What should a stair builder calculate before cutting a stringer?
Published · By Sai Nath
A stair builder works out the riser count from the finished floor-to-floor rise, then the going, the run, the stairwell opening and the baluster spacing, and checks each against the code. A 2700 mm rise with a 250 mm going gives 15 risers of 180 mm and 14 treads, inside the NCC's 115–190 mm riser and 240–355 mm going.
What does a stair builder need to calculate?
Six things, in this order, all from the finished floor-to-floor rise:
- Total rise, finished floor to finished floor.
- Risers: the rise ÷ a target riser, rounded to a whole number. Riser height = rise ÷ risers, decimals kept.
- Treads and going: treads = risers − 1. You choose the going, or a fixed run sets it: run ÷ treads.
- Run and stringer line: treads × going, and √(rise² + run²).
- The checks: riser, going, 2R + G and the pitch of the nosing line against your code.
- Stairwell opening and balusters: the hole the headroom needs, and the gaps in the balustrade.
The free stair calculator on this page and the Stairs calculator in MeisterCalc do steps 1 to 5 and the opening with the same formulas; Baluster Spacing does the balusters. Laying the steps out on the board is in how to lay out a stair stringer. For other trades, see calculators by trade and country, or all the construction calculators.
Does a 2700 mm floor-to-floor stair pass the NCC?
Yes, with 15 risers of 180 mm and a 250 mm going. Worked through:
- Target riser from the 2R + G rule: (600 − 250) ÷ 2 = 175 mm.
- Risers: 2700 ÷ 175 = 15.43, which rounds to 15. Riser = 2700 ÷ 15 = 180 mm.
- Treads: 15 − 1 = 14. They take 14 × 250 = 3500 mm of floor, bottom riser face to top riser face.
- 2R + G: 2 × 180 + 250 = 610 mm. Pitch: arctan(180 ÷ 250) = 35.75°.
| Check | This stair | NCC 2025, house stair (not spiral) | Result |
|---|---|---|---|
| Riser | 180 mm | 115 to 190 mm | Passes |
| Going | 250 mm | 240 to 355 mm | Passes |
| 2R + G | 610 mm | 550 to 700 mm | Passes |
| Risers in the flight | 15 | 2 to 18 | Passes |
| Variation | All 180 mm | Adjacent risers within 5 mm; largest to smallest in a flight within 10 mm | Passes |
| Headroom | Set by the opening | 2.0 m above the nosing line | See below |
Sources: NCC 2025 Housing Provisions, Table 11.2.2a and clause 11.2.2(1)(a) and (c), and clause 10.3.1(1)(f) for headroom, checked on 1 November 2026. The same stair fails in New Zealand, where Acceptable Solution D1/AS1 (Table 6) asks for a going of 280 mm or more on a main private stairway.
One thing to read carefully in the app: in Rise Only, Total Run is risers × going, 15 × 250 = 3750 mm, one going more than the treads cover. Its Stringer Length, √(2700² + 3750²) = 4620.9 mm at 35.75°, runs to the upper floor one going past the top riser. Use 3500 mm for the floor space.
How do I fit a stair into a fixed run?
Choose the riser count and let the run set the going. If the going comes out under the minimum, the stair needs more run or a turn, not more risers.
Worked example: a 2600 mm rise with 3000 mm from the face of the bottom riser to the trimmer.
- Risers at a 190 mm target: 2600 ÷ 190 = 13.68, so 14 risers of 185.7 mm.
- Going: 13 treads share 3000 mm, so 230.8 mm, under the NCC's 240 mm. 2R + G is 602.2 mm and passes, which hides the problem.
- Fewer risers? 13 risers are 200.0 mm each, over 190 mm.
- Shortest straight flight for this rise under the NCC: 13 goings of 240 mm = 3120 mm. Find 120 mm more, or turn the stair with a landing.
With 3400 mm of run the going is 261.5 mm and 2R + G 633.0 mm, and everything passes. In Rise + Run mode the app's Incline Angle is arctan(2600 ÷ 3400) = 37.41°, the line from the foot of the bottom riser to the top of the last one. The pitch codes limit is the nosing line, arctan(185.7 ÷ 261.5) = 35.38°. More options for a short run are in how to fit a staircase into a fixed length.
How long does the stairwell opening need to be?
