Start with one measurement
Everything comes from the floor-to-floor height: finished floor below to finished floor above. Measure it after you know the floor finishes, because 20 mm of tile at the top changes every step. Measure in two or three places if the floor is old; use the largest.
Number of steps and the exact rise
- Divide the height by a comfortable rise, around 175 to 190 mm, and round to a whole number. That is the number of risers.
- Divide the height by that number for the exact rise.
A 2,800 mm height ÷ 180 = 15.6, so 16 risers. 2,800 ÷ 16 = 175 mm each. Every riser has to be identical; an odd step at the top or bottom is the single most common cause of falls on a stair.
Risers and treads: one fewer
The top riser lands on the upper floor, which is its own tread, so a flight with 16 risers has 15 treads. Getting this wrong by one is the classic mistake in both ordering timber and setting out.
Comfort rules
Several rules of thumb all describe the same thing, that a deep step needs a low rise:
- 2 × rise + going between about 550 and 700 mm.
- rise + going around 450 mm.
- rise × going around 45,000 mm².
Going is the horizontal depth from one nosing to the next. The tread board is usually deeper than the going, because the nosing overhangs the riser below by 20 to 25 mm.
What the regulations set
Limits are legal, not advisory, and they differ by country. Two examples:
- England and Wales, private stairs: maximum rise 220 mm, minimum going 220 mm, maximum pitch 42°, with 2,000 mm of headroom.
- The US residential code: maximum riser 7¾ in (197 mm), minimum tread 10 in (254 mm), headroom 6 ft 8 in (2,032 mm).
Treat these as a reminder to look up the code where you are building, including the rules on handrails, baluster spacing and landings, which are just as enforceable.
Total length and the stringer
total run = treads × going, and the stringer along the slope is √(height² + run²). That 16-riser flight at 280 mm going runs 15 × 280 = 4,200 mm and needs a stringer of about 5,048 mm, cut from 5.4 m stock. People routinely find the stair does not fit the room at this step, which is the moment to consider a landing and a turn.
Headroom and the opening
Headroom is measured vertically from the pitch line to the ceiling or trimmer above. Check it at the top of the flight where the opening starts, since that is where it runs out. Enlarging the floor opening is usually the fix, and it is far cheaper to find on paper than after the joists are cut.
Tools
The Stair Calculator gives the number of steps, the exact rise, the total run, the stringer length and the comfort check from the floor-to-floor height. Roof Pitch Calculator converts the stair angle between degrees and a ratio, Board Feet Calculator prices the timber, and the Tile Calculator covers finishing the treads.