🪜Stair Calculator

About this tool

Enter the total rise from finished floor to finished floor, the maximum riser height you want and the tread depth, called the going. Choose millimetres or inches and the rules to check against. The calculator shows the number of risers and their exact height, the treads, the total run, the stringer length and the angle, with a pass or fail for each rule.

How stairs are calculated

Divide the total rise by your maximum riser height and round up to get the number of risers. Every riser is then exactly the same height, which matters for safety because a single uneven step can trip people. A straight flight has one fewer tread than risers, because the top step is the upper floor. A 2,600 mm floor-to-floor rise with a 190 mm maximum needs 14 risers of 185.7 mm and 13 treads. With a 250 mm going, the run is 3,250 mm, the pitch is 36.6°, and 2 × rise + going is 621 mm. That passes all the UK Part K checks for a private stair: rise up to 220 mm, going at least 220 mm, pitch up to 42° and 2R + G between 550 and 700 mm. The stringer is roughly √(rise² + run²), about 4.16 m here before cutting. In the US, the International Residential Code allows risers up to 7¾ in and treads of at least 10 in. Honest limits: headroom, handrails, nosings, landings, winders and local amendments are not checked, so confirm the design with your building inspector. The roofing calculator uses the same rise-and-run maths, the unit converter switches between inches and millimetres and the board foot calculator prices the timber.

Frequently asked questions