October 23, 2026 · Building & Materials · 6 min read

Stair Design Basics: Rise, Run and the Rules That Keep Stairs Safe

People can't tell you why a staircase feels good, but their legs can. The difference between a stair you climb without thinking and one that makes everyone stumble is a handful of numbers — and those numbers are all connected.

The comfort rule: 2R + T

Stair designers aim for the relationship:

A riser of 18 cm with a 26 cm tread gives 62 cm — comfortable. Steepen it to a 20 cm riser and the tread should grow to about 24 cm to stay in range. The rule encodes a real physiological fact: a step's effort is dominated by lift (riser), but your foot needs landing room (tread).

What the codes allow

Regulations vary by country, but typical limits are:

• Riser height: 12–19 cm (4.7–7.5 in), uniform within 1 cm across the flight
• Tread depth: at least 25–28 cm (10–11 in)
• Headroom: at least 2 m measured vertically from the tread nosing
• Handrail height: roughly 85–100 cm above the nosing line

The uniformity requirement is the one most DIY stairs fail: a single 5 mm inconsistency in riser height is enough to trip people, because legs learn a rhythm within two steps.

From floor to floor: the worked design

Suppose your floor-to-floor height is 275 cm. Divide by a target riser of 17.5 cm → 15.7 steps, so round to 16 risers of 17.19 cm each (always divide exactly — equal risers are non-negotiable). With 16 risers there are 15 treads; at 27 cm each, the total run is 405 cm. Check the horizontal space you have before committing. The stringer runs diagonally: length = √(275² + 405²) ≈ 490 cm, at an angle of atan(275 ÷ 405) ≈ 34° — comfortably inside the typical 25–40° range.

Common design mistakes

• Measuring finished floor to finished floor with only rough floor in place — the missing floor thickness ruins every riser.
• Forgetting tread nosing overhang and nosing-to-nosing headroom under the upper floor.
• Ignoring landing thickness at the top, which eats into available headroom.

Building as well as measuring? How Much Concrete Do You Need? covers ordering the right amount for stair foundations and landings.