MM to Fraction Inches Converter
Enter a size in millimeters to get the nearest fractional inch, from 1/2″ down to 1/64″, along with the exact decimal inches and how far off the fraction is. Choose round down or up when a part has to fit, and print the 1–100 mm chart.
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How to use the MM to Fraction Inches Converter
- Type the length in millimeters, or tap one of the example sizes.
- Pick the precision: 1/16″ matches most tape measures; 1/64″ suits drill bits.
- Choose rounding: nearest, or down/up when a part must fit or clear.
- Read the fraction and the error in mm, plus the same size at other precisions.
- Use the 1–100 mm chart below the result; download it as CSV or copy the results.
How to convert mm to fractional inches
Fractional inches are how US tape measures, rulers, drill bits and lumber are marked, so a metric size usually has to be rounded to a fraction rather than converted exactly. The method is the same for any precision:
- Divide by 25.4 to get decimal inches. The inch is defined as exactly 25.4 mm, so this step is exact. Example: 19 mm ÷ 25.4 = 0.74803 in.
- Multiply by the denominator you want (2, 4, 8, 16, 32 or 64). 0.74803 × 16 = 11.968.
- Round to a whole number: 11.968 → 12. That count is the number of 1/16ths: 12/16.
- Simplify by dividing top and bottom by their greatest common divisor: 12/16 = 3/4″.
- Check the error: 3/4″ = 0.75 in = 19.05 mm, so the fraction is 0.05 mm larger than 19 mm.
Use round down when the part has to fit inside an opening (a board in a slot, a bolt in a hole) and round up when you need at least the metric size, such as choosing a drill bit that clears a 6 mm dowel. The tool shows the error so you can judge whether the nearest fraction is close enough.
Common mm to fraction conversions
| Millimeters | Decimal inches | Nearest 1/16″ | Error (mm) | Nearest 1/64″ | Error (mm) |
|---|---|---|---|---|---|
| 1 mm | 0.0394 | 1/16″ | +0.59 | 3/64″ | +0.191 |
| 2 mm | 0.0787 | 1/16″ | -0.41 | 5/64″ | -0.016 |
| 3 mm | 0.1181 | 1/8″ | +0.17 | 1/8″ | +0.175 |
| 4 mm | 0.1575 | 3/16″ | +0.76 | 5/32″ | -0.031 |
| 5 mm | 0.1969 | 3/16″ | -0.24 | 13/64″ | +0.159 |
| 6 mm | 0.2362 | 1/4″ | +0.35 | 15/64″ | -0.047 |
| 8 mm | 0.3150 | 5/16″ | -0.06 | 5/16″ | -0.062 |
| 10 mm | 0.3937 | 3/8″ | -0.48 | 25/64″ | -0.078 |
| 12 mm | 0.4724 | 1/2″ | +0.70 | 15/32″ | -0.094 |
| 13 mm | 0.5118 | 1/2″ | -0.30 | 33/64″ | +0.097 |
| 15 mm | 0.5906 | 9/16″ | -0.71 | 19/32″ | +0.081 |
| 16 mm | 0.6299 | 5/8″ | -0.12 | 5/8″ | -0.125 |
| 19 mm | 0.7480 | 3/4″ | +0.05 | 3/4″ | +0.050 |
| 20 mm | 0.7874 | 13/16″ | +0.64 | 25/32″ | -0.156 |
| 22 mm | 0.8661 | 7/8″ | +0.22 | 55/64″ | -0.172 |
| 25 mm | 0.9843 | 1″ | +0.40 | 63/64″ | +0.003 |
| 30 mm | 1.1811 | 1 3/16″ | +0.16 | 1 3/16″ | +0.162 |
| 32 mm | 1.2598 | 1 1/4″ | -0.25 | 1 17/64″ | +0.147 |
| 35 mm | 1.3780 | 1 3/8″ | -0.07 | 1 3/8″ | -0.075 |
| 38 mm | 1.4961 | 1 1/2″ | +0.10 | 1 1/2″ | +0.100 |
| 40 mm | 1.5748 | 1 9/16″ | -0.31 | 1 37/64″ | +0.084 |
| 45 mm | 1.7717 | 1 3/4″ | -0.55 | 1 49/64″ | -0.153 |
| 50 mm | 1.9685 | 1 15/16″ | -0.79 | 1 31/32″ | +0.006 |
| 60 mm | 2.3622 | 2 3/8″ | +0.32 | 2 23/64″ | -0.072 |
| 75 mm | 2.9528 | 2 15/16″ | -0.39 | 2 61/64″ | +0.009 |
| 100 mm | 3.9370 | 3 15/16″ | +0.01 | 3 15/16″ | +0.012 |
How precise do you need to be?
| Precision | Step size | Worst-case error (nearest) | Typical use |
|---|---|---|---|
| 1/2″ | 12.7 mm | ±6.35 mm | Rough estimates |
| 1/4″ | 6.35 mm | ±3.18 mm | Framing lumber, garden work |
| 1/8″ | 3.175 mm | ±1.59 mm | Rough carpentry, fabric |
| 1/16″ | 1.5875 mm | ±0.79 mm | Finish carpentry, most tape measures |
| 1/32″ | 0.794 mm | ±0.40 mm | Woodworking, machinist rulers |
| 1/64″ | 0.397 mm | ±0.20 mm | Drill bit sizes, fine metalwork |
Below about 0.4 mm, fractions stop being practical; machinists switch to decimal inches (thousandths, "thou") instead.
Frequently asked questions
How do you convert mm to fractional inches?
Divide the millimeters by 25.4 to get decimal inches, multiply by the denominator you want (for example 16), round to a whole number and put it over that denominator. 10 mm ÷ 25.4 = 0.3937 in; × 16 = 6.3 → 6/16 = 3/8 inch.
What is 19 mm in inches as a fraction?
19 mm is 0.748 inches, which rounds to 3/4 inch at 1/16 precision. 3/4 inch is 19.05 mm, so it is only 0.05 mm larger.
What is 10 mm in fractional inches?
10 mm = 0.3937 in. The nearest 1/16 is 3/8 inch (9.525 mm, 0.475 mm short); the nearest 1/64 is 25/64 inch (9.922 mm).
Which precision should I use?
Use 1/16 inch for tape-measure work and general carpentry, 1/32 for fine woodworking, and 1/64 for drill bits and metalwork. Coarser fractions like 1/4 or 1/8 are fine for rough estimates.
Is 25 mm the same as 1 inch?
Not quite. 1 inch is exactly 25.4 mm, so 25 mm is 0.984 in, or 63/64 inch. At 1/16 precision it rounds to 1 inch with an error of 0.4 mm.
Why does the tool show an error?
Most metric sizes do not land exactly on a fraction of an inch. The error is the fraction minus the true size: positive means the fraction is larger than your mm value, negative means it is smaller.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.