A US contractor opening a set of architectural drawings stamped in Berlin, Tokyo, or Vancouver tends to react the same way: every dimension is a four-digit number, the door head is at "2100", the slab thickness is "150", and the columns are "300 × 300 @ 6000 o.c." Those are millimeters, and the only remaining problem is that the lumber yard down the road sells nothing in millimeters and the building inspector references an imperial code.
This guide walks through how metric construction documents are organized, what the conventions mean, how to translate them quickly in the field, and — the part that actually costs money if you get it wrong — how to spec metric-dimensioned drawings against US-supplied materials whose "sizes" are not what their labels say.
Why the rest of the world drafts in millimeters
Outside the United States, Liberia, and Myanmar, construction has standardized on the metric system. Within metric construction, the convention is even tighter: linear dimensions on architectural and structural drawings are given in millimeters, with no unit symbol, while levels and elevations are given in meters, usually to three decimal places (e.g. "+12.450" means twelve and four hundred fifty-thousandths of a meter above datum). Site plans switch back to meters for distances longer than about ten meters, where four- and five-digit millimeter numbers stop being readable.
The reason for the millimeter convention is purely practical: millimeters are small enough to express any reasonable construction tolerance as an integer, eliminating decimal points and the misreadings they cause. A 92-and-five-eighths-inch stud height has six characters and a fraction; the same dimension in millimeters is "2354" — four digits, no ambiguity, no fraction-stack misread on a rolled-up sheet at 6 a.m. ISO 128, the international standard for technical drawings, codifies this — millimeters by default unless explicitly stated otherwise.
Reading metric scales
US drawings are scaled in inches per foot — 1/4" = 1'-0" is the archetypal floor-plan scale. Metric drawings use ratio scales: 1:50, 1:100, 1:200. The number on the right is how many real-world units one drawing unit represents, with no bias toward any particular unit. A wall drawn 30 mm long on a 1:50 sheet is 30 × 50 = 1500 mm in the real building, or about 4 ft 11 in.
The most common metric scales and their rough imperial equivalents:
| Metric scale | Typical use | Closest imperial scale |
|---|---|---|
| 1:5, 1:10 | Detail drawings, joinery | 1-1/2" = 1'-0", 1" = 1'-0" |
| 1:20, 1:25 | Wall sections, stair details | 1/2" = 1'-0", ~3/4" = 1'-0" |
| 1:50 | Floor plans, sections (residential) | ~1/4" = 1'-0" |
| 1:100 | Floor plans (commercial), elevations | ~1/8" = 1'-0" |
| 1:200, 1:500 | Site plans | 1" = 20', 1" = 40' |
Note "closest" — 1:50 is not exactly 1/4" = 1'-0". One quarter inch per foot is 1:48 (since 12 × 4 = 48). The discrepancy is small enough that you can scale a 1:50 drawing with an imperial architect's scale and be off by only ~4%, but you should never quote dimensions you scaled off a print as authoritative — always read the printed dimension. ISO 128 and ASME Y14.5 both make the same point: scaled measurements are for verification, never for fabrication.
The conversion shortcuts that matter on site
You will not have a calculator out every minute. A handful of mental anchors will get you through 90% of field translations.
| If you see… | Think… | Exact value |
|---|---|---|
| 25.4 mm | 1 inch | 1 in = 25.4 mm exactly |
| 300 mm | ~1 foot | 1 ft = 304.8 mm |
| 600 mm | ~2 ft (roughly stud spacing) | 2 ft = 609.6 mm |
| 1200 mm | ~4 ft (sheet width) | 4 ft = 1219.2 mm |
| 2400 mm | ~8 ft (sheet length, ceiling height) | 8 ft = 2438.4 mm |
| 3000 mm | ~10 ft | 10 ft = 3048 mm |
| 1 m | ~3 ft 3-3/8 in | 1 m = 3.2808 ft |
The 25.4-mm-equals-one-inch identity is exact by international agreement (1959 International Yard and Pound Agreement, codified in the US through NIST SP 811). Every other "round" metric dimension is an approximation — 1200 mm is not a sheet of plywood, it is almost a sheet of plywood, short by 19 mm (about 3/4 inch). That shortfall is the single most common source of trouble when American sheet goods are substituted into a metric design.
For one-step on-site conversion, divide millimeters by 25.4 to get decimal inches, or by 304.8 to get decimal feet. To go the other direction quickly: 1 ft = 305 mm and 1 in = 25 mm are accurate enough for layout and only ~1.6% off — fine for "where do I cut the rough opening", not fine for "how long is the steel beam".
Nominal vs. actual: the lumber trap
US softwood lumber sizes are nominal — they refer to the rough-sawn green dimension before the board was kiln-dried and surfaced. The American Softwood Lumber Standard (DOC PS 20), administered through NIST, defines the actual surfaced ("S4S") dimensions. The differences are not small.
| Nominal (in) | Actual (in) | Actual (mm) |
|---|---|---|
| 1 × 4 | 3/4 × 3-1/2 | 19 × 89 |
| 2 × 4 | 1-1/2 × 3-1/2 | 38 × 89 |
| 2 × 6 | 1-1/2 × 5-1/2 | 38 × 140 |
| 2 × 8 | 1-1/2 × 7-1/4 | 38 × 184 |
| 2 × 10 | 1-1/2 × 9-1/4 | 38 × 235 |
| 2 × 12 | 1-1/2 × 11-1/4 | 38 × 286 |
If a German drawing specifies a stud at 38 × 89 mm, that is precisely the actual size of a US 2 × 4 — they are interchangeable. If the drawing instead says 50 × 100 mm (a common European nominal), that is a true 50 × 100 stud, and a US 2 × 4 is undersized by 12 × 11 mm relative to the design. That difference can matter for spans, fire resistance, and connection details — never assume metric framing members come out at convenient US imperial actual sizes.
