Stair calculator

Start from the total rise

The one measurement that cannot be adjusted is the total rise: finished floor to finished floor. Everything else follows from dividing it into equal steps.

Divide the total rise by your maximum acceptable riser height and round up to get the number of risers. Then divide the total rise by that count to get the actual riser height, which will be slightly less than your maximum. Rounding up is what keeps every riser equal and under the limit.

Note that a flight has one more riser than it has treads, because the top riser lands on the upper floor and there is no tread there.

Every riser must be identical

This is the rule that matters most for safety. People climb stairs by feel after the first two steps, and a single riser differing by even 6-8 mm is a genuine trip hazard. Most building codes limit the variation across a flight to around 5 mm.

The usual way this goes wrong on site is forgetting floor finishes. If the bottom step sits on bare concrete that will later receive 20 mm of tile, that riser ends up 20 mm shorter than the rest. Always work from finished floor levels.

The three classical rules

Comfort depends on the relationship between riser and going, not either alone. Three old formulas encode it, and a good stair passes all three:

• 2R + G = 600 to 660 mm — the stride rule, derived from an average pace. The most widely used.
• R + G = 430 to 460 mm — the comfort rule.
• R × G ≈ 45,000 mm² — the product rule.

A stair with a 190 mm rise and 250 mm going gives 2R + G = 630, R + G = 440, R × G = 47,500. All three pass, which is why that combination feels normal underfoot.

Typical limits

Domestic limits vary by jurisdiction, but the common ranges are a maximum riser around 190-200 mm, a minimum going around 220-250 mm, and a maximum pitch of about 42°. Headroom of 2 m is a widespread requirement, measured vertically from the pitch line.

Check your local regulations before building. These are the values most codes converge on, but the details — handrail heights, landing requirements, tapered treads — differ enough to matter for anything requiring approval.

Nosing and stringers

The nosing is the overhang of the tread past the riser below. It adds effective foot room without adding to the total run, and most codes require between 16 and 25 mm where the going is at the minimum. Going is measured nosing to nosing, not the physical board width, which is a common source of error.

The stringer length is the hypotenuse of total rise and total run — Pythagoras again. Add extra when ordering timber for the cuts at each end, and remember the stringer must be deep enough that the notches do not cut away too much of it.

Common questions