What does a construction calculator need to do in Australia?

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Australian building maths is in millimetres, checked against the NCC Housing Provisions. The sums that come up most are stairs (risers 115 to 190 mm, goings 240 to 355 mm, 2R + G 550 to 700 mm), studs and joists at 450 or 600 mm centres, concrete in m³ and 20 kg bags, and the 1:500 gutter fall.

What units and rules does Australian building work use?

Millimetres for every length, cubic metres for concrete and soil, tonnes for gravel and road base. Plans are drawn in millimetres, timber is sold by the lineal metre and ready-mix by the cubic metre, so a calculator that starts in feet and inches makes you convert twice.

The rules for a house come from the National Construction Code (NCC), Housing Provisions, as your state adopts it. NCC 2022 and NCC 2025 are both published, so check which edition your state applies. Timber framing goes through the NCC to AS 1684 (NCC 2022 Volume Two, clause H1D6(4)), whose span tables are sold by Standards Australia. Your plans, your certifier and the council decide what applies to a particular job; this page and the app don't make work compliant.

How do I check a stair against the NCC?

Work out the risers and the going, then check four numbers against NCC 2025 Housing Provisions Table 11.2.2a and clause 11.2.2(1)(a): each riser 115 to 190 mm, each going 240 to 355 mm, twice the riser plus the going (2R + G) 550 to 700 mm, and 2 to 18 risers in a flight. Headroom is 2.0 m above the nosing line (clause 10.3.1(1)(f)).

Worked example: a 2700 mm floor-to-floor height (finished floor to finished floor) with a 240 mm going.

  1. Target riser: (600 − 240) ÷ 2 = 180 mm.
  2. Risers: 2700 ÷ 180 = 15, so 15 risers of 2700 ÷ 15 = 180 mm, and 14 treads.
  3. Going length the treads take: 14 × 240 = 3360 mm. The app's Total Run in Rise Only is 15 × 240 = 3600 mm, one going more, because it runs on to the upper floor.
  4. Stringer line: √(2700² + 3600²) = 4500 mm at 36.87°.
  5. Check: 180 mm is inside 115 to 190, 240 mm sits on the 240 minimum, 2R + G = 2 × 180 + 240 = 600 mm, and 15 risers is under 18. It passes, with no room to shorten the going.

Two Australian traps. First, the riser cap: put the same 2700 mm into 14 risers and each is 2700 ÷ 14 = 192.9 mm, which fails the 190 mm limit. The app's amber riser warning fires above 190 mm, so it flags this one. Second, the 18-riser limit: a 3600 mm rise needs at least 3600 ÷ 190 = 18.9, so 19 risers, one more than a single flight may have. That stair needs a landing, and the app works one straight flight at a time, so do each flight on its own.

Free stair calculator: it opens on the Australia (NCC 2025) check, so type your own floor-to-floor height and going and read the riser, going, 2R + G and riser-count lines underneath.

For the counts behind this, see how many steps a 2.7 m or 3 m floor needs and the 2R + G stair formula. Landings, open risers, winders, handrails and slip resistance, each with its NCC clause, are in stair requirements in Australia.

How many 20 kg bags of concrete does a shed slab take?

Volume in cubic metres, plus 10 percent, times the bags per m³ printed by the maker. A garden shed slab 3000 × 2400 mm and 100 mm thick:

  1. Volume: 3 × 2.4 × 0.1 = 0.72 m³ (720 litres).
  2. Order with 10 percent: 0.72 × 1.10 = 0.792 m³.
  3. Boral Concrete Mix 20 kg, 108 bags per m³ on its data sheet: 0.792 × 108 = 85.5, so 86 bags.
  4. Cement Australia PROStrength 20 kg, about 110 per m³: 0.792 × 110 = 87.1, so 88 bags.

That is 86 × 20 = 1720 kg of dry mix to carry and mix, so price a ready-mix delivery as well. The bag count changes with the product, so don't use one "bags per cubic metre" figure for every brand. The makers' yields are compared in how many 20 kg bags make a cubic metre, and the volume step for slabs, footings and holes is in how much concrete do I need. Slab thickness, mesh and footing sizes come from your engineer's drawings, not a calculator.

Free concrete calculator: enter 3000, 2400 and 100 mm and choose Boral Concrete Mix, 20 kg under Count bags of to see the 86 bags above.

