Stair Calculator: Risers, Treads and Code Check (US, Australia)
Risers, treads, total run and angle for a staircase, checked against the US IRC or Australia's NCC.
Inputs
- Working
- 270 cm ÷ 18 cm = 15 → 15 risers of 18 cm; 14 treads × 28 cm = 3.92 m of run
- Each riser
- 18 cm
- Treads
- 14
- Total run
- 3.92 m
- Stair angle
- 32.7°
- IRC R311.7.5 (US homes)
- riser 7.09 in, max 7¾ ✓ · tread 11.02 in, min 10 ✓
- The top riser lands on the upper floor, so there is one fewer tread than risers. Codes also govern headroom, handrails and landings, which this does not check.
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What each Stair input means
- Total rise (floor to floor)
- The height from finished floor to finished floor, in metres or feet — including the flooring still to be laid.
- Target riser height
- The riser height you are aiming for, in cm or inches. The page rounds up to whole risers and shares the rise equally, so the real riser is a little lower: 7–7½ in, or 170–180 mm, is a comfortable target.
- Tread depth (going)
- The tread (or going) depth, front to back without the nosing, in cm or inches. The IRC sets a 10-inch minimum; the NCC 240–355 mm.
- Check against
- Which rules to check: the US International Residential Code, Australia's NCC Housing Provisions, or none.
- Units
- Metric (m and cm) or US units (feet and inches). The rise, riser and tread already typed are recalculated in the new units.
Stair formula
risers = total rise ÷ target riser, rounded up · each riser = total rise ÷ risers · treads = risers − 1 · run = treads × tread depth
Measure the total rise from finished floor to finished floor. Divide it by the riser height you are aiming for and round up — every step must be the same height, so the rise is then shared equally: 270 cm over 15 risers is exactly 18 cm each. The top riser lands on the upper floor, so a stair has one fewer tread than risers, and the total run is the treads times the tread depth. Building codes set the limits. In the US, the International Residential Code allows a riser of at most 7¾ inches and a tread of at least 10. In Australia, the NCC's Housing Provisions want risers of 115–190 mm, goings of 240–355 mm, a comfort rule of 2R + G between 550 and 700 mm, and no more than 18 risers before a landing. The page checks whichever you choose.
Stair worked example
2.7 m floor to floor with an 18 cm target riser: 270 ÷ 18 = 15 risers of exactly 18 cm, and 14 treads of 28 cm make 3.92 m of run, at an angle of 32.7°. Under the US IRC it passes — 18 cm is 7.09 inches (max 7¾) and 28 cm is 11.02 inches (min 10). Under Australia's NCC it passes too: riser 180 mm, going 280 mm, and 2R + G = 640, inside 550–700.
More Stair examples
A 9 ft (108 in) floor-to-floor rise aiming for 7½ in risers: 108 ÷ 7.5 = 14.4, rounded up to 15 risers of 7.2 in each, and 14 treads of 10½ in make 12.25 ft of run. Both pass the IRC.
An Australian house with 2.6 m between floors and a 175 mm target riser: 15 risers of 173 mm, 14 goings of 250 mm and 3.5 m of run. The NCC check passes on all four counts, with 2R + G at 597.
Where the Stair calculator can give the wrong answer
- Measure the rise between finished floor levels. Flooring added later changes the top and bottom risers, which codes allow to differ by only 3/8 inch from the rest (IRC) or 5 mm from the next one (NCC).
- The check covers riser, tread and the NCC's slope rule and flight length. Headroom, handrail height, landings and nosing projections are also code items and are not checked.
- Winders and spiral stairs have their own rules in both codes.
- Commercial and public stairs follow different, stricter tables than homes.
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Stair reference table
| Floor to floor | Risers (IRC, max 7¾ in) | Each riser | Floor to floor (metric) | Risers (NCC, max 190 mm) | Each riser (metric) |
|---|---|---|---|---|---|
| 96 in (8 ft) | 13 | 7.38 in | 2,400 mm | 13 | 184.6 mm |
| 102 in | 14 | 7.29 in | 2,600 mm | 14 | 185.7 mm |
| 108 in (9 ft) | 14 | 7.71 in | 2,700 mm | 15 | 180 mm |
| 114 in | 15 | 7.6 in | 2,800 mm | 15 | 186.7 mm |
| 120 in (10 ft) | 16 | 7.5 in | 3,000 mm | 16 | 187.5 mm |
Stair FAQ
- Total rise ÷ the riser height you want, rounded up. 108 inches (9 ft) ÷ 7½ = 14.4, so 15 risers of 7.2 inches, and 14 treads.
- 7¾ inches (196 mm) under the US International Residential Code. Australia's NCC allows 115–190 mm for houses.
- The NCC 2022 Housing Provisions, Table 11.2.2a: risers 115–190 mm, goings 240–355 mm, 2R + G between 550 and 700 mm, and between 2 and 18 risers in a flight. Choose 'Australia' in the code check.
- Twice the riser plus the going, a measure of how comfortable the step is. Australia's NCC requires it to be between 550 and 700 mm: a 180 mm riser with a 280 mm going gives 640.
- The last step up lands on the upper floor itself, which acts as the top tread. Fifteen risers need only fourteen treads.