About the MM to Inches Converter
Metric and imperial measurements meet in the workshop: drill bits, bolts, plumbing parts and wood are sold in millimetres in some places and in inches in others. This converter changes millimetres to inches, or inches to millimetres, and also finds the nearest fraction of an inch that you can find on a tape measure or a drill set: eighths, sixteenths, thirty-seconds and sixty-fourths. For each fraction it tells you how far it is from the exact value in millimetres, so you can judge whether it is close enough for your job. A table of common sizes is included.
Updated October 2026
How to use the MM to Inches Converter
- 1Choose millimetres to inches or inches to millimetres.
- 2Enter the measurement.
- 3Read the converted value in the large result.
- 4Check the nearest fractions and the errors.
- 5Use the table for common sizes.
Common sizes
- 3 mm = 0.118 in (about 1/8 in).
- 6 mm = 0.236 in (about 1/4 in).
- 10 mm = 0.394 in.
- 12 mm = 0.472 in (about 15/32 in).
- 19 mm = 0.748 in (about 3/4 in).
- 25 mm = 0.984 in (about 1 in).
Working across systems
Never mix metric and imperial parts when an exact fit is needed, such as bolts and nuts. A 1/4 inch bolt will go into a 6 mm hole loosely, and it may not thread at all. Buy parts for the system your project was designed in, and use conversion only for estimating or for choosing the closest sensible stock size.
Choose your accuracy to suit the work. In joinery, a sixteenth of an inch, 1.6 mm, is often the finest practical mark on a tape, so the nearest sixteenth is usually enough. For metalwork and machining, thousandths of an inch or hundredths of a millimetre matter, and you should use the decimal results and work in one system only. When you must mix systems, convert every measurement of the part, not just the one you are cutting, and keep notes of which numbers came from which unit.
Drill bits give a good example. A 6.5 mm bit is 0.2559 inches, close to 1/4 inch (0.25) but a little bigger, and 8 mm is 0.3150 inches, between 5/16 (0.3125) and 21/64 (0.328). Sets in each system overlap but rarely match, which is why a tradesperson with a metric set may still keep a few imperial bits. The error column shows how close the nearest imperial fraction is, so you can pick the bit that gives the fit you want.
Reading fractions
Sixteenths are the smallest division on most tape measures. Thirty-seconds and sixty-fourths appear on precision rules and drill bit sets. To simplify, reduce the fraction: 8/16 is 1/2, and 12/64 is 3/16. The calculator reduces the fraction for you so that it matches the markings on the tool.
Related tools
The Inches to CM Converter accepts fractions as input, the Feet to Meters Converter converts longer lengths, the Wire Gauge Calculator relates gauge numbers to millimetres, and the Unit Converter handles other units.
Features
- Both directions
- Fractions to 1/8, 1/16, 1/32 and 1/64
- Error in millimetres for each fraction
- Table of common sizes
- Exact 25.4 factor
- Free and private
Frequently asked questions
How do I convert mm to inches?
Divide the millimetres by 25.4. A 19 mm measurement is 0.748 inches, very close to 3/4 inch (0.75).
How do I convert inches to mm?
Multiply the inches by 25.4. 1/4 inch is 6.35 mm and 1 inch is exactly 25.4 mm.
What is 10 mm in inches?
0.3937 inches, between 25/64 (0.3906) and 13/32 (0.4063). The nearest 1/16 is 3/8 inch, though a 10 mm hole is a little larger than 3/8 inch (9.525 mm).
Why do I need a fraction?
Imperial tools such as spanners, drill bits and rulers use fractions of an inch. Knowing the nearest fraction tells you which one to pick. If the error is larger than your tolerance, choose a metric tool instead.
How close is close enough?
It depends on the job. For rough carpentry, an error of 0.5 mm is fine. For machine fits, you need tenths of a millimetre, and a fraction won't do. The error column shows the difference.
What is the thread size 6 mm in inches?
6 mm is about 0.236 inches, near 1/4 inch (6.35 mm), but the two are not interchangeable for threaded fasteners.