Millimeters to Inches Calculator
Enter millimeters to get inches as a decimal and as a tape-measure fraction. Choose the nearest 1/16, 1/32 or 1/64 inch, or switch the direction to turn inches into millimeters. Exactly 25.4 mm make 1 inch.
Last updated November 3, 2026
0 or more, up to 1,000,000.
Result
10 mm = 0.3937 in ≈ 3/8 in
| Unit | Length |
|---|---|
| Millimeters | 10 |
| Centimeters | 1 |
| Inches (decimal) | 0.3937 |
| Inches (nearest 1/16) | 3/8 in |
| Fraction rounding error | 0.475 mm |
The fraction is reduced (8/16 shows as 1/2). A tape measure is usually marked in 1/16 inch, so check the finest marks on yours.
Show the work
Formula: inches = millimeters ÷ 25.4 (exact). Fraction = round(inches × denominator) ÷ denominator, reduced.
- You entered 10 mm.
- Decimal inches: 10 ÷ 25.4 = 0.3937 in.
- Rounding to the chosen fraction: 0.3937 × 16 = 6.2992, rounded to 6, so 6/16 in.
- Reduced and carried into whole inches: 3/8 in.
- Difference from the exact value: 0.475 mm.
Result: 10 mm = 0.3937 in ≈ 3/8 in
How millimeters convert to inches
inches = millimeters ÷ 25.4
millimeters = inches × 25.4
fraction = round(inches × n) ÷ n
Since the international yard and pound agreement of 1959, the inch has been defined as exactly 25.4 millimeters. The factor is not an approximation, so dividing by 25.4 gives the true decimal value and multiplying by 25.4 reverses it.
A tape measure does not show decimals, so the calculator also gives a fraction. It multiplies the decimal inches by the denominator you chose, 16, 32 or 64, and rounds to the nearest whole number. That number is the numerator. The fraction is then reduced, so 8/16 shows as 1/2 and 12/64 as 3/16.
If the rounded fraction reaches a full inch, the extra inch is carried into the whole number. For example, 0.99 inch rounded to the nearest 1/16 is 16/16, which is displayed as 1.
See how we calculate for the formulas, sources and review schedule behind every calculator.
Also useful alongside this calculator: KM to Miles Calculator and CM to Inches Calculator.
Choosing 1/16, 1/32 or 1/64 inch
Choose the precision that matches the marks on your ruler. Most tape measures show 1/16 inch, and woodworking rules often show 1/32. Machinist and engineering work may call for 1/64, though decimals or millimeters are common there too.
The finer the fraction, the smaller the rounding error. The result list shows that error in millimeters, so you can see what you give up. A 1/16 inch step is 1.5875 mm, a 1/32 step is 0.7938 mm and a 1/64 step is 0.3969 mm, which means the worst rounding error is half of that.
Common metric sizes in inches
Many hardware sizes are sold in millimeters, while plans may use inches. A 10 mm socket is 0.3937 inch, which is 3/8 inch to the nearest 1/16 and 25/64 inch to the nearest 1/64. A 6 mm bolt is 0.2362 inch, about 1/4 inch. A 19 mm board is 0.748 inch, which is 3/4 inch to the nearest 1/16.
Exact matches are rare. Only multiples of 25.4 mm, such as 25.4, 50.8 and 76.2 mm, land on whole inches. Fractions such as 1/2 inch need exactly 12.7 mm.
Converting inches back to millimeters
Set Direction to inches to millimeters and type decimal inches. To use a fraction such as 3/8, divide it first: 3 ÷ 8 = 0.375, so 0.375 inch is 9.525 mm.
The fraction line in the result then repeats your input at the precision you chose, which is a quick way to confirm that a decimal reading matches a mark on the ruler.
Edge cases and limits
The length must be a finite number from 0 up to 1,000,000. Negative values are rejected. Zero gives 0 inches.
Decimal results are rounded to four places for display only. The fraction is always calculated from the exact value, not from the rounded decimal, and an exact tie rounds up.
How to use the Millimeters to Inches Calculator
- Type the length you want to convert.
- Choose Millimeters to inches or Inches to millimeters under Direction.
- Pick Fraction precision: nearest 1/16, 1/32 or 1/64 inch.
- Read the decimal inches, the reduced fraction and the rounding error.
- Open Show the work for each step, or use Copy, Print or Share.
Worked example
A 10 mm socket is 10 ÷ 25.4 = 0.3937 inch. Multiplying by 16 gives 6.299, which rounds to 6, so the nearest 1/16 inch is 6/16, reduced to 3/8. At 1/64 the answer is 25/64.
| Millimeters | Inches | Nearest 1/16 | Nearest 1/64 |
|---|---|---|---|
| 3 | 0.1181 | 1/8 | 1/8 |
| 5 | 0.1969 | 3/16 | 13/64 |
| 6 | 0.2362 | 1/4 | 15/64 |
| 8 | 0.315 | 5/16 | 5/16 |
| 10 | 0.3937 | 3/8 | 25/64 |
| 12 | 0.4724 | 1/2 | 15/32 |
| 19 | 0.748 | 3/4 | 3/4 |
| 25.4 | 1 | 1 | 1 |
Frequently asked questions
How many millimeters are in an inch?
Exactly 25.4 millimeters make one inch. This is a definition, not a measurement, so the factor has no rounding error.
How many inches is 10 mm?
10 mm is 0.3937 inch. As a fraction it is 3/8 inch to the nearest 1/16, 13/32 to the nearest 1/32 and 25/64 to the nearest 1/64.
Why does the fraction not match the decimal exactly?
A fraction has to be a whole number of 1/16, 1/32 or 1/64 steps, so it is rounded. The result list shows the difference in millimeters.
How do I convert inches to millimeters?
Multiply by 25.4, or choose Inches to millimeters in the Direction menu. For example, 2 inches is 50.8 mm and 0.375 inch is 9.525 mm.
Which fraction precision should I use?
Use 1/16 for a standard tape measure, 1/32 for finer woodworking rules and 1/64 when you need the closest fractional match to a metric size.