By Sam Alex, civil engineer · About the author
Australia went metric in the 1970s, but imperial units never quite left the building trade. Older house plans are drawn in feet and inches, real estate agents still talk about "squares", some American products list coverage in square feet, and the timber yard sells "four by two". Here are the conversions I use most, and the traps that catch people out.
Length
| Imperial | Metric |
|---|---|
| 1 inch | 25.4 mm (exactly) |
| 1 foot | 304.8 mm (0.3048 m) |
| 1 yard | 0.9144 m |
| 1 mile | 1.609 km |
Quick mental versions: - Inches to millimetres: multiply by 25 and add a bit (10 in ≈ 254 mm) - Feet to metres: multiply by 0.3, then add 1.6% (10 ft ≈ 3.05 m) - Metres to feet: multiply by 3.28 (3 m ≈ 9.84 ft)
The feet to metres converter and inches to centimetres converter do the exact maths.
Area: the most common source of mistakes
Area conversions use the square of the length factor, which is where people go wrong: - 1 m = 3.28 ft, but 1 m² = 3.28 × 3.28 = 10.76 ft² - 1 ft² = 0.0929 m² - 1 "square" (old Australian house plans) = 100 ft² = 9.29 m² - 1 acre = 4,047 m², and 1 hectare = 10,000 m² = 2.47 acres
So a "25-square house" is about 232 m², not 250. Our square metres to square feet converter handles the conversion.
Volume
| Imperial / US | Metric |
|---|---|
| 1 cubic foot | 0.0283 m³ (28.3 L) |
| 1 cubic yard | 0.7646 m³ |
| 1 US gallon | 3.785 L |
| 1 imperial gallon | 4.546 L |
Watch the gallons. American product labels use US gallons, which are about 17% smaller than the old British imperial gallon. If a US paint tin says "covers 400 ft² per gallon", that's about 37 m² per 3.785 L, or roughly 9.8 m² per litre.
Weight and force
- 1 pound (lb) = 0.4536 kg; 1 kg = 2.205 lb
- 1 US ton (short ton) = 2,000 lb = 907 kg
- 1 tonne (metric) = 1,000 kg
- 1 psi = 6.895 kPa, so a tyre at 32 psi is about 220 kPa
Timber: nominal vs actual sizes
This one catches everyone. In North America, a "2 × 4" is named from its rough-sawn size, but after drying and planing it's actually about 1½ × 3½ inches (38 × 89 mm).
Australian framing timber is sold by its actual metric size, such as 90 × 45 mm or 70 × 35 mm, in lengths that go up in 0.3 m steps (2.4, 2.7, 3.0 m and so on). A "4 by 2" in Australian slang is roughly a 90 × 45, but when you're following an American plan, don't assume the sizes swap directly. Check the span tables for the timber you're actually using.
Pipe and fittings
Pipe sizes are often nominal too. A "DN20" or "¾-inch" water pipe doesn't have a 20 mm or 19 mm bore exactly. Match fittings by the system and standard, not by measuring with a tape.
Temperature
°C = (°F − 32) × 5 ÷ 9. Concrete and paint instructions often give temperature limits. For example, avoid pouring below about 5 °C (41 °F) without precautions.
My rules for working across both systems
- Pick one unit system per job and convert everything into it at the start.
- Convert lengths before calculating areas and volumes. Don't try to convert the final area or volume with a length factor.
- Write the unit next to every number on your sketch. "3.6" means nothing; "3.6 m" is clear.
- Sense-check the answer. If a room comes out at 180 m², it's probably 180 ft².
Frequently asked questions
How many square feet are in a square metre?
10.764 ft². One square foot is 0.0929 m².
What is a "square" in Australian real estate?
An old imperial unit of 100 ft², about 9.29 m². Divide square metres by 9.29 to get squares.
Is a US gallon the same as a UK gallon?
No. A US gallon is 3.785 L and an imperial gallon is 4.546 L.