Length
mm to Inches Converter
Convert millimeters to inches and back instantly — type in either box, with a reference chart for the sizes people look up most.
How to convert mm to inches
One inch is defined as exactly 25.4 millimeters, so converting either direction is a single step:
- inches = mm ÷ 25.4
- To go the other way: mm = inches × 25.4
For example, 100 mm ÷ 25.4 = 3.937 inches, and a 12-inch ruler is exactly 304.8 mm long. Type a value into either box above and the paired field, the 0–200 mm progress bar and the chart all update instantly — clear a box and the other clears too.
mm to inches conversion chart
These are the millimeter values people convert most often, from a single millimeter up to 200 mm. Handy for reading engineering drawings, screen and print specs, or hardware sizes that list only metric measurements:
| Millimeters (mm) | Inches (in) | Inches (approx.) |
|---|---|---|
| 1 mm | 0.0394" | ~1/32" |
| 5 mm | 0.1969" | ~3/16" |
| 10 mm | 0.3937" | ~3/8" |
| 25 mm | 0.9843" | ~1" |
| 50 mm | 1.9685" | ~2" |
| 100 mm | 3.9370" | ~4" |
| 150 mm | 5.9055" | ~6" |
| 200 mm | 7.8740" | ~8" |
Why 25.4 is the exact factor
Since the international yard-and-pound agreement of 1959, the inch has been fixed at exactly 25.4 millimeters, which makes this conversion precise rather than approximate. Because 1 cm = 10 mm, you could convert millimeters to centimeters first and then to inches, but dividing straight by 25.4 is quicker and just as exact. As a rough mental estimate, 25 mm is very close to one inch.
Frequently asked questions
How many inches is 1 mm?
1 mm equals 0.03937 inches. Divide the millimeter value by 25.4 to get inches.
How many mm is 1 inch?
1 inch equals 25.4 mm exactly, fixed by the international yard-and-pound agreement of 1959.
How do I convert inches to mm?
Multiply the number of inches by 25.4 to get millimeters.
Is mm to inches the same formula as cm to inches?
They are related but not identical. Since 1 cm = 10 mm, you can convert mm to cm then to inches, but dividing millimeters directly by 25.4 is faster.
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