Australian jobs have been metric since the 1970s, but imperial lingers: "4×2" for 90 × 45 mm timber, fence posts on 2.4 m (8 ft) centres, older houses built to imperial sizes, and US product data in feet and gallons. Knowing the conversions, and the traps, saves expensive mistakes.
Exact factors
| From | To | Multiply by |
|---|---|---|
| inches | mm | 25.4 |
| feet | metres | 0.3048 |
| yards | metres | 0.9144 |
| sq ft | m² | 0.0929 |
| m² | sq ft | 10.764 |
| cu yd | m³ | 0.7646 |
| m³ | cu yd | 1.308 |
| lb | kg | 0.4536 |
| US gallon | litres | 3.785 |
| UK gallon | litres | 4.546 |
| psi | N/mm² (MPa) | 0.006895 |
The inch is defined as exactly 25.4 mm, so length conversions are exact. Rounding errors creep in when you round intermediate results, so convert once, at the end, if you can.
The traps
- Tons: a US (short) ton is 2,000 lb (907 kg). A metric tonne is 1,000 kg (2,205 lb). A UK long ton is 2,240 lb. "Ton" on a quote could mean any of them, so ask.
- Gallons: US gallons are about 17% smaller than UK gallons. A "gallon" of paint from a US product sheet covers less than the same number of UK gallons.
- Nominal timber: a "4×2" is actually 90 × 45 mm, and a "3×2" is 70 × 35 mm. Use actual sizes for spacing and layout.
- Sheet sizes: 8 × 4 ft is 2440 × 1220 mm, but Australian sheets are 2400 × 1200 mm. They're not interchangeable on a stud layout at 450 or 600 mm centres.
- Skips: skip bins are sold in cubic metres in Australia, but some suppliers and older tradies still talk in yards: 1 cu yd is about 0.76 m³.
- Fence panels: Colorbond panels are 2.365 m wide on 2.4 m post centres, a legacy of 8 ft spacing.
- Fractions: 3/16" is about 4.8 mm, not 5 mm. Close enough for a grout joint, but not for machined parts.
Handy approximations
- 1 m ≈ 3 ft 3⅜"
- 1 m² ≈ 10¾ sq ft
- 1 m³ ≈ 1.3 cu yd
- 1 inch ≈ 25 mm (exactly 25.4)
- 1 kg ≈ 2.2 lb
- 1 litre ≈ 0.26 US gal ≈ 0.22 UK gal
- 10 m² ≈ 108 sq ft
Mixed-unit jobs
When a job mixes systems, for example metric tiles on an imperial floor, or US-made windows in a local opening, pick one system for the takeoff and convert everything into it before you start. Mark the system on every page of your notes. Mixed-unit arithmetic is where the biggest mistakes happen: 12 multiplied by 4 means something very different in feet and inches than in metres and millimetres.
Tape measures
Dual-scale tapes (imperial on one edge, metric on the other) are useful on mixed jobs, but take care to read the same edge every time. Many tradespeople keep two tapes so they never misread. Many tapes sold in Australia highlight 450 mm and 600 mm centres for stud and joist layout.
Converting a takeoff
Every TakeoffYard calculator accepts mixed units: pick the unit beside each field, and switch the whole site between the US, UK and Australian versions at the top. Product sizes, bag yields and trade terms follow the region, so you get quantities in the units you actually buy. The unit converter handles anything else, and the fraction calculator turns millimetres into the nearest fraction on a tape.
Common questions
Why are some Australian products still sold in imperial sizes?
Many standard sizes date from before metrication and were kept, then given metric equivalents. Plasterboard, sheet materials and some timber sizes follow old imperial grids, which is why metric and imperial versions don't always line up exactly.
How do I convert a concrete order?
Multiply cubic yards by 0.7646 to get cubic metres, or cubic metres by 1.308 to get cubic yards. Australian suppliers sell premix by the cubic metre, usually in 0.2 m³ steps.
Writing dimensions clearly
Ambiguous notation causes real mistakes. Write feet and inches as 12' 6" (or 12 ft 6 in), never 12.6, which could mean 12.6 feet. Write metric lengths in millimetres on drawings (3600, not 3.6), and in metres in quotes and conversations. On mixed jobs, put the unit next to every number in your notes and on cut lists, even when it feels obvious.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.