Fraction To mm Converter
A plan calls for a 5/16 inch hole, your drill set is metric, and the question is which millimetre size comes closest.
Quick answer: turn the fraction into a decimal and multiply by 25.4. So 1/16 in is 1.5875 mm, 1/8 in is 3.175 mm, 5/16 in is 7.9375 mm and 1/2 in is 12.7 mm.
The inch fraction to millimetres formula
Because one inch is exactly 25.4 millimetres, a fraction of an inch converts by multiplying the fraction as a decimal by 25.4.
If the size has whole inches too, add them to the fraction first, such as 1 3/8 for 1 + 3/8.
For 5/16 in, 5 ÷ 16 is 0.3125, and 0.3125 × 25.4 is 7.9375 mm.
For 1 3/8 in, 1 + 3/8 is 1.375, and 1.375 × 25.4 is 34.925 mm.
Fractions of an inch and the millimetre
American tools, drill bits, wrenches and lumber are marked in fractions of an inch, usually down to 1/16, 1/32 or 1/64. The millimetre is the working unit for metric parts, drawings and imported hardware.
The inch is defined as exactly 25.4 mm, a value fixed in the 1959 agreement between the United States, the United Kingdom, Australia, Canada, New Zealand and South Africa (NIST: SI units, length; Wikipedia: International yard and pound).
That exactness means the conversion has no rounding error, only the rounding you choose when you display the millimetres.
How to convert fractions to mm step by step
- Write the size as whole inches plus a fraction.
- Divide the numerator by the denominator and add the whole inches.
- Multiply the decimal by 25.4.
- Round to two decimals for fitting or four for machining.
Example 1: a 5/16 inch drill
- Start with 5/16 in.
- 5 ÷ 16 = 0.3125.
- 0.3125 × 25.4 = 7.9375.
5/16 in = 7.9375 mm
Example 2: a 3/4 inch pipe
- Start with 3/4 in.
- 3 ÷ 4 = 0.75.
- 0.75 × 25.4 = 19.05.
3/4 in = 19.05 mm
Example 3: a mixed number
- Start with 1 3/8 in.
- 1 + 3 ÷ 8 = 1.375.
- 1.375 × 25.4 = 34.925.
1 3/8 in = 34.925 mm
The 5/16 in drill at 7.9375 mm sits very close to an 8 mm metric bit, which is why the two are often swapped when a precise fit is not required.
fractions to mm conversion table
| Fraction | Decimal inches | Millimetres |
|---|---|---|
| 1/64 in | 0.015625 in | 0.3969 mm |
| 1/32 in | 0.03125 in | 0.7938 mm |
| 1/16 in | 0.0625 in | 1.5875 mm |
| 3/32 in | 0.09375 in | 2.3813 mm |
| 1/8 in | 0.125 in | 3.175 mm |
| 5/32 in | 0.15625 in | 3.9688 mm |
| 3/16 in | 0.1875 in | 4.7625 mm |
| 1/4 in | 0.25 in | 6.35 mm |
| 5/16 in | 0.3125 in | 7.9375 mm |
| 3/8 in | 0.375 in | 9.525 mm |
| 7/16 in | 0.4375 in | 11.1125 mm |
| 1/2 in | 0.5 in | 12.7 mm |
| 9/16 in | 0.5625 in | 14.2875 mm |
| 5/8 in | 0.625 in | 15.875 mm |
| 11/16 in | 0.6875 in | 17.4625 mm |
| 3/4 in | 0.75 in | 19.05 mm |
| 13/16 in | 0.8125 in | 20.6375 mm |
| 7/8 in | 0.875 in | 22.225 mm |
| 15/16 in | 0.9375 in | 23.8125 mm |
| 1/1 in | 1 in | 25.4 mm |
The highlighted row is 1/16 in, the smallest step on many tape measures. The last row shows the exact 25.4 mm for one inch.
A quick mental shortcut for fractions to millimetres
Remember that 1/16 in is about 1.6 mm, then count the sixteenths. For 5/16 in, five times 1.6 is 8 mm, against an exact 7.94 mm.
For eighths, multiply 3.2 mm by the numerator, and for quarters multiply 6.35 mm.
Fractions to mm for drill bits and hardware
Drill sets, socket sets and fasteners come in both fractional and metric sizes, and the sizes often fall close without matching. A 1/4 in bit is 6.35 mm, close to a 6 mm bit, and a 3/8 in bit is 9.525 mm, near a 9.5 mm one.
Always test the fit when the difference matters.
More worked examples
3/8 inch is 9.525 mm, so a 9.5 mm bit is very close. 3/16 inch is 4.7625 mm, so a 4.8 mm bit is close.
Choosing a bit
For a clearance hole, choose the next size up. For a pilot hole, choose the next size down.
Check the fit with a test piece before you drill the real part.
A drill set example
A 5/16 inch hole is 7.9375 mm. An 8 mm bit is about 0.06 mm larger, so the hole would be slightly loose for a snug fit.
Check your answer by converting back
Divide the millimetres by 25.4. 9.525 mm divided by 25.4 is 0.375, which is 3/8 inch.
Quick conversion answers
| Conversion | Result |
|---|---|
| 1/16 inch to mm | 1.5875 mm |
| 1/8 inch to mm | 3.175 mm |
| 1/4 inch to mm | 6.35 mm |
| 5/16 inch to mm | 7.9375 mm |
| 3/8 inch to mm | 9.525 mm |
| 1/2 inch to mm | 12.7 mm |
| 3/4 inch to mm | 19.05 mm |
| 1 inch to mm | 25.4 mm |
Where this conversion is used
Mechanics and woodworkers convert fractional bit and bolt sizes to millimetres for metric tooling.
Builders and makers translate American plan dimensions into metric for imported materials.
Students and engineers use it for unit conversion and tolerance checks.
Common mistakes to avoid
- Forgetting the whole inches. 1 3/8 in is 1.375 in, not 0.375 in.
- Dividing by 25.4. That converts millimetres to inches.
- Using 25 as the factor. It is about 1.6 percent low.
- Swapping numerator and denominator. 5/16 is 0.3125, not 3.2.
Related converters
- Millimeters to fractions
- Inches to decimal
- Decimal to inches
- Inches to millimeters
- Optical Zoom to mm
Frequently asked questions
How do I convert a fraction of an inch to mm?
Turn the fraction into a decimal and multiply it by 25.4.
How many mm is 1/16 inch?
1/16 in is exactly 1.5875 mm.
How many mm is 5/16 inch?
5/16 in is 7.9375 mm.
How do I convert 1 3/8 inches to mm?
Add 1 and 3/8 to get 1.375 in, then multiply by 25.4 to get 34.925 mm.
Why are fractions used for inch measurements?
Tape measures, bits and lumber are marked in fractions, which are easier to read than long decimals.
Is the conversion exact?
Yes. The inch is exactly 25.4 mm, so the only rounding is in how many decimals you show.
Sources
- NIST: SI units, length (nist.gov)
- Wikipedia: International yard and pound (en.wikipedia.org)
- Inch Calculator: Decimal to fraction calculator (inchcalculator.com)
