Why 2·rise + going ≈ 61–63 cm, and how to derive the number of risers and the going depth from the floor height. Blondel's pace-measure rule is still the basis for a comfortable stair.
Introduction
A staircase feels comfortable when the ratio between riser height and going (tread depth) matches the human stride length. François Blondel formalised this in 1675 in the rule we still use.
The rule
You always start with the floor-to-floor height (vertical distance between two finished floors). The number of risers is found by dividing the floor-to-floor height by a desired riser height and rounding up — that way the individual riser is never higher than desired.
Example: a storey height of 280 cm and a desired riser of 17.5 cm gives 280 / 17.5 = 16 risers. Actual riser = 280 / 16 = 17.5 cm. With a stride measure of 63 cm, the going is 63 − 2·17.5 = 28 cm. Number of treads = 16 − 1 = 15.
Number of steps
- 01The number of risers is the number of steps up from floor to floor.
- 02The number of treads (goings) is one fewer than the number of risers in a straight flight — the top step is the floor deck itself.
BR18's binding stair dimensions
Blondel's formula gives comfort; Building Regulations give minimum dimensions. BR18 § 57 sets the binding requirements for stairs in common stairwells — free width, free height over the steps, maximum rise and minimum run. The requirements depend on the stair type and whether the stairs are in a dwelling or a common stairwell.
| Target | Krav (BR18 § 57) | Note |
|---|---|---|
| Clear width | ≥ 1,0 m | Stairs in common access routes |
| Free height above step edge | ≥ 2,10 m | Measured vertically in the stairwell line |
| Step (rise) | ≤ 0,18 m | Largest vertical step height |
| Ground, level / quarter / half turns | ≥ 0,28 m | Applies to common access routes; internal residential stairs have no binding minimum in BR18 |
| Ground, spiral/helical staircase | ≥ 0,20 m | Measured in centreline |
Source: Building Regulations BR18 § 57 (bygningsreglementet.dk). Note that these dimensions apply to stairs in common access routes. A staircase internally in one dwelling is not subject to the same width and run requirements, but Blondel comfort and secure handrail still apply.
Protection and handrails
A staircase must be secured against falls. BR18 §§ 58–60 set minimum guarding heights, and a staircase must have a handrail that is easy to grip and hold. The guarding height is measured vertically from the top edge of the step or landing.
| Location | Minimum protection height | Reference |
|---|---|---|
| Along stairs and ramps | 0,80 m | BR18 § 58 |
| Over breaks | 0,90 m | BR18 § 58 |
| General protection (e.g. at floor level) | 1,0 m | BR18 § 58 |
| Stairs with openings > 0.30 m, elevated walkways | 1,20 m | BR18 § 58 |
Stair types and their geometry
- 01Straight staircase: one straight flight from floor to floor — simplest to install and calculate, but requires most space lengthwise.
- 02Flight with landing: two runs with an intermediate landing — used for large storey heights and provides rest en route.
- 03Quarter-turn stair: turns 90° via quarter-turn step or landing.
- 04Half-turn staircase (U-staircase): turns 180° often around a landing — space-saving in height.
- 05Spiral/helical staircase: revolves around a spindle; the tread is measured on the walking line and must be at least 0.20 m there.
On turned steps (quarter, half-turn and winding), the tread changes in width — it is narrower at the inner stringer and wider outward. Therefore, the tread requirement is measured along the line of travel, typically some distance in from the inner stringer.
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