Sheet goods follow the same pattern. North American plywood and OSB ship in nominal 4 ft × 8 ft (1219 × 2438 mm) sheets. Most metric designs are coordinated to a 1200 × 2400 module. The 19 mm of width and 38 mm of length difference will translate into a visible joint offset across a wall if you do not adjust for it. Drywall is similar: US 4 × 8 sheets are 1219 × 2438 mm and metric "1200 × 2400" sheets sold in Europe are exactly that.
Dimensioning conventions on the drawing itself
Beyond the unit, metric drawings follow ISO 128 / ISO 129 conventions that differ from the ASME Y14.5 conventions familiar to US drafters:
- Decimal separators. Continental European drawings use a comma as the decimal mark ("12,450 m" means 12.450 m). UK, Australian, and most Asian metric drawings use a period.
- No trailing zeros for whole millimeters. A dimension of 1500 is written "1500", not "1500.0". A dimension of one and a half millimeters is written "1.5".
- Levels in meters with three decimals. "+0.000" is the project finished-floor datum; "+12.450" is twelve meters and four hundred fifty millimeters above it. A leading "+" or "−" indicates above or below the datum.
- Window and door schedules in width × height. "1500/2100" means a 1500 mm wide, 2100 mm tall opening. The width is always first.
- Centerline-to-centerline column grids. A grid line spacing labeled "6000" means the columns are 6.000 m on center — not face-to-face. This convention is the same as US grid practice.
- Hatching and section conventions. Materials are hatched per ISO 128 standard patterns, which differ slightly from the ASME conventions and from the American Institute of Architects (AIA) layering standards. Concrete, in particular, uses a stippled pattern in ISO drawings rather than the diagonal-line pattern common in US drawings.
Specifying metric drawings against US materials
When you are the contractor sourcing materials in the US for a metric-dimensioned design, you have three reasonable strategies, and choosing among them is one of the most underrated decisions in the project.
- Hard convert the design. Translate every dimension to imperial and reissue. Pros: matches US material sizes exactly. Cons: slow, error-prone, and creates two sets of drawings to maintain. Only worth doing on a long project with a single general contractor.
- Soft convert (work bilingual). Keep the metric drawings authoritative, but tag critical dimensions with imperial equivalents in parentheses (e.g. "2400 (7'-10-1/2")"). The Construction Specifications Institute MasterFormat divisions all accommodate dual-unit specs, and this is the most common approach on mixed-unit projects in North America.
- Substitute compatible materials. Wherever possible, source materials whose actual size matches the metric dimension exactly. A US 2 × 4 truly is 38 × 89 mm; a US 4-inch schedule-40 PVC pipe truly is ~114 mm OD; a US 1/2-inch plywood is ~12.7 mm. Many fasteners and threaded fittings have exact metric equivalents in the M-series. Where no match exists — sheet sizes being the obvious example — you must adjust the layout.
The International Building Code, which is the basis for most US state and municipal building codes, is published in dual units (imperial primary, metric in parentheses). Code-required clearances, egress widths, and stair geometry can be checked directly against either unit. That makes IBC compliance the easy part; material coordination is the hard part.
Common mistakes to avoid
- Treating 1200 mm as 4 ft. It is not — it is 3 ft 11-1/4 in. On an interior wall the 19 mm error is invisible; on a curtain-wall mullion grid across a 30-meter façade it accumulates to nearly half a meter.
- Confusing the unit on level callouts. "+3.000" is three meters above datum (about 10 feet), not three millimeters. More than one inexperienced reader has put a slab three thousandths of a meter higher than designed.
- Treating European 50 × 100 as a US 2 × 4. They are off by 12 × 11 mm. For nailing patterns, hangers, and joist hangers this matters.
- Forgetting that fasteners are still primarily imperial in the US. Drywall screws, framing nails, and lag bolts come in #6, #8, 16d, 1/2", etc. Specifying an "M5 wood screw" on a US project will be expensive to source even when the product nominally exists.
- Reading a comma as a thousands separator. A European drawing showing "1,5" means 1.5 mm, not 15 mm and not 1,500. Always check the project's title-block notes for decimal-separator convention before pricing anything.
- Mixing drawing units on the same sheet. If the drawing is supposed to be metric, every dimension must be metric. Mixed-unit sheets are the leading single-source cause of dimension disputes on international projects.
The mindset shift
The hardest part of working from a metric drawing is not the arithmetic — it is internalizing a different unit feel. A wall is not "eight feet tall," it is "twenty-four hundred." A door head is not "six-eight," it is "twenty-one hundred." A stud bay is not "sixteen on center," it is "four hundred." After a few weeks on a metric job, most US carpenters and project managers stop translating in their heads and start thinking natively in millimeters for fine work and meters for layout — at which point the drawings stop fighting you and start telling you, in fewer characters, exactly what to build.
The goal is not to abandon imperial — your suppliers, your inspector, and your building code are all still partly there. The goal is to be bilingual: read the drawing in its own language, order the materials in theirs, and translate cleanly across the boundary so the building actually fits together.