Calculate stairs: risers, rise, tread depth and slope

A staircase must climb a fixed vertical height while fitting into limited horizontal space. Changing the number of risers therefore changes the rise, tread relationship, run and slope.

Measurements to start with

Measure the total finished-floor-to-finished-floor rise and the available horizontal run. For a 280 cm rise with 16 risers:

280 ÷ 16 = 17.5 cm per riser

With 17 risers, the value becomes about 16.47 cm. The stair calculator makes it easy to compare several configurations.

Rise and tread depth

The rise is the vertical difference between consecutive levels. Tread depth is the useful horizontal distance between successive nosings. These dimensions must be considered together rather than chosen independently.

The Blondel relationship

A classic consistency check is 2h + g, where h is riser height and g is tread depth. A commonly used comfort zone is around 60–64 cm, often summarized by a target close to 63 cm.

With h = 17.5 cm and g = 28 cm:

2 × 17.5 + 28 = 63 cm

This is a geometric rule of thumb, not a universal building-code approval.

Check the available run

Fifteen treads of 28 cm require 15 × 28 = 420 cm, or 4.20 m. If only 3.50 m is available, the apparently comfortable dimensions do not fit and the configuration must change.

The slope calculator is useful when you want to inspect rise, horizontal distance, percentage and angle separately.

Compare alternatives

Risers	Rise for 280 cm
15	18.67 cm
16	17.50 cm
17	16.47 cm
18	15.56 cm

More risers reduce individual rise but generally require more horizontal development at a comparable tread depth. The best option is therefore a compromise between geometry and available space.

Example

For a total rise of 270 cm, 16 risers and a 28 cm tread:

rise = 270 ÷ 16 = 16.875 cm

Blondel = 2 × 16.875 + 28 = 61.75 cm

With 15 treads, the horizontal run is 15 × 28 = 420 cm.

Common mistakes

Do not round too early, measure between unfinished levels, treat Blondel as the only rule, or forget the available run. Finished floor thickness can make the first or last riser different if it was omitted from the initial measurement.

Straight-flight formulas also do not fully describe winders, quarter-turn stairs, landings or constrained openings. Those layouts require a complete geometric and regulatory check.

Use the stair calculator to explore alternatives, then verify the actual site dimensions and applicable requirements. For the broader takeoff method, see how to plan material quantities.