Almost every other country in the world uses the metric system for construction. The United States and (officially) Liberia and Myanmar are the holdouts on imperial. Canada is the interesting case: officially metric since 1970, but construction practice is heavily imperial — lumber dimensions, drawing scales, plywood sheet sizes, common dimensions like 2×4 and 9×13 — because the supply chain runs north-south from US mills and factories.
This guide is the practical reference for working between the two systems. It covers what you will see on which kind of drawing, where the systems mix (with examples), and the conversion rules that actually matter when you are on a job site or estimating a quote.
What system is on which drawing
US architectural drawings: always imperial. Lengths in feet and inches (with the inches usually as fractions: 1/2", 3/4", 7/8"). Scales like 1/4" = 1'-0", 1/8" = 1'-0", 1/2" = 1'-0". Areas in square feet. Volumes in cubic feet or cubic yards (concrete). Lumber by nominal name (2×4, 2×6, 2×10).
Canadian architectural drawings: mostly metric for federal, institutional, and large commercial projects (metres, millimetres). Residential and most renovation drawings are imperial because the materials are imperial — drywall comes in 4×8 sheets, lumber in 2×4 and 2×6, plywood in 4×8. A drawing that uses metric dimensions and then specifies "2×6 framing at 16" o.c." is mixing systems intentionally because the underlying materials are imperial regardless of which units the architect labels.
European drawings: metric exclusively. Lengths in metres or millimetres (not centimetres — Europeans use millimetres for precision work and metres for room-sized dimensions). Scales as ratios: 1:50, 1:100, 1:200. Areas in square metres. Volumes in cubic metres.
Engineering drawings (civil, structural, mechanical): in the US, imperial. In Canada, almost always metric. In the rest of the world, metric. Civil drawings in Canada often use the "engineering scale" 1:200 or 1:500 for site plans, with all dimensions in metres.
The lumber problem
North American framing lumber is sized in nominal imperial dimensions that do not match the actual finished dimensions. A 2×4 is actually 1.5″ × 3.5″ once it has been milled. A 2×6 is 1.5″ × 5.5″. A 2×10 is 1.5″ × 9.25″. The nominal name persists because it is shorthand for the rough-sawn size before planing.
When a metric drawing specifies "2×6 framing," the dimensions are not metric at all — they are imperial nominal sizes whose actual cross-section happens to be 38 × 140 mm. The same is true of 4×8 plywood sheets (1,219 × 2,438 mm) and 4×4 ceiling tiles (610 × 610 mm). The dimensions used on the drawing might be metric, but the underlying products are imperial.
This is the practical reason Canadian construction keeps imperial alive: the supply chain is dominated by US mills that sell imperial lumber to North American specs. A pure-metric Canadian project would have to import European lumber, which is rare and expensive.
See the Lumber Nominal vs Actual Calculator for the lookup table.
The conversion rules that matter
Linear measurements. 1 inch = 25.4 mm exactly. 1 foot = 304.8 mm = 0.3048 m exactly. 1 yard = 914.4 mm. These are exact conversions by international agreement, not rounded.
Practical thumb-rules used on site:
- 3 ft ≈ 0.9 m (actual 0.9144)
- 4 ft ≈ 1.2 m (actual 1.219)
- 8 ft ≈ 2.4 m (actual 2.438)
- 10 ft ≈ 3 m (actual 3.048)
- 25 mm ≈ 1 inch (actual 25.4)
- 1 m ≈ 39.4 inches ≈ 3.28 feet
Area measurements. Area conversion factors are the linear factor squared. 1 ft² = 0.0929 m². 1 m² = 10.764 ft². For practical site work: 100 ft² ≈ 9.3 m²; 1,000 ft² ≈ 93 m².
Volume measurements. Cubed. 1 ft³ = 0.0283 m³. 1 m³ = 35.31 ft³. Practical: 100 ft³ ≈ 2.83 m³; 1 cubic yard = 27 ft³ = 0.7646 m³.
