that keep every riser at or under that height, so all risers are equal: riser = total rise / number of risers. A straight flight has one tread fewer than it has risers, because the last riser lands on the upper floor.
Total run = treads × tread depth (the going). The stringer length is the diagonal of the total rise and total run, and the pitch is the angle whose tangent is riser / tread. 2R + G, two risers plus one going, is the comfort rule used in the UK and elsewhere: it should land between 550 and 700 mm.
The checks compare your riser and tread with the selected code: IRC R311.7 for US homes (riser at most 7¾ in, tread at least 10 in), IBC 1011 for other US buildings (riser 4 to 7 in, tread at least 11 in), UK Approved Document K for a private stair (rise 150 to 220 mm, going 220 to 300 mm, 2R + G 550 to 700 mm, pitch at most 42°) and India NBC 2016 for residential buildings (riser at most 190 mm, tread at least 250 mm, at most 12 risers in a flight — NBC 2016 Part 4, internal stairs). It also checks the rise of one flight: 151 in under IRC R311.7.3 and 12 ft under IBC 1011.8. Local amendments can be stricter.
For a reinforced concrete stair, give the waist-slab thickness: the concrete is the stair width × (the triangular steps + the inclined waist slab), for one straight flight without landings.
- Number of risers
- n = ⌈H / R_max⌉
- Riser and run
- R = H / n, Run = (n − 1) × G
- Stringer and pitch
- L = √(Run² + H²), θ = atan(R / G)
- Concrete in one flight
- V = b × (n − 1) × (R × G / 2 + t × √(R² + G²))
Follows: IRC R311.7.5 · IBC 1011.5 · UK Approved Document K, Section 1 · NBC 2016 (India)
Worked example: a concrete stair for a 3 m floor-to-floor height
Given
- Floor-to-floor 3000 mm, maximum riser 175 mm, tread 250 mm
- Width 1000 mm, waist slab 150 mm
Steps
- Risers: 3000 / 175 = 17.1, rounded up to 18, so each riser is 3000 / 18 = 166.7 mm; 17 treads.
- Run: 17 × 250 = 4,250 mm. Stringer: √(4250² + 3000²) = 5,202 mm. Pitch: atan(166.7 / 250) = 33.7°. 2R + G = 583 mm.
- Steps: 17 × 0.1667 × 0.250 / 2 × 1.0 = 0.354 m³. Waist slab: 17 × √(0.1667² + 0.250²) × 0.150 × 1.0 = 0.766 m³.
- Checks against NBC 2016 residential: riser 166.7 ≤ 190 mm and tread 250 ≥ 250 mm pass, but 18 risers in one flight exceed the 12 that NBC 2016 Part 4 allows for internal stairs, so the stair needs a mid landing: two flights of 9.
Result: 18 risers of 166.7 mm and 17 treads of 250 mm: run 4,250 mm, stringer 5,202 mm, pitch 33.7°, 1.120 m³ of concrete.
Enter the same values in the calculator above to check each step.
Frequently asked questions
How many stairs do I need for 9 feet?
For 9 ft (108 in) floor to floor: 108 / 7.75 = 13.9, so 14 risers of 7.71 in, which meets the IRC 7¾ in maximum. With 10 in treads that is 13 treads and a total run of 130 in (10 ft 10 in).
What is the maximum riser height?
IRC (US homes): 7¾ in. IBC (other US buildings): 7 in, with a 4 in minimum. UK private stair under Approved Document K: 220 mm. India NBC 2016, residential buildings: 190 mm.
What is the 2R + G rule?
Two risers plus one going approximates a comfortable stride on a stair. Approved Document K requires 550 to 700 mm. For example, 170 mm risers with 270 mm goings give 2 × 170 + 270 = 610 mm.
How do I calculate stringer length?
Stringer = √(run² + rise²). For 14 risers of 7.71 in and 13 treads of 10 in: √(130² + 108²) = 169.0 in, about 14 ft 1 in, before trimming at the top and bottom.
How much concrete is in a staircase?
Add the triangular steps and the inclined waist slab. A 1 m wide flight of 18 risers of 166.7 mm with 250 mm treads and a 150 mm waist takes 0.354 + 0.766 = 1.120 m³, plus any landings.