How to Calculate Stairs: Rise, Run & Number of Steps

The method, in one line

Divide your total rise by 190 and round up. That gives your number of risers. Divide the rise back by that count to get the riser height. You get one fewer going than risers. Pick a going between 240 mm and 355 mm that fits your run. Then check that 2R+G lands between 550 mm and 700 mm.

Your two starting numbers are already on site. The total rise is the floor-to-floor height. The available run is how much floor length you have to fit the stair into. Every other number comes from those two.

Here’s what each term means:

• Rise: the total height, floor to floor.
• Run: the total floor length the stair takes up.
• Riser (R): the height of one step.
• Going (G): the depth of one step, nose to nose.
• 2R+G: a comfort check. Two step heights plus one step depth must land in a set range.

And here are the limits the rule book sets (NCC 2025, clause 11.2):

Two things to get straight before you start. A run of stairs always has one fewer going than it has steps. The top tread is the floor above, so it gets no going of its own. And every step has to be the same height, every going the same depth. The odd step out is where people trip.

How to calculate stairs by hand

Say your floor-to-floor height is 2700 mm. You have 3600 mm of floor length to fit the stair into. Here’s how it goes, step by step.

1. Work out the number of steps. Divide the total rise by the top step height, then round up: 2700 ÷ 190 = 14.2. You can’t have part of a step, so round up to 15 risers.
2. Work out the step height. Divide the rise back by that number so every step is even: 2700 ÷ 15 = 180 mm per step. Check it: 180 is under the 190 mm top limit and over the 115 mm bottom limit. Good.
3. Work out the number of goings. One fewer than the number of steps: 15 − 1 = 14 goings.
4. Pick a going that fits your run. The deepest going you can fit is your floor length divided by the number of goings: 3600 ÷ 14 = 257 mm. So anything up to about 257 mm fits. Pick a round 250 mm. Check it: 250 is inside the 240 mm and 355 mm limits. Good.
5. Work out the total run. Number of goings times the going: 14 × 250 = 3500 mm. That fits inside the 3600 mm you had. Good, with 100 mm to spare.
6. Run the 2R+G comfort check. (2 × 180) + 250 = 610 mm. That’s inside the 550–700 range. Comfortable.

So: 15 steps at 180 mm, 14 goings at 250 mm, 3500 mm of run. Every number checks out against NCC 2025 HP 11.2.

If step 6 had come out under 550, the stair would feel cramped and steep. Over 700 and it’s a stretch to walk. The fix is to change the going a little. Or, if you’re tight on floor length, add a step to drop the step height. And if your step count had come out over 18, you’d need to break the stair with a landing.

Do it in 5 seconds

Keep reading

• NCC stair requirements: riser, going & the 2R+G rule has every limit this method checks against, in full.
• Maximum & minimum stair riser heights in Australia covers the step-height numbers on their own. It also covers what happens when a stair is a few millimetres out.
• Part of the stairs cluster. Start here for the whole NCC stairs picture.

Sources

• NCC 2025, ABCB Housing Provisions Standard, clause 11.2 — stairway construction (riser, going, 2R+G, risers per flight). Confirm every figure at ncc.abcb.gov.au. (Clause number and values are confirmed against the NCC 2025 Housing Provisions Standard, verified 2026-07-28.)