Are studs and joists at 450 or 600 centres in Australia?

Both are common, and neither is a national rule. The framing limit comes from AS 1684's tables for the member size, grade, height, load and wind class, and what is fixed to the frame sets a second limit that the makers publish:

  • Wall linings: Gyprock's Red Book 2 allows 600 mm framing centres for 10 mm Gyprock Plus and 13 mm Gyprock Standard on walls in wind classes N1 to N4.
  • Particleboard floors: 19 mm STRUCTAflor needs joists at 450 mm centres at most, 22 mm allows 600 mm.
  • Decks: WoodSolutions TDG 21 gives 500 mm for 19 mm hardwood boards (Medium Feature or Standard grade) and 400 mm for 19 mm Standard grade treated softwood.

Worked example: a 5400 mm wall with 2400 mm studs. Studs = wall length ÷ spacing, rounded up, plus one. At 450 mm: 5400 ÷ 450 = 12, + 1 = 13 studs, 13 × 2.4 = 31.2 lineal metres. At 600 mm: 5400 ÷ 600 = 9, + 1 = 10 studs, 24 lineal metres. Stud Counter gives the same 13 studs and 31200.0 mm. Corner, opening and junction studs, noggings and plates come on top, and the app doesn't count those. The sources and the full tables are in stud spacing in Australia and joist spacing in Australia.

How much road base and crusher dust do I order?

Area × compacted depth gives cubic metres; cubic metres × your supplier's compacted density gives tonnes. Work each layer on its own. Under a 6 × 3 m paved area with 100 mm of road base and 30 mm of crusher dust bedding (example depths: take yours from the paver maker or installer):

  • Road base: 6 × 3 × 0.1 = 1.8 m³.
  • Crusher dust: 6 × 3 × 0.03 = 0.54 m³.

Ask the quarry for each product's compacted tonnes per cubic metre and multiply, then add about 10 percent. Densities vary by product and supplier, so this page quotes none. In the app, Gravel takes any bulk material once you type its density: it starts at 1500 kg/m³, an app default, not a road base figure. More in how much road base do I need.

What falls and balustrade gaps does the NCC set?

The ones that come up on a house job, from the Housing Provisions:

WhatFigureClause
Eaves gutter fallNot less than 1:500 (2 mm per metre)7.4.4(1)(a)
Ground or paving beside a slab-on-ground house50 mm over the first 1 m; 25 mm for concrete or clay paving in a low rainfall intensity area3.3.3(a)
Wet-area floor to a wasteBetween 1:80 and 1:5010.2.12, 10.2.14
Openings in a barrier or balustradeMust not let a 125 mm sphere through11.3.4(4)
Barrier height on a deck where a fall of 1 m or more is possibleAt least 1 m; 865 mm above stair nosings11.3.3(1), 11.3.4(2)

A 7.2 m gutter to one downpipe needs at least 7200 ÷ 500 = 14.4 mm of fall. The 125 mm is a clear gap, not a centre spacing: with 42 mm balusters the centres must be under 167 mm. Details in the minimum fall for a gutter and the balustrade gap in Australia; for any other ratio, the slope calculator converts 1:500 to millimetres per metre and degrees.

Which MeisterCalc calculators suit Australian work?

The app was made metric first, for Australian, New Zealand and UK trades, and runs on Android 7.0 or later with every calculation on the phone, so it works with no signal on site. Stairs, Concrete and Check Square are free. Stud Counter, Running Measurements (marks at 450 or 600 read off one tape, with a Speak button), Baluster Spacing (its Max Spacing starts at 125 mm), Pitch, Decking, Gravel, Soil and Dumpy Level are among the other 13, which unlock with a weekly subscription that starts with a 3-day free trial, or a one-time lifetime unlock; Google Play shows the prices in your currency. It doesn't do rafter or hip cuts, stair landings or roof area. For quick sums in a browser, see all the construction calculators, and for other trades and countries, calculators by trade and country.

Doing your Cert III? The maths in each core unit of CPC30220 and its replacement CPC30326 is worked through in mm in Cert III Carpentry maths, unit by unit.

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Codes and sources

Checked on 1 November 2026. The code, the approved drawings and your certifier or council decide what a job needs; the figures here are quoted so you can check them.

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