How it works
The height is split into equal risers. With an available run, the calculator tries every riser count and picks the one whose stride 2R + G (two risers plus one going) is closest to your target while staying within your riser and going limits. With a fixed count or riser height, the going follows from the step formula: G = target − 2R.
There is always one tread fewer than risers — the last step is the upper floor. Total run = treads × going; the tread board is the going plus the nosing.
Pitch = arctan(riser / going). On a cut stringer, the plumb cut is the pitch off square and the level cut is 90° minus the pitch; on a framing square that is the riser on the tongue and the going on the blade.
The stringer length is the shortest board that holds the whole sawtooth outline: from the bottom front corner on the floor to the last notch under the upper floor. The throat is what remains of the board below the notches.
Headroom is measured vertically from the line through the tread nosings to the underside of the ceiling at the edge of the opening — the same way the US IRC and the UK Approved Document K measure it.
What the calculator assumes
- This is a geometry and quantities tool, not structural design. Stringer size, span, fixings, handrails and guards must follow your local building rules or an engineer.
- Limits differ by country and building type, so they are inputs. Common references: the step formula 2R + G of about 600–650 mm (630 mm in Czech and Slovak practice), the US IRC (max riser 7 3/4 in, min tread 10 in, headroom 6 ft 8 in), the UK Approved Document K for private stairs (2R + G 550–700 mm, rise up to 220 mm, going at least 220 mm, pitch up to 42°, headroom 2.0 m). Check the rules that apply to you.
- Straight flight with two cut stringers; the treads sit on the notches and run the full stair width. The tread thickness is taken off every notch, so all finished risers come out equal.
Questions
- What is the 2R + G rule?
- Two risers plus one going should match an average stride of about 600–650 mm (24–25.5 in). 175 mm risers with a 280 mm going give 630 mm, a comfortable house stair.
- Why is there one tread fewer than risers?
- You step up from the lower floor onto the first tread and from the last tread onto the upper floor. A stair with 16 risers has 15 treads.
- How do I check headroom?
- Turn on the headroom check and enter the length of the ceiling opening and the thickness of the floor above. The calculator shows the clear height and the shortest opening that gives the headroom you need.
Planning aid only. It does not size structural members or check loads — follow the manufacturer's instructions and your local building rules.