Last updated July 28, 2026
A CAD drawing has no units. It has numbers. The units live in an agreement between everyone who touches the file, and when that agreement breaks you get a door 900 millionths of a millimetre wide, or a site plan that arrives a thousand times too small.
This is the convention for metric work, and what to do when a file turns up and you cannot tell what it is in.
The rule: millimetres for almost everything
The millimetre is the primary unit for metric CAD across most disciplines:
- Mechanical engineering — parts, assemblies and machinery, where tolerances are specified in fractions of a millimetre.
- Architectural detailing — construction details, joinery, fixtures and fittings.
- Structural detailing — steel and concrete, where every standard section and bar size is quoted in mm.
- Product design — the same reasoning as mechanical.
The reason is decimal points. In millimetres, most building and machine dimensions are whole numbers — 90, 450, 2400, 3600. A decimal point is a place an error can hide, and a dimension without one cannot be misread by a factor of ten.
Metres, for the big picture
- Architectural plans — overall floor plans, elevations and sections, where 10.5 m reads better than 10500. Detail blow-ups within the same set usually revert to millimetres.
- Site plans and landscaping — boundaries, building placement, levels.
- Civil engineering — road layouts, drainage, earthworks.
Note the split: on many jobs the drawing is modelled in millimetres and only the annotation is in metres. Levels are quoted as 41.250 while the geometry sits at 41250. That is normal and it is not a units conflict — but it does mean “what units is this drawing in” and “what units are the numbers on it” can have different answers.
Kilometres, and centimetres
Kilometres appear on major infrastructure — highways, pipelines, rail, large surveys — for overall layout and chainage, not for drawing geometry.
Centimetres are avoided, and it is worth being firm about it. They sit between the two units everyone else is using, they force decimals on detail work, and a drawing that mixes cm with mm produces errors of exactly the factor of ten that is easiest to miss on a check. Some furniture and interiors work uses them; formal drafting practice does not.
How to tell what units a drawing is in
You will be handed files with no note and no one to ask. The fastest checks, in order:
- Measure something you know. A standard door leaf, a brick, a road lane, a parking bay. If a door measures 900 you are in millimetres; 0.9 and you are in metres. This takes five seconds and beats every other method.
- Check an existing dimension. If the drawing is dimensioned, the dimension text tells you what the author intended, and comparing it against the measured distance tells you whether a scale factor has crept in.
- Look at INSUNITS. This is the drawing’s declared unit, used when it is inserted into or referenced by another drawing. It is a declaration, not a measurement — it can be set to Millimeters on a drawing modelled in metres, and often is.
Trust the tape measure over the setting. INSUNITS records what someone said; measuring records what is there.
Where mismatched units actually bite
Not usually in the drawing you are working on. It is at the boundaries.
- Inserting a block. AutoCAD scales an inserted block by the ratio of its INSUNITS to the drawing’s. Two files that disagree give you a block a thousand times too big or too small — the classic symptom of a metre-based block landing in a millimetre drawing.
- Attaching an xref. Same mechanism, larger consequence, because the whole consultant drawing arrives at the wrong scale and it is not always obvious at first glance.
- Importing survey or GIS data. Almost always in metres, almost always going into a millimetre drawing.
- Round-tripping to a different package. Revit, SolidWorks and the rest each have their own convention, and the exporter’s assumption is where the factor of a thousand appears.
The habit that prevents most of this: set INSUNITS honestly on every drawing you issue, even though it changes nothing you can see. It costs nothing and it is the only thing that tells the next file what yours is.
Changing units on an existing drawing
Two different jobs that are easy to confuse:
- UNITS changes how numbers are displayed and the drawing’s declared insertion unit. It does not touch the geometry. Use it to correct a wrong declaration.
- Scaling the geometry is what you need if the model really is in the wrong units — and it has to take the annotation, dimension styles and text heights with it, or you will have a correct model covered in wrong-sized text.
Do the second one on a copy. It is the change most likely to look fine and be wrong.
Standards and practice
- ISO provides the foundations — ISO 128 for presentation in technical drawings, ISO 5455 for recommended scales.
- National standards adapt them: AS 1100 in Australia, ASME in the United States.
- Consistency beats correctness. Once a primary unit is chosen for a drawing it should be used throughout. Where a sheet genuinely needs both — a site plan in metres with a detail in millimetres — say so on the sheet, next to the dimensions, not in a general note nobody reads.
- Check the project standard first. If one exists, it outranks everything above.
Draw at 1:1 in your chosen unit and scale only for plotting. Drawing at a scale is a habit from the drawing board and it makes everything downstream harder.
References
- Loading acadiso.lin by default — the related metric/imperial setting for linetypes and hatch patterns
- Standard paper sizes
- Reduction ratios and scale change
- CAD line weights
- Standards Australia — AS 1100.101, Technical drawing: General principles