Staircase Calculator — Risers & Treads

Stairs are laid out backwards from one fixed number: the height from one finished floor to the next. Everything else bends to fit it, because every riser in a flight has to be the same height. This Staircase Calculator divides that height into equal risers close to the size you want, then works out the treads, the horizontal run and the angle.

It also checks the result. Pick US residential (IRC), US commercial (IBC) or one of two UK Approved Document K categories, and each limit shows a green tick or a red cross with the permitted range beside it. A side view of the flight is drawn to scale.

Worked Example: A 2.7 m Storey Under Part K

The page opens in millimetres with a total rise of 2700, a target riser of 175 and a going of 250, checked as a UK private stair. The result:

• Risers: 2700 ÷ 175 = 15.4, rounded to 15 risers of exactly 180 mm
• Treads: 14, since the upper floor serves as the last step
• Total run: 14 × 250 = 3,500 mm, at a pitch of 35.8°
• 2 × riser + going: 610 mm, inside the 550–700 mm Part K band and the comfort band

Load Sample moves on to a 9-foot US house checked against the IRC, which gives 14 risers. The − and +buttons beside the riser count override the rounding: try 16 risers here and each one drops to 168.8 mm while the run grows to 3,750 mm. Editing any dimension returns to the automatic count.

Method: Rounding the Riser Count

Risers = round(total rise ÷ target), and the actual riser is the total rise divided by that whole number. Treads = risers − 1. Pitch = atan(riser ÷ going), and the pitch-line length is the hypotenuse of the total rise and run, a starting figure for a stringer before you add the top and bottom cuts.

In imperial mode heights are rounded to the nearest 1/16 inch for the tape measure, so 108 ÷ 14 = 7.714 in prints as 7 11/16″ with the three-decimal value next to it. Switching units converts what you have typed rather than reinterpreting it, and moves UK rules to the IRC or back. Out-of-range inputs are stopped with a message such as “Target riser height must be between 100 and 250 mm.”

When Not to Use It Without Building Control

• Headroom, handrail height, nosing projection and balustrade gaps are not checked
• Winders, spiral stairs and landings mid-flight need their own geometry
• Local amendments to the IRC, IBC or Part K can tighten any of these limits

Use it to size a flight and see whether it fits the space. Then confirm the design with your local building control office or permit reviewer before cutting timber.