Imperial vs Metric for DIY Projects: A Practical Conversion Guide
Plans in feet, products in millimeters, a tape with both — mixing systems is normal in DIY. Knowing a handful of exact conversions and the common traps keeps estimates right.
The exact conversions
| Quantity | Imperial → metric | Metric → imperial |
|---|---|---|
| Length | 1 in = 25.4 mm (exact); 1 ft = 0.3048 m (exact) | 1 m = 3.281 ft = 39.37 in |
| Area | 1 sq ft = 0.0929 m² | 1 m² = 10.764 sq ft |
| Volume | 1 cu ft = 28.32 L; 1 cu yd = 0.7646 m³ | 1 m³ = 35.31 cu ft = 1.308 cu yd |
| Liquid | 1 US gal = 3.785 L | 1 L = 0.264 US gal |
| Weight | 1 lb = 0.4536 kg; 1 short ton = 0.907 t | 1 kg = 2.205 lb; 1 t = 1.102 short tons |
Area and volume traps
Conversions multiply in area and volume. One foot is 0.3048 m, but one square foot is 0.3048² = 0.0929 m², and one cubic foot is 0.3048³ = 0.0283 m³. Converting a length and then forgetting to square or cube the factor is a classic mistake.
Mixed units within a calculation are the other trap: length in feet, depth in inches. Always convert depth to feet (÷ 12) or everything to meters before multiplying.
US vs UK vs Australian products
| Product | US | UK | Australia |
|---|---|---|---|
| Drywall / plasterboard | 4 × 8 ft | 1200 × 2400 mm | 1200 × 2400 mm + |
| Paint | gallon, quart | 5 L, 2.5 L, 1 L | 4 L, 2 L, 1 L |
| Bagged concrete | 40/60/80 lb | 20/25 kg | 20 kg |
| Brick | 7⅝ × 2¼ in | 215 × 65 mm | 230 × 76 mm |
| Timber | nominal 2×4 | 47 × 100 mm sawn | 90 × 45 mm |
Conversion gets you the right quantity; the product list must still use what your store sells. That is why DIYCalcLab switches bag sizes, tin sizes and sheet sizes with the unit system.
The UK and "imperial-ish" habits
The UK is officially metric for trade, but people still talk about feet and inches for room sizes and heights, and timber lengths are sometimes quoted in feet. Gallons in the UK are imperial gallons of 4.546 L — larger than the US gallon of 3.785 L. Canada mixes systems too: lumber in imperial, many specifications in metric.
Fractions and precision
Imperial tapes read to 1/16 in (about 1.6 mm). Metric tapes read to 1 mm. When converting, round to a precision you can actually mark: 1,000 mm is 39⅜ in to the nearest sixteenth, not 39.3701 in. The feet-and-inches calculator handles fraction arithmetic and conversion to millimeters.
Worked example: converting a garden plan
- Plan: a 12 × 8 ft bed with 3 in of mulch.
- Area: 12 × 8 = 96 sq ft = 96 × 0.0929 = 8.92 m².
- Depth: 3 in = 76.2 mm = 0.0762 m.
- Volume: 8.92 × 0.0762 = 0.68 m³ — or in imperial, 96 × 3 ÷ 12 = 24 cu ft = 0.89 cu yd.
- Check: 0.89 cu yd × 0.7646 = 0.68 m³ ✓.
Temperatures, pressures and other units
Some product instructions use units beyond length and volume. Concrete and mortar cure times and paint application limits are often given in both °F and °C (°C = (°F − 32) × 5 ÷ 9; 50 °F is 10 °C). Tyre-style pressures for sprayers may be in psi or bar (1 bar ≈ 14.5 psi). Fastener lengths are in inches or millimeters, and screw gauges differ between systems. When in doubt, use the figure printed on the product in front of you rather than converting from another market's instructions.
Rounding: when it matters and when it does not
For bulk materials, rounding conversions to two or three significant figures is fine — nobody can spread mulch to the nearest millimeter. For cut lengths, rounding errors matter because they repeat: a shelf unit with six shelves converted from 300 mm to 11¾ in rather than 11.81 in loses about ⅜ in overall. Convert the total once, then divide, rather than converting each part.
Products themselves are often "soft converted". A US product labelled 4 × 8 ft may be sold in Canada with a metric equivalent printed on it, while a UK plasterboard of 1200 × 2400 mm is about 47¼ × 94½ in — close to, but not the same as, 4 × 8 ft. Framing spacing follows the same logic: 16 in centers are 406 mm, while metric framing uses 400 mm or 600 mm. Mixing the two systems in one wall leads to boards that do not land on studs.
Metric habits on site
In metric construction, everything is usually measured in millimeters — a wall is 2400 mm high, not 2.4 m or 240 cm — which avoids decimal points on drawings and cutting lists. Centimeters are common in DIY stores but rarely appear on plans. If you are working from metric drawings, set your tape and calculator to millimeters and convert only the final material totals to square or cubic meters.
A good habit
- Pick one system per project and stick to it.
- Write units next to every number on your sketch.
- Convert at the end, once, rather than step by step.
- Check a result against a known figure — a cubic yard covers 81 sq ft at 4 in.
Calculators for this guide
- Feet and Inches Calculator — Add, subtract, multiply and divide feet-and-inch measurements with fractions to 1/16″, and convert to decimal feet, inches and millimeters.
- Square Footage Calculator — Find the area of a room or yard in square feet, square meters and square yards — rectangles, triangles, circles and trapezoids.
- Cubic Yard Calculator — Convert length, width and depth into cubic yards, cubic feet, cubic meters and liters for soil, mulch, gravel or concrete orders.
More guides
Estimates only. This guide explains general methods. Verify quantities with your supplier and follow local building codes; for structural or safety-critical work consult a licensed professional. Editorial policy · Report an error.