Decimal and fractional length
MM to Inches Converter
Convert millimeters to decimal and fractional inches for hardware, parts, and fabrication references. Values stay on this device.
How to convert mm to inches
The standard inch is exactly 25.4 millimeters. Decimal conversion is exact before display rounding; a listed fraction is the nearest step at the selected denominator.
inches = millimeters ÷ 25.4
millimeters = inches × 25.4
1 inch = 25.4 millimeters exactly
Example: 10 mm ÷ 25.4 = 0.393700… in. Of the displayed options, 25/64 in is closest: it is 0.003076 in (0.0781 mm) under the exact value.
When decimal and fractional inches help
A decimal result preserves the calculation, while common fractions help compare inch-marked tools and catalogs.
Fasteners, drill bits, and hand tools
Compare a metric diameter with nearby inch sizes before checking the manufacturer's fit, clearance, and thread specification.
3D printing and digital fabrication
Translate model dimensions for an inch-based workspace while keeping decimal inches available for software input.
Electronics and small parts
Review enclosures, spacers, lenses, and component dimensions when a supplier mixes metric and U.S. customary units.
Common mm to inches and fractions chart
Decimal inches are rounded to four places. Each fraction column is rounded independently to the nearest named resolution.
| Millimeters (mm) | Inches (in) | Nearest 1/16 in | Nearest 1/32 in | Nearest 1/64 in |
|---|---|---|---|---|
| 0.1 | 0.0039 | 0 in | 0 in | 0 in |
| 0.5 | 0.0197 | 0 in | 1/32 in | 1/64 in |
| 1 | 0.0394 | 1/16 in | 1/32 in | 3/64 in |
| 2 | 0.0787 | 1/16 in | 3/32 in | 5/64 in |
| 3 | 0.1181 | 1/8 in | 1/8 in | 1/8 in |
| 5 | 0.1969 | 3/16 in | 3/16 in | 13/64 in |
| 6 | 0.2362 | 1/4 in | 1/4 in | 15/64 in |
| 8 | 0.3150 | 5/16 in | 5/16 in | 5/16 in |
| 10 | 0.3937 | 3/8 in | 13/32 in | 25/64 in |
| 12 | 0.4724 | 1/2 in | 15/32 in | 15/32 in |
| 16 | 0.6299 | 5/8 in | 5/8 in | 5/8 in |
| 20 | 0.7874 | 13/16 in | 25/32 in | 25/32 in |
| 25 | 0.9843 | 1 in | 31/32 in | 63/64 in |
| 25.4 | 1.0000 | 1 in | 1 in | 1 in |
| 50 | 1.9685 | 1 15/16 in | 1 31/32 in | 1 31/32 in |
| 100 | 3.9370 | 3 15/16 in | 3 15/16 in | 3 15/16 in |
How the nearest inch fraction is chosen
For 1/16-inch output, the decimal measurement is multiplied by 16 and rounded to the nearest whole step. The 1/32 and 1/64 results repeat that process with 32 or 64 steps per inch.
Each result reports absolute error and signed delta in both inches and millimeters. A negative delta is under the exact decimal value, a positive delta is over it, and the smallest absolute error is marked as the closest match.
The displayed fraction is reduced for readability, so 4/16 appears as 1/4. Its column still tells you that the selection was made on a 1/16-inch grid.
A nearest fraction is an approximation unless the metric value lands exactly on that fraction. Parts with specified tolerances, threads, press fits, or safety requirements must be checked against the drawing and manufacturer data, not chosen from a rounded fraction alone.
Sources and method
The unit constants come from the primary standards below. ToolNeko applies those constants in a browser-only calculation, then rounds only the displayed result.
- NIST — SI Units: Length Gives the exact standard-inch relationship of 1 inch to 25.4 millimeters.
- NIST — Writing with SI (Metric System) Units Explains conversion-result precision and why extra digits should not imply unsupported accuracy.
Frequently asked questions
How many millimeters are in one inch?
There are exactly 25.4 millimeters in one inch. This exact definition is the basis for both conversion directions.
What is 10 mm in inches and fractions?
10 mm is approximately 0.3937 inch. It rounds to 3/8 inch at 1/16 resolution, 13/32 inch at 1/32, and 25/64 inch at 1/64.
Why do the 1/16, 1/32, and 1/64 results differ?
Each denominator divides an inch into a different number of steps. A finer grid can land closer to the decimal measurement, but it still remains a rounded reference unless the values match exactly.
Which fraction resolution should I use?
Use the resolution shown on the ruler, tool, drawing, or product specification you are comparing. The closest-match badge identifies the smallest displayed approximation error, but a finer denominator does not create accuracy that the original measurement did not have.
Is a fractional-inch result exact?
Not always. The decimal inch value comes from the exact 25.4 mm definition, but the fraction is explicitly the nearest step at 1/16, 1/32, or 1/64 inch.
Can I select a drill bit or fastener from this result alone?
Use the conversion as a comparison aid only. Thread pitch, clearance, tolerance class, material, and manufacturer specifications can matter even when nominal diameters look close.