Imperial and metric on site: conversions every trade needs

Canada went metric in the 1970s, but the building trades never fully followed: houses are framed in feet and inches with 2×4s and 4×8 sheets, while concrete is ordered in cubic metres, products are labelled in both, and engineers’ drawings are often metric. Knowing the conversions, and the traps, saves expensive mistakes.

Exact factors

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 lumber: a 2×4 is actually 1½" × 3½" (38 × 89 mm), and a 2×6 is 1½" × 5½" (38 × 140 mm). Use actual sizes for spacing and layout.
• Sheet sizes: 8 × 4 ft is 2440 × 1220 mm, but imported metric sheets are 2400 × 1200 mm. They're not interchangeable on a stud layout at 16" or 24" centres.
• Bins: bins and dump trucks are rated in cubic yards in Canada: a 20-yard bin holds about 15 m³.
• Fence panels: wood fence sections are usually 8 ft long, so posts land at 2.44 m centres.
• 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
• 100 sq ft ≈ 9.3 m²

Mixed-unit jobs

When a job mixes systems, for example metric tiles on an imperial floor, or US-made windows in a metric-drawn 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. Most tapes sold in Canada mark 16" centres for stud layout, and many are dual-scale, with imperial on the top edge and metric below.

Converting a takeoff

Every TakeoffYard calculator accepts mixed units: pick the unit beside each field, and switch the whole site between the US, Canadian, 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 Canadian products still sold in imperial sizes?

Many standard sizes date from before metrication and were kept, then given metric equivalents. Drywall, sheet materials and some lumber 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. Canadian plants sell by the cubic metre, even when the plans are in feet.

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 local codes and your inspector for your project.