Millimeters to inches is essential for anyone working with metric engineering drawings, imported hardware, or specifications from international manufacturers. Bearings, fasteners, electronic components, and 3D printer nozzles all come spec’d in mm, but if your tooling is imperial, you need the inch equivalent. The division by 25.4 is exact, but fractional approximations matter for selecting the closest standard tool.
How to Calculate Millimeters to Inches (Step by Step)
Step One: Divide millimeters by 25.4
Since 1 inch = exactly 25.4 mm, dividing any millimeter value by 25.4 gives the precise inch equivalent.
Inches = Millimeters / 25.4
Step Two: Worked example with default value (150 mm)
Inches = 150 / 25.4 = 5.905512 in
150 millimeters is approximately 5.906 inches, or just under 5 and 29/32 inches.
Step Three: Find the nearest standard fraction
For selecting tools or matching hardware, round to the nearest 16th or 32nd:
Nearest 16th = round(5.905512 x 16) / 16 = round(94.488) / 16 = 94/16 = 5.8750 in
Nearest 32nd = round(5.905512 x 32) / 32 = round(188.976) / 32 = 189/32 = 5.90625 in
The 32nd approximation (5.90625”) is within 0.001” of the true value, plenty close for most applications.
What Your Results Mean
The Inches (decimal) result is your precise conversion with six decimal places for CNC programming and calculations. The Nearest 1/16 inch and Nearest 1/32 inch results show the closest standard fractional sizes, which is what you’d actually select from a drill index or measure on a ruler.
Understanding precision loss
Converting mm to fractional inches always introduces some rounding. The maximum error for each precision level:
- 16ths: +/-0.794 mm (+/-1/32 inch)
- 32nds: +/-0.397 mm (+/-1/64 inch)
- 64ths: +/-0.198 mm (+/-1/128 inch)
For precision machining where you need better than +/-0.01”, use the decimal inch value directly rather than rounding to a fraction.
Common Metric-to-Imperial Hardware References
| Metric (mm) | Decimal inches | Nearest fraction |
|---|---|---|
| 1 | 0.0394 | 1/32 |
| 2 | 0.0787 | 5/64 |
| 3 | 0.1181 | 1/8 |
| 4 | 0.1575 | 5/32 |
| 5 | 0.1969 | 13/64 |
| 6 | 0.2362 | 15/64 |
| 7 | 0.2756 | 9/32 |
| 8 | 0.3150 | 5/16 |
| 9 | 0.3543 | 23/64 |
| 10 | 0.3937 | 25/64 |
| 11 | 0.4331 | 7/16 |
| 12 | 0.4724 | 15/32 |
| 13 | 0.5118 | 33/64 |
| 14 | 0.5512 | 9/16 |
| 15 | 0.5906 | 19/32 |
| 16 | 0.6299 | 5/8 |
| 17 | 0.6693 | 43/64 |
| 18 | 0.7087 | 23/32 |
| 19 | 0.7480 | 3/4 |
| 20 | 0.7874 | 25/32 |
| 22 | 0.8661 | 7/8 |
| 24 | 0.9449 | 15/16 |
| 25 | 0.9843 | 1 |
| 30 | 1.1811 | 1 3/16 |
| 32 | 1.2598 | 1 1/4 |
| 35 | 1.3780 | 1 3/8 |
| 38 | 1.4961 | 1 1/2 |
| 40 | 1.5748 | 1 9/16 |
| 45 | 1.7717 | 1 3/4 |
| 50 | 1.9685 | 2 |
| 60 | 2.3622 | 2 3/8 |
| 65 | 2.5591 | 2 9/16 |
| 70 | 2.7559 | 2 3/4 |
| 75 | 2.9528 | 2 15/16 |
| 80 | 3.1496 | 3 5/32 |
| 90 | 3.5433 | 3 17/32 |
| 100 | 3.9370 | 3 15/16 |
For the reverse conversion (imperial to metric), use the inches to mm converter. If you’re starting with a fractional inch measurement and need to compare it to a metric spec, the inches fraction to decimal calculator gives you the decimal inch first. For larger measurements, the meters to inches converter may be more appropriate.