A stair is two dimensions that have to agree. The total rise is fixed by the building — floor to floor — and dividing it by a comfortable riser gives the number of steps, always rounded up to a whole number, which means the actual riser is slightly smaller than the target. The going is then whatever horizontal room you have divided by that same number, and the two together are checked against the old rule that twice the riser plus the going should land between 600 and 650 millimetres, which is roughly the length of a comfortable stride. Get it wrong in either direction and people notice on the first step and never stop noticing.
Rise, run and the walk line
The going is what is left after the risers have taken their share.
Finished floor to finished floor, not to the underside of the joists.
From the face of the top landing to where the flight lands at the bottom.
Clear width between the walls or strings.
Result
The rounding changes the riser
Two and a half metres of rise at a 170 millimetre target is 14.7, which is not a number of steps. It becomes fifteen, and fifteen steps over the same rise means the actual riser is not 170 millimetres but 167. That is a three-millimetre difference and it is the whole point: the riser is a consequence of the step count, not an input to it.
It matters because the riser is the dimension that decides whether a stair is comfortable, and because every riser has to be identical. A flight with one riser twenty millimetres different from the others is a trip hazard, because the foot is calibrated after two steps and then lies on the third. This is why every building regulation on stairs is about consistency as much as about dimension.
The walk line, and why it is the real test
Twice the riser plus the going is an old rule and a good one. It approximates the distance a person covers in one comfortable stride on a slope, and it has the useful property of catching both failure modes: a stair that is too steep fails because the going is too short for the riser, and one that is too shallow fails because the going is too long and the stride becomes a stretch.
The range is 600 to 650 millimetres for a domestic stair. Below 600 the stair feels mean and people descend it sideways; above 650 it feels like a ramp with steps in it and people take two at a time, which is where falls happen. The rule is not a regulation everywhere, but it is a very good predictor of whether a stair gets complained about.
A stair needs at least two metres of clear headroom measured vertically above the pitch line, and on an existing stair that is the constraint that usually bites before the going does.
If the headroom is short, the answer is usually to move the top of the flight or to alter the opening, not to steepen the stair.
What the regulations ask for
Domestic stairs in most places want a maximum riser of around 220 millimetres, a minimum going of around 220, a maximum pitch of about 42 degrees, and consistent risers and goings throughout the flight. A stair with a tapered tread, a winder or a curved section has additional rules about the going at the narrow end, because that is where people actually put their feet.
The other rule that matters is about consecutive flights: a flight of more than about sixteen to eighteen steps wants a landing, both for rest and to stop a fall becoming a long one. And a handrail is required on at least one side, on both sides if the stair is wide.
| Stair | Riser | Going | Pitch |
|---|---|---|---|
| Domestic, standard | 180 mm | 260 mm | 35° |
| Domestic, tight plan | 200 mm | 220 mm | 42° |
| Domestic, generous | 170 mm | 280 mm | 31° |
| External, garden | 150 mm | 300 mm | 27° |
| Basement, steep | 210 mm | 210 mm | 45° |
| Public, accessible | 160 mm | 320 mm | 27° |
A worked example
- Steps2.7 m at a 180 mm target is 15 exactly, so 15 steps.
- Riser2.7 over 15 is 180 mm — the target, exactly.
- Going3.9 m of room over 15 treads is 260 mm each.
- Walk lineTwice 180 plus 260 is 620 mm, inside the 600 to 650 rule.
- PitchThat is 34.7 degrees, over 4.74 m of sloping length.
Fifteen steps at 180 by 260 is as standard as a domestic stair gets, and it is what a 2.7 metre rise and 3.9 metres of room produce almost automatically. Change either measurement and the walk line is the figure that tells you whether the result is acceptable.
Setting out and building
Measure the rise to the finished floor levels on both floors, not to the structural surfaces, because the floor finishes on the two floors are rarely the same thickness and a fifteen-millimetre error over fifteen steps is a visible discrepancy at the top.
Use a stair gauge — a pair of clamped battens set to the riser and going — rather than measuring each step. It makes every step identical by construction rather than by care, and it is the single thing that separates a stair that feels right from one that does not.
Then the surrounding work: the mortar figure if the stair is being built in masonry, the stud figure for the wall beside it, and the trim figure for the skirting that follows the rake, which is one of the more awkward cuts in a house.
Do
- Measure the rise to finished floor level on both floors.
- Round the step count up and accept the smaller riser.
- Use a stair gauge so every step is identical by construction.
- Check the headroom before accepting the geometry.
- Aim for a walk line between 600 and 650 mm.
Avoid
- Mixing riser heights, even by a few millimetres.
- Measuring to structural surfaces rather than finished floors.
- Steepening a stair to save half a metre of plan.
- Forgetting the landing on a long flight.
- Setting out without a gauge and hoping they come out the same.
Where the numbers stop being true
This sets out a straight flight of identical steps, which is the easy case. A winder, a kite tread, a curved flight or a staircase with a turn all have goings that vary across the width of the stair, and the going is measured on the walk line — usually the centre, or 270 millimetres from the inner handrail on a curved stair — not at the narrow end.
Nothing here covers the structure: the thickness of the treads, the depth of the strings, the trimmer at the top of the flight, or the fact that the total rise includes the thickness of the tread material itself. All of those change the setting-out in the second decimal place, and on a tight flight that is enough.
And this is geometry, not compliance. A stair in a real building has to meet the regulations that apply where it is, and those cover handrails, guarding, headroom, landings and the maximum number of steps in a flight as well as the dimensions here.
How the figures are produced
Steps are the total rise divided by the target riser, rounded up, so the actual riser is slightly less than the target. The going is the horizontal room divided by the number of treads. The walk line is twice the riser plus the going, and the pitch is the arctangent of riser over going. The sloping length is the square root of the rise squared plus the horizontal run squared.
Questions people ask
How many steps for a 2.7 m rise?
Fifteen at a 180 mm riser. The actual riser comes out at exactly 180 mm.
What is the standard riser?
180 mm domestically, with a going around 260 mm. The walk line at that combination is 620 mm.
Can I make the stair steeper to save space?
Only a little. Above about 42 degrees it stops being a stair and starts being a ladder, and most regulations stop there too.
Why do all the risers have to be the same?
Because the foot is calibrated after two steps. One different riser is a trip hazard for the life of the stair.
How much headroom do I need?
At least two metres, measured vertically above the pitch line. It is usually the constraint that bites first on an existing stair.
When do I need a landing?
After about sixteen to eighteen steps, and at the top and bottom of every flight in any case.