Mixed inch-foot dimensions. Drawings often write dimensions as "3'-6 1/2"" — that is 3 feet 6.5 inches = 42.5 inches = 1,079.5 mm. The hyphen is part of the imperial convention, not a minus sign. Type this into the Mixed-Unit Calculator and it parses correctly to millimetres.
Where the systems mix on a real site
Materials in imperial, dimensions in metric. A Canadian project specifies a wall as 3,000 mm long. The wall is framed with 2×6 lumber (imperial nominal, actually 38 × 140 mm). Studs are placed at 406 mm on centre — which is exactly 16 inches, the standard imperial stud spacing. The drawing is metric; the material and spacing are imperial.
Imperial concrete in metric documents. Concrete is sold in cubic yards across most of North America, including Canada. Even when a metric drawing specifies "5.5 m³" of concrete, the ready-mix supplier delivers in 10-yard (7.65 m³) truck loads. Order in yards, document in cubic metres.
Steel beam designations. Steel is sold by nominal section names that mix systems. A W12×35 beam (US) is a 12-inch deep, 35 lb/ft I-beam. In Canada, the equivalent is a W310×52 — a 310 mm deep, 52 kg/m beam. The numbers are different but refer to the same family of sections.
Drywall thickness. Standard drywall is 1/2" (12.7 mm) or 5/8" (15.9 mm). Specifications can say either; suppliers will accept either spec for the same product.
Plumbing pipe. Always nominal sizing — NPS 1/2", NPS 3/4", NPS 1" — regardless of whether the drawing is imperial or metric. The actual outside diameter is fixed by ASME B36.10 / B36.19. A "1/2 inch" copper pipe has 5/8" OD; a "1/2 inch" steel pipe has 0.840" OD. See the Pipe Size Converter.
When does it matter most
Three places where unit confusion costs real money:
Concrete orders. Order in yards or order in cubic metres, but never mix. A "10 m³" order misread as "10 yd³" is 23 percent short — the truck does not have enough concrete to finish the pour. Always confirm units verbally with the dispatcher.
Material take-offs. A plywood schedule that mixes "ten 4×8 sheets" (imperial) with "1.2 × 2.4 m sheets" (metric, same product, slightly rounded) is a paperwork nightmare. Standardize on one convention per project, and convert all quantities to that convention at the start.
Rebar. Rebar is sized in eighths of an inch in the US (#4 = 4/8" = 1/2"). In Canada, metric: 10M (10 mm), 15M (15 mm), 20M (20 mm). The numbers are not interchangeable. Always check which system the structural engineer specified.
Reference tables
Length: 1 in = 25.4 mm | 1 ft = 304.8 mm | 1 yd = 914.4 mm | 1 mi = 1.609 km | 1 m = 39.37 in = 3.281 ft
Area: 1 ft² = 0.0929 m² | 1 m² = 10.764 ft² | 1 acre = 4,047 m² | 1 hectare = 2.471 acres
Volume: 1 ft³ = 28.32 L = 0.0283 m³ | 1 m³ = 35.31 ft³ = 264.2 US gal | 1 US gal = 3.785 L
Weight: 1 lb = 0.4536 kg | 1 kg = 2.205 lb | 1 ton (short) = 907.2 kg
Calculators on this site
- Mixed-Unit Calculator — parse and add lengths like "3'-7 1/2"" or "5 m 30 cm".
- Architectural Scale Converter — convert between imperial and metric drawing scales.
- Lumber Nominal vs Actual — actual finished dimensions of 1×, 2×, 4× lumber.
- Pipe Size Converter — NPS to OD to DN.
- Dimensions Converter — quick L × W × H in either system.
Sources
- ISO 1000:1992 SI units and recommendations for the use of their multiples (international metric standard).
- NIST Special Publication 811 (US definitive metric conversion factors).
- ASTM A6 (steel section designations).
- CSA G30.18 (Canadian rebar designations).
- ASME B36.10 / B36.19 (pipe nominal sizes).