Rise, riser, going, run
A stair is defined by two numbers: the riser, which is the height of each step, and the going, which is the depth of each tread from one nosing to the next. Everything else follows. Divide the total rise by the riser you would like, round up to a whole number of risers, and divide again to get the exact riser. Treads are one fewer than risers, because the top floor is the last step. Going times treads is the run, the horizontal length the stair takes up.
risers = total rise ÷ target riser, rounded up
riser = total rise ÷ risers run = going × (risers − 1)
A 2,700 mm floor-to-floor rise at a target of 175 mm gives 16 risers of 168.75 mm, 15 treads, and at a 250 mm going a run of 3,750 mm. Change the target to 180 and you get 15 risers of 180 mm and a run of 3,500. Both comply; the second is a little steeper and 250 mm shorter.
The NCC limits
The National Construction Code sets limits for stairs in Australian houses: risers between 115 and 190 mm, goings between 240 and 355 mm, and the sum of twice the riser plus the going between 550 and 700 mm. Non-residential buildings tighten the riser to 180 mm and the going to at least 250. All risers in a flight must be the same height, within 5 mm, and all goings the same depth, which is why the calculator divides the rise evenly rather than leaving an odd step at the top. The 2R + G rule is what makes a stair feel right to walk: a 190 riser with a 240 going scrapes in at 620, but a 190 riser with a 355 going is 735 and fails, because nobody can stride that far while climbing that high.
| Riser | Going | 2R + G | Feel |
|---|---|---|---|
| 165 | 280 | 610 | Gentle, takes room |
| 175 | 250 | 600 | Typical house stair |
| 180 | 250 | 610 | Typical, slightly steep |
| 190 | 240 | 620 | Steepest allowed in a house |
| 190 | 355 | 735 | Fails 2R + G |
Headroom, landings and width
Headroom is 2,000 mm measured vertically above the nosing line, all the way up. That fixes how long the opening in the floor above must be: the stair has to have climbed enough, by the point where it passes under the floor, to leave two metres above your head. A landing lets a stair turn a corner or break a long flight; it counts as a tread for the run and it does not change the riser count. The NCC also sets minimum widths, requires a handrail on a stair that rises more than a metre, and requires a balustrade wherever there is a fall of a metre or more. Those rules are outside this calculator and are worth a read before the stringers are cut.
Stringers
The stringer length shown is the straight-line distance along the nosings, which is what the stair actually spans. Timber for cut stringers needs to be longer than that to allow for the plumb and level cuts at each end and the depth of the sawtooth, so add 300 to 400 mm, and buy stringers from a length that suits: a 3,750 run and 2,700 rise is a 4.6 m stringer line, so a 5.4 m length.
Worked example
Floor to floor 2,700 mm, target riser 175, going 250, house. 2,700 ÷ 175 = 15.4, so 16 risers of 168.75 mm and 15 treads. Run is 15 × 250 = 3,750 mm. 2R + G is 587.5, pitch 34 degrees, stringer line 4.62 m. All three checks pass. If the room only allows a 3,400 mm run, try 15 risers of 180 with a 240 going: run 3,360, 2R + G 600, pitch 37 degrees, still compliant but noticeably steeper.
Common questions
What is the maximum riser height?
190 mm in a house, 180 mm in other buildings, minimum 115 mm. All the same within 5 mm.
What is the minimum going?
240 mm in a house, 250 in other buildings, maximum 355. And 2R + G between 550 and 700.
How do I work out the number of steps?
Rise ÷ target riser, rounded up. Then rise ÷ that number for the exact riser.
How much headroom?
2,000 mm above the nosing line all the way up.
Geometry only, checked against the NCC limits for straight flights. Widths, handrails, balustrades, headroom, landings and winders have their own rules in the NCC and must be checked by whoever designs and certifies the stair.