Stair Calculator

There is always one fewer tread than there are risers

A flight climbing to a landing has a riser at the top but no tread — the landing is the tread. So treads are always steps minus one, and a staircase ordered by counting steps arrives with one board too many. The other rule worth knowing is Blondel's: twice the rise plus the going should land around 600 to 650 mm, which is what makes a flight feel right underfoot.

How it works

• Divides the total rise by your preferred riser height and rounds to whole steps.
• Recomputes the actual riser height, since the total must divide exactly.
• Checks the comfort rule, twice the rise plus the going.

steps = round(total rise ÷ preferred rise)
actual rise = total rise ÷ steps
treads = steps − 1
total run = treads × going
comfort = 2 × actual rise + going, ideally about 600-650 mm

Worked example

A 2,800 mm total rise with a preferred riser of 180 mm.

1. 2800 ÷ 180 = 15.6, which rounds to 16 steps
2. the actual rise becomes 2800 ÷ 16 = 175 mm, not the 180 you asked for
3. treads = 16 − 1 = 15, because the landing is the sixteenth
4. with a 280 mm going, 2 × 175 + 280 = 630 mm
5. 630 sits inside the comfortable band, so the flight will walk well

Sixteen risers, fifteen treads. Every riser in a flight must be identical — an uneven one is the classic trip hazard, and it is why the height gets recomputed rather than left at your preference.

Reading the result

• Every riser in a flight has to be the same height, which is why the calculator divides the total rather than taking your preferred height literally. A single riser differing by even a few millimetres is what people catch their foot on, usually the last one.
• Building regulations set limits on maximum rise, minimum going and minimum headroom, and they differ by country and by whether the stair is private, shared or public. Treat the comfort rule as a starting point and the local regulation as the constraint.
• The total rise is floor finish to floor finish, not the structural opening. Forgetting the thickness of the upstairs floor covering is a common way to end up with one riser slightly wrong at the top.
• The run this produces is the horizontal space the flight needs. It is often the figure that decides whether a straight flight fits at all, or whether the design has to turn with a winder or a half-landing.

Common questions

How many stairs do I need for a 3 m rise?

17 steps if you aim for 18 cm risers: 300 ÷ 18 = 16.7, which rounds to 17, so each riser comes out at 17.65 cm. That gives 16 treads, and with a 28 cm going the flight needs 4.48 m of horizontal run. The comfort check is 2 × 17.65 + 28 = 63.3 cm, inside the 61–65 cm band the calculator treats as comfortable.

What is the maximum step height allowed for stairs?

It depends on the country and on the type of building, so check the rules that apply where you are building. Two widely used examples: in England, Approved Document K allows a private stair up to 220 mm rise with at least 220 mm going and a pitch of no more than 42°; the US International Residential Code sets a maximum riser of 7¾ in (about 196 mm) and a minimum tread of 10 in (254 mm). Local rules for shared or public stairs are usually stricter.

How can I make a staircase take up less floor space?

Use fewer, taller steps and a shorter going, while keeping the comfort check in range. For a 2.8 m rise, 18 cm risers and 28 cm goings give 16 steps and 4.2 m of run; allowing 19 cm risers with 25 cm goings gives 15 steps of 18.7 cm and only 3.5 m of run, and 2 × 18.7 + 25 still lands at 62.3 cm. Check the steeper figures against your local regulations, and remember the run does not include the landing space at the top and bottom.