(Floor thickness + headroom) × going ÷ riser, measured along the flight from the trimmer at the top. For the 2700 mm stair, with a 300 mm floor (joist, flooring and ceiling lining) and the NCC's 2.0 m headroom: (300 + 2000) × 250 ÷ 180 = 3194.4 mm. With the app's default 200 mm floor it is 3055.6 mm, so every 100 mm of floor adds 139 mm of opening. Measure the real floor build-up. Width and trimmer sizes are separate checks: see how big a stairwell opening needs to be.
How do I space balusters on the rake?
Space them on the level distance between the newels, because the gap between two upright balusters is measured level. Then multiply by the slope factor to mark them along the handrail or string.
Worked example: 3300 mm level between newels on the 180 × 250 mm stair, 32 mm balusters, and Australia's 125 mm sphere rule.
- Baluster Spacing with Run 3300, Baluster Width 32 and Max Spacing 125: ⌈3300 ÷ 125⌉ − 1 = 26 balusters, gaps of (3300 − 26 × 32) ÷ 27 = 91.4 mm, 123.4 mm on centres, level.
- Slope factor, worked by hand: √(1 + (180 ÷ 250)²) = 1.2322. Along the rail the centres are 123.4 × 1.2322 = 152.1 mm apart, and the first three leading edges in the running table, 91.4, 214.8 and 338.2 mm level, mark at 112.6, 264.7 and 416.8 mm along the rake.
- Fewer balusters? The app's count keeps every gap under the maximum but isn't the fewest: (3300 − 125) ÷ (32 + 125) = 20.2, so 21 balusters with 119.5 mm gaps would also stay under 125 mm. Many builders keep the margin.
On a stair the 125 mm opening is measured above the nosing line (NCC 2025 Housing Provisions, Part 11.3). Heights and the other balustrade rules are in the maximum gap between balusters in Australia.
Where do stair builders go wrong with the numbers?
- Measuring before the floor finishes. 2700 mm to the bare slab, then 20 mm of tile and bed: the finished rise is 2680 mm, and a stair built as 15 × 180 ends with a 160 mm bottom riser, 20 mm short of the next. The NCC allows 5 mm between adjacent risers. Built right: 2680 ÷ 15 = 178.7 mm each.
- Rounding the riser. Marking 179 on every step leaves the last at 2680 − 14 × 179 = 174 mm, already the full 5 mm out before any site error.
- Forgetting headroom. Cut the opening at 2400 mm instead of 3194.4 and the clearance under the trimmer is 2400 × 180 ÷ 250 − 300 = 1428 mm, not 2000.
- Using the tread as the going. With a 20 mm nosing, a 250 mm going needs a 270 mm tread. Step the stringer at 270 and the flight grows 14 × 20 = 280 mm. See what the going of a stair is.
- Reading Total Run as floor space in Rise Only: 3750 mm against the 3500 mm the treads take.
- Taking the app's warnings as the code. Its amber warnings use fixed limits: a riser over 190 mm or under 115 mm, a going under 150 mm or over 450 mm, an incline over 45°. The riser limits are the NCC Housing Provisions' for houses, but the going check is far wider than the NCC's 240 mm minimum and 2R + G isn't checked, so a 185 mm riser with a 230 mm going shows no warning and still fails.
Which stair rules apply outside Australia?
Each country sets its own, and the free calculator checks four of them. New Zealand's D1/AS1: a main private stair has risers of 190 mm or less, goings of 280 mm or more and a pitch of 37° or less. England's Approved Document K: a private stair has risers of 150 to 220 mm, goings of 220 to 300 mm and a pitch of 42° or less. In the US, the IRC caps risers at 7 3/4" and asks for treads of 10" or more, as the City of Boise handout quotes it. The full table, with headroom, is in stair rules by country. Your council, certifier or building department decides what applies; the app and this page do the arithmetic and don't make a stair compliant.
Is the stair calculator free, and what doesn't it do?
Stairs is one of three free calculators in MeisterCalc, with Concrete and Check Square; Baluster Spacing unlocks with the subscription or the lifetime unlock. It runs on Android 7.0 or later, offline. It works one straight flight at a time: no landings, winders or spiral stairs, no stringer cuts, and no deduction for tread or riser board thickness, which you take off by hand when you mark the stringer.