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Why Engineers Use Millimeters Instead of Centimeters

June 16, 2025 โ€ข 4 min read

If you have ever looked at an engineering drawing, you will notice all dimensions are in millimeters โ€” not centimeters, not meters, not inches. A bracket might be labelled "45.5" without any unit, and everyone in the field knows it means 45.5 mm. Why is this the convention?

The Practical Reasons

  • โ–ถ Whole numbers for most parts: Most manufactured parts have dimensions between 1 and 1000 mm. Using mm means you rarely need decimal points for standard tolerance work (a 25 mm bolt, not 2.5 cm or 0.025 m)
  • โ–ถ Eliminates decimal confusion: Writing "2.5 cm" risks misreading the decimal as "25 cm" โ€” a 10x error. Writing "25 mm" has no ambiguity
  • โ–ถ One unit throughout: When everything is in mm, you never mix up cm and m on the same drawing
  • โ–ถ International standard: ISO 129 and ISO 80000 specify mm as the preferred unit for engineering drawings

The Decimal Problem

Consider three ways to write the same dimension:

  • โ–ถ 0.0375 m โ€” four decimal places, easy to misread
  • โ–ถ 3.75 cm โ€” requires a decimal point
  • โ–ถ 37.5 mm โ€” clean, readable, fewer digits

In a workshop environment with grease, poor lighting, and time pressure, the clearest notation wins. Millimeters give you whole numbers or simple one-decimal values for almost everything you would machine, print, or fabricate.

When Precision Goes Further

For high-precision work, engineers use decimal millimeters:

  • โ–ถ Standard machining: ยฑ0.1 mm tolerance
  • โ–ถ Precision machining: ยฑ0.01 mm tolerance
  • โ–ถ Grinding/lapping: ยฑ0.001 mm (1 micrometre)

Even at micrometre precision, it is written as 25.003 mm rather than switching to a different unit. Consistency reduces errors.

What About Other Industries?

  • โ–ถ Architecture: Uses mm in metric countries (a wall might be 2400 mm tall)
  • โ–ถ Electronics: PCB design uses mm (component pads at 0.5 mm pitch)
  • โ–ถ 3D Printing: Slicers and CAD software default to mm
  • โ–ถ Construction (US): Still uses feet, inches, and fractions (1/16")
  • โ–ถ Aerospace: Often uses inches in the US, mm elsewhere

Converting Between Units

  • โ–ถ 1 mm = 0.1 cm = 0.001 m
  • โ–ถ 1 mm = 0.03937 inches
  • โ–ถ 1 inch = 25.4 mm (exact)
  • โ–ถ 10 mm = 1 cm

The relationship between mm and inches is especially important because many legacy parts are still dimensioned in imperial. The exact conversion (25.4 mm per inch) makes dual-dimensioning straightforward.

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