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Method and sources checked on by the Brikalo editorial team · Our method
A stair is worked out from the total rise: the rise of each step is that height divided by the number of risers, and the going is chosen so that twice the rise plus the going falls between 550 and 700 mm, as Approved Document K sets out.
The three measurements of a stair
Everything starts from the total rise, measured from finished floor to finished floor. Divided by the number of risers, it gives the rise of each step, which must be the same all the way up. The going is the depth of a tread, measured from nosing to nosing. The total going is the length the stair takes up on plan.
A stair with 15 risers has only 14 goings: the top riser lands on the upper floor, which is not a tread. For a 2,700 mm total rise and 4,000 mm of space, 15 risers give 2,700 ÷ 15 = 180 mm and a going of 4,000 ÷ 14 = 285.7 mm.
The 2R + G rule: between 550 and 700 mm
The figures on this page come from Approved Document K, 2013 edition, which applies in England; Wales and Scotland publish their own guidance, so check it there. The document gives the normal relationship between rise and going: twice the rise plus the going should be between 550 mm and 700 mm. It reflects the length of a stride: the higher the step, the shorter the tread. The calculator looks for the number of risers whose 2R + G is closest to 625 mm, the middle of that range.
In the example, 2 × 180 + 285.7 = 645.7 mm. With 16 risers, each would be 168.8 mm and the going 4,000 ÷ 15 = 266.7 mm, giving 604.2 mm: also within the range, and a gentler stair. The table below gives, for each total rise, the fewest risers that a private stair can have.
Fewest risers for each floor-to-floor height
Private stair under Approved Document K: rise 150 to 220 mm, going 220 to 300 mm, 2R + G between 550 and 700 mm and a pitch of 42° at most. Each row gives the fewest risers that comply, with the going set for 2R + G = 625 mm, the middle of the range. The stairwell opening is worked out for 2 m headroom and a 200 mm floor. Example: a 2,700 mm rise is climbed in 14 risers of 193 mm with a 239 mm going.
| Total rise | Risers | Rise | Going | Total going | Opening |
|---|---|---|---|---|---|
| 2,400 mm | 12 | 200 mm | 225 mm | 2,475 mm | 2,475 mm |
| 2,500 mm | 13 | 192 mm | 240 mm | 2,885 mm | 2,750 mm |
| 2,600 mm | 13 | 200 mm | 225 mm | 2,700 mm | 2,475 mm |
| 2,700 mm | 14 | 193 mm | 239 mm | 3,111 mm | 2,730 mm |
| 2,800 mm | 14 | 200 mm | 225 mm | 2,925 mm | 2,475 mm |
| 2,900 mm | 15 | 193 mm | 238 mm | 3,337 mm | 2,712 mm |
| 3,000 mm | 15 | 200 mm | 225 mm | 3,150 mm | 2,475 mm |
Private, utility and general access stairs
| Stair category | Rise | Going | Other limits |
|---|---|---|---|
| Private stair (within one dwelling) | 150 to 220 mm | 220 to 300 mm | Pitch 42° at most |
| Utility stair | 150 to 190 mm | 250 to 400 mm | 16 risers per flight at most |
| General access stair | 150 to 170 mm | 250 to 400 mm | 12 risers per flight, exceptionally 16 |
The 42° pitch matters in a house: a 220 mm rise with a 220 mm going would be 45°, too steep, even though both values sit within the table. With a 220 mm rise the going must be at least 240 mm to stay under 42°, giving 42.5°: still too steep, so the rise has to come down. For a 2,600 mm total rise and a 250 mm going, 14 risers of 185.7 mm give 2R + G = 621.4 mm and a pitch of 36.6°.
Utility and general access stairs apply to buildings other than dwellings and to common areas in blocks of flats. A general access stair rising 2,700 mm with a 280 mm going needs 16 risers of 168.8 mm, more than 12 in one flight: it has to be split by a landing.
Total going, stairwell opening and headroom
The total going is the number of goings times the going, plus any landing. The opening in the upper floor must be long enough to keep the headroom, measured above the pitch line: at least 2 m. In a loft conversion where there is not enough space, Approved Document K accepts 1.9 m at the centre of the stair, falling to 1.8 m at the side.
The pitch line climbs by one rise at each going. Under the upper floor you need the headroom plus the floor depth, so the opening is (headroom + floor depth) × going ÷ rise. For 15 risers of 180 mm, a 285.7 mm going, 2 m headroom and a 200 mm floor: (2,000 + 200) × 285.7 ÷ 180 = 3,492 mm.
Quarter-turn stairs and tapered treads
On tapered treads the going changes across the width. Approved Document K measures it at the centre of the tread when the stair is less than 1 m wide, and 270 mm in from each side when it is 1 m or wider; consecutive tapered treads keep the same going, and that going must not be less than the going of the straight treads. Each tapered tread keeps at least 50 mm at the narrow end.
For a quarter turn, the calculator adds the two straight runs and the quarter circle followed by that line. In a 3,000 × 2,000 mm stairwell with an 800 mm wide stair, the line runs 400 mm from the inner edge: (3,000 − 800) + (2,000 − 800) + 2π × 400 ÷ 4 = 4,028 mm. For a 1,000 mm wide stair, the line moves to 270 mm from the narrow side and the developed length drops to 3,424 mm. The setting-out of the tapered treads is drawn afterwards and is not calculated here.
Common mistakes
- Measuring the rise from the bare slab and forgetting the floor finishes: 20 mm of tiles at the top changes the last step.
- Counting as many goings as risers: the total going comes out one tread too long.
- Forgetting the floor depth in the opening: in the example, 200 mm of floor adds 317 mm of opening.
- Checking rise and going but not the pitch: 220 mm and 220 mm are both allowed, together they are not.
To carry on, a ramp gradient sometimes replaces a few steps, the cutting angle helps with strings and skirting, the concrete volume of a cast stair comes from the same dimensions, and the room area helps you place the opening.
Method and sources
References used:
This tool is checked by automated tests against known cases. Spotted a mistake? Report it on the contact page.
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