OneTaskBy Tim King · Updated
Stair Calculator
Number of Steps, Riser Height and Stringer Length
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How the Stair Calculator works
This stair calculator works out how many steps a staircase needs and how tall each one is. Enter the total rise from finished floor to finished floor, the riser height you are aiming for and the going (tread depth), and it returns the number of risers and treads, the exact riser height, the total run, the pitch of the stair and the rough stringer length.
Every riser in a flight must be the same height, because people's feet expect it; a single odd step is a trip hazard. So the calculator divides the total rise by your target riser, rounds up to a whole number of steps, and then shares the rise equally between them. There is always one fewer tread than risers, because the top floor forms the last step.
How the stair calculator works
The formula is: risers = total rise ÷ target riser, rounded up; riser height = total rise ÷ risers; treads = risers − 1; run = treads × going. The stringer length is √(rise² + run²). For example, a 2,600 mm floor-to-floor height with a 190 mm target gives 14 risers of 186 mm and 13 treads, a run of 3,250 mm at 250 mm going, a 36.6° pitch and a stringer of about 4.4 m. A second example in imperial: a 9 ft rise at 7.5 in gives 15 risers of 7.2 in and a 140 in run.
The comfort check is 2 × riser + going. Between 550 and 700 mm (about 24 to 25.5 in in US practice) a stair feels natural to climb. Building regulations also cap the riser and the pitch, and set minimum goings and headroom: in England a private stair may have a riser up to 220 mm and a pitch up to 42°, while the US IRC allows 7.75 in risers and 10 in treads. Check your local rules to see how many millimetres you are allowed before you cut stringers.
Stair riser count and height by total rise, at a 190 mm target riser and 250 mm going
| Total rise | Risers | Riser height | Treads | Total going |
|---|---|---|---|---|
| 2000 mm | 11 | 181.8 mm | 10 | 2,500 mm |
| 2050 mm | 11 | 186.4 mm | 10 | 2,500 mm |
| 2100 mm | 12 | 175.0 mm | 11 | 2,750 mm |
| 2150 mm | 12 | 179.2 mm | 11 | 2,750 mm |
| 2200 mm | 12 | 183.3 mm | 11 | 2,750 mm |
| 2250 mm | 12 | 187.5 mm | 11 | 2,750 mm |
| 2300 mm | 13 | 176.9 mm | 12 | 3,000 mm |
| 2350 mm | 13 | 180.8 mm | 12 | 3,000 mm |
| 2400 mm | 13 | 184.6 mm | 12 | 3,000 mm |
| 2450 mm | 13 | 188.5 mm | 12 | 3,000 mm |
| 2500 mm | 14 | 178.6 mm | 13 | 3,250 mm |
| 2550 mm | 14 | 182.1 mm | 13 | 3,250 mm |
| 2600 mm | 14 | 185.7 mm | 13 | 3,250 mm |
| 2650 mm | 14 | 189.3 mm | 13 | 3,250 mm |
| 2700 mm | 15 | 180.0 mm | 14 | 3,500 mm |
| 2750 mm | 15 | 183.3 mm | 14 | 3,500 mm |
| 2800 mm | 15 | 186.7 mm | 14 | 3,500 mm |
| 2850 mm | 15 | 190.0 mm | 14 | 3,500 mm |
| 2900 mm | 16 | 181.2 mm | 15 | 3,750 mm |
| 2950 mm | 16 | 184.4 mm | 15 | 3,750 mm |
| 3000 mm | 16 | 187.5 mm | 15 | 3,750 mm |
FAQ
How many steps do I need for a 9 foot ceiling?
The floor-to-floor rise is the ceiling height plus the floor thickness, usually about 9 ft 10 in (118 in) for a 9 ft ceiling. At a 7.5 in target that is 16 risers of about 7.4 in each and 15 treads. Measure your real finished floor to finished floor height.
What is a good riser height for stairs?
Around 170–190 mm (6.75–7.5 in) is comfortable for most people. Taller risers make a steeper, shorter stair; lower ones need more treads and more floor space. Keep 2 × riser + going between 550 and 700 mm so the stair matches a natural stride.
How long should a stair stringer be?
Roughly the hypotenuse of the total rise and total run, √(rise² + run²), plus a little extra for the top and bottom cuts. Buy a board at least 300 mm (12 in) longer than the figure the calculator gives, and cut the plumb and seat lines from the actual riser and going.
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