MM to Inches Converter
Millimeters ⇄ inches, decimals & fractions
How to Use
- 1Type the length you want to convert.
- 2Pick the direction: mm to inches, or inches to mm.
- 3Choose how many decimal places you need.
- 4Read the result plus the nearest ruler fraction (e.g. 15/16″).
Conversion Formula
- Exact definition1 inch = 25.4 mm
- mm → inchesin = mm ÷ 25.4
- Inches → mmmm = in × 25.4
- Quick estimate1 mm ≈ 0.0394 in (≈ 1/25 in)
Quick Examples
Tap an example to load it above.
Everything about MM to Inches
The exact 25.4 rule, ruler fractions, and the values worth memorising.
Millimeters and inches meet everywhere: screws and wrenches, TV screens, paper sizes, 3D-printer parts and plumbing fittings. The conversion is exact by definition: since 1959, one inch equals precisely 25.4 millimeters. This mm to inches converter applies that rule both ways and also gives the nearest ruler fraction, because imperial measurements are usually written in 16ths.
Rule of thumb: divide mm by 25 for a quick estimate, so 50 mm ≈ 2 inches. For the exact answer, divide by 25.4: 50 ÷ 25.4 = 1.9685 in.
How the conversion works
To convert millimeters to inches, divide by 25.4: 100 mm ÷ 25.4 = 3.9370 in. To go back, multiply: 3.9370 × 25.4 = 100 mm. Because 25.4 is exact, no accuracy is ever lost, only the rounding you choose.
Common values worth remembering
| Millimeters | Inches (decimal) | Inches (fraction) |
|---|---|---|
| 1 mm | 0.0394 in | ≈ 3/64″ |
| 5 mm | 0.1969 in | ≈ 13/64″ |
| 10 mm | 0.3937 in | ≈ 25/64″ |
| 12.7 mm | 0.5 in | 1/2″ exactly |
| 25.4 mm | 1 in | 1″ exactly |
| 50 mm | 1.9685 in | ≈ 1 31/32″ |
| 100 mm | 3.9370 in | ≈ 3 15/16″ |
| 300 mm | 11.8110 in | ≈ 11 13/16″ |
Tips for accurate work
- 1Round at the end
If a job needs several conversions, keep full precision throughout and round only the final answer.
- 2Know your tolerance
Woodwork is fine at 1/16″ (≈1.6 mm); machining often needs 0.001″ (0.0254 mm). Choose decimals to match.
- 3Watch fastener sizes
M6 ≈ 15/64″ and 1/4″ ≈ 6.35 mm, close but NOT interchangeable. Convert before you buy.
- 4Use fractions for rulers
Imperial tape measures mark 16ths, and the converter's fraction result maps straight onto the tape.
Frequently Asked Questions
How many inches is 1 mm?
1 mm = 0.0394 inches (exactly 1 ÷ 25.4). As a fraction that is about 3/64 of an inch, roughly the thickness of a credit card.
How do I convert mm to inches by hand?
Divide the millimeter value by 25.4. Example: 80 mm ÷ 25.4 = 3.1496 inches. To convert inches to mm, multiply instead: 3.15 × 25.4 = 80.01 mm.
Is 25.4 mm per inch exact or approximate?
Exact. Since the international yard agreement of 1959, one inch is defined as exactly 25.4 mm, so conversions are perfectly precise and the only error comes from rounding the answer.
How many mm are in an inch, half inch and quarter inch?
1″ = 25.4 mm, 1/2″ = 12.7 mm, 1/4″ = 6.35 mm, 1/8″ = 3.175 mm and 1/16″ = 1.5875 mm. These five cover most workshop situations.
What is 100 mm in inches?
100 mm = 3.9370 inches, just under 4 inches (about 3 15/16″). Similarly 10 cm, since 10 mm = 1 cm.
Is this mm to inches converter free and does it work on mobile?
Yes. Completely free, no sign-up, works in any modern browser on phones, tablets and desktops. Calculations run locally in your browser and nothing is stored.