Millimeters to Inches Converter
Enter millimeters to convert to inches, or enter inches to convert to millimeters.
Enter a value