Millimetres to inches converter
Drawing sets moving between metric and imperial offices are the single most common source of dimension errors in CAD. One inch is exactly 25.4 mm — the conversion is defined, not measured — so the only real risk is rounding. This converter shows the exact decimal value plus the nearest 1/16in fraction that US construction documents normally use.
mm to inches converter
- Inches
- 39.3701in
- Nearest 1/16 inch
- 39 3/8"
- Feet and inches
- 3' 3.37"
- Centimetres
- 100cm
Formula
inches = millimetres ÷ 25.425.4 is exact by international definition (1959 international yard and pound agreement), so no precision is lost in the conversion itself.
Worked examples
| Input | Result |
|---|---|
| 2400 mm door height | 94.4882 in ≈ 7'-10 1/2" |
| 300 mm module | 11.811 in ≈ 11 13/16" |
| 12 mm plasterboard | 0.4724 in ≈ 15/32" |
Questions
How many millimetres are in an inch?
Exactly 25.4 mm. It is a defined value, not a measurement, so 1 in = 25.4 mm holds at any precision.
Should I round metric dimensions to whole inches on a drawing?
No. Round to the nearest 1/16 in on architectural sheets and to three decimal places on machined parts. Rounding a 2400 mm dimension to 94 in loses almost 12 mm — enough to break a door schedule.
How do I set inches in a metric AutoCAD drawing?
Leave the model in millimetres and change only the dimension style: set DIMLFAC to 1/25.4 (0.03937) on an inch-annotated layout, or use a separate imperial dimension style. Rescaling the geometry itself is what corrupts coordinated models.