Convert Millimeters to Thou
1 millimeter is 39.37 thou. One millimeter is exactly 39.3700787402 thou.
One millimeter is 39.370079 thou. To convert millimeters to thou, multiply by 39.370079; to go the other way, divide by 39.370079 or multiply by 0.0254. The relation is exact but the figure is not: it is a rounding of a number that does not terminate, and it follows from NIST. One thousandth of a meter.
Factor checked against the defining standard · How we check
- Millimeters × 39.370079
1 * 39.37007939.37 thou
1 millimeter in every length unit
| Unit | Value | Exact factor |
|---|---|---|
| inches | 0.03937007874 in | × 0.0393700787402 |
| feet | 0.003280839895 ft | × 0.00328083989501 |
| yards | 0.001093613298 yd | × 0.00109361329834 |
| miles | 0.0000006213711922 mi | × 0.000000621371192237 |
| millimeters (as entered) | 1 mm | — |
| decimetres | 0.01 dm | × 0.01 |
| centimeters | 0.1 cm | × 0.1 |
| meters | 0.001 m | × 0.001 |
| kilometers | 0.000001 km | × 0.000001 |
| nautical miles | 0.0000005399568035 nmi | × 0.000000539956803456 |
| micrometers | 1,000 µm | × 1,000 |
| nanometers | 1,000,000 nm | × 1,000,000 |
| thou | 39.37007874 thou | × 39.3700787402 |
Factors are entered by hand from NIST rather than derived from a units library, so the ones that terminate are exact rather than rounded.
The same converter answers this pair however it is written: millimetres to thou is the British spelling of the same units — the metre and the litre are spelled that way outside the United States and the quantity does not change.
How do you convert millimeters to thou? The converter and the working
- 1.Take the measurement in millimeters.
- 2.Multiply by 39.370079 — that is how many thou fit in one millimeter.
- 3.To go back the other way, divide by 39.370079, or multiply by 0.0254.
Worked example: 25 millimeters to thou
- Formula
- mm × 39.370079 = thou
- Arithmetic
- 25 × 39.370079 = 984.25 thou
- Answer
- 25 mm = 984.25 thou
The factor is rounded for display; the figure above is computed from the full value.
Millimeters to thou conversion table
| Millimeters | Thou |
|---|---|
| 1 mm | 39.37 thou |
| 2 mm | 78.74 thou |
| 3 mm | 118.11 thou |
| 4 mm | 157.48 thou |
| 5 mm | 196.85 thou |
| 6 mm | 236.22 thou |
| 7 mm | 275.59 thou |
| 8 mm | 314.96 thou |
| 9 mm | 354.33 thou |
| 10 mm | 393.7 thou |
| 12 mm | 472.44 thou |
| 15 mm | 590.55 thou |
| 20 mm | 787.4 thou |
| 25 mm | 984.25 thou |
| 50 mm | 1,968.5 thou |
| 100 mm | 3,937.01 thou |
| 200 mm | 7,874.02 thou |
| 300 mm | 11,811.02 thou |
| 500 mm | 19,685.04 thou |
| 1000 mm | 39,370.08 thou |
| 2000 mm | 78,740.16 thou |
| 5000 mm | 196,850.39 thou |
| 10000 mm | 393,700.79 thou |
A mil is a thousandth of an inch, except where it means a millimeter
In an American workshop a mil is 0.001 in — 25.4 micrometers — and the same drawing read in Europe may be taken as a millimeter, forty times larger. The unambiguous word is thou. Plastic film, plating thickness and printed-circuit copper are all specified in it, and the ambiguity is why exported drawings usually state micrometers instead.
NIST Handbook 44, Appendix C — general tables of units of measurement
What is a millimeter?
One thousandth of a meter. The working unit of metric engineering drawings, which avoid decimals by using it throughout.
What is a thou?
A thousandth of an inch, exactly 25.4 micrometers, and called a mil in American workshops — the same word the metric world uses for a millimeter, which is why a drawing marked "5 mil" is worth checking twice.
How many thou are in a millimeter?
One millimeter is 39.370079 thou. Going the other way, one thou is 0.0254 millimeters. Both figures come from the same defined relation rather than from a measurement, which is why they are the same today as they were in 1959.
Every length unit here
The whole group — 13 units with their symbols and the size of each one in meters — is on the conversion index, along with every pair it makes. The table above already gives each of them relative to a millimeter.
Source: NIST — international yard and pound agreement (inch = 25.4 mm exactly)
Cite this page
- APA
- Rule Calculator. (2026). Convert Millimeters to thou. Rule Calculator. https://rulecalculators.com/convert/millimeter-to-thou
- MLA
- "Convert Millimeters to thou." Rule Calculator, August 16, 2026, https://rulecalculators.com/convert/millimeter-to-thou.
- Chicago
- Rule Calculator. "Convert Millimeters to thou." Rule Calculator. Last reviewed August 16, 2026. https://rulecalculators.com/convert/millimeter-to-thou.
The date is when the figures on this page were last checked against their source, not the day you opened it. Add your own access date if your style needs one.
Questions people actually ask
- How many thou are in a millimeter?
- 39.370079. A thousandth of an inch, exactly 25.4 micrometers, and called a mil in American workshops — the same word the metric world uses for a millimeter, which is why a drawing marked "5 mil" is worth checking twice.
- How do I convert thou back to millimeters?
- Divide by 39.370079, or multiply by 0.0254 — they are the same operation.
Other length conversions
Thou to millimeters — the same conversion the other way.