Stair calculator — riser height and tread depth

Start from the floor-to-floor height, including the floor structure. Divide it by your desired riser height and round to the nearest whole number — that is the number of risers. The actual riser height is then the floor-to-floor height divided by that count: 2,600 mm with a desired riser of 175 mm gives 15 risers at 173.3 mm each. The number of treads is always one fewer than the number of risers, because the top “step” is the upper floor itself.

The tread depth then follows from the comfort rule 2h + d ≈ 630 mm (range 600–660 mm), which the calculator uses. With a 173 mm riser, the comfortable tread depth is about 284 mm, and a stair with 14 treads then needs just under 4 meters of total run. If your run is limited, enter it and the tool calculates the actual tread depth instead — and flags the stair if it lands outside the comfort range.

Beyond risers and treads, a few dimensions decide whether the stair is safe and comfortable in practice. Common European practice is 150–180 mm risers and at least 250 mm treads in homes, at least 2.0–2.1 m of headroom along the whole flight, a handrail at about 0.9 m above the nosings — preferably on both sides — and guard openings no larger than 100 mm so small children can’t slip through. Exact requirements vary by country and building type, so always check the building code that applies to your project.

A straight stair with comfortable dimensions between two normal floors often needs 3.5–4.5 meters of run, plus space to step on and off at each end. If that doesn’t fit, split the stair with a landing: the landing divides the fall height and makes the stair safer. Winders and spiral stairs save floor area but give an uneven tread depth in the walking line, so keep them for secondary spaces where you can.