Stair calculator
Steps, tread and riser from the space you have
risers, treads
Passes the basic CTE check. Width, nosing and railing are not checked here.
No straight stair meets the CTE in this space. This is the closest.
- Riser
- Tread
- 2R + T
- Run used
- Pitch
- Landings
- Tread
- Riser
- 2R + T
- Rise of a flight
- Risers per flight
- Landing
No straight stair fits this run. Try a longer run or a stair that turns.
Not enough room for a straight stair? Stairo also designs L, U, winder and spiral stairs, fits the width to the material, checks the railing and gives you the cut list and a PDF drawing.
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How it works
Every step climbs the same: the riser (R) is the floor-to-floor height divided by the number of risers. The calculator tries every number of risers that gives a riser between 12 and 23 cm and, for each one, finds the most comfortable tread (T) that fits in the run. The last riser lands on the upper floor, so there is one tread fewer than risers.
To pick the most comfortable one it uses Blondel's formula: a step is comfortable when 2R + T is close to 63 cm, the length of a normal stride. The ideal stair has a 17.5 cm riser and a 28 cm tread.
What the Spanish code asks
Spain's building code (Código Técnico de la Edificación) covers stairs in Basic Document SUA 1, section 4, with two cases:
- Dwelling (restricted use, section 4.1): a stair inside a home. Treads of at least 22 cm and risers of at most 20 cm.
- General use (section 4.2): common areas and premises. Treads of 28 cm or more; risers between 13 and 17.5 cm; 2R + T between 54 and 70 cm; each flight climbs at most 2.25 m and has at least 3 risers; landings between flights of at least 1 m.
The calculator only checks the geometry of a straight stair. Width, railing, nosing and curved flights also count: see the official CTE text or Stairo, and have a professional review the project. Other countries have their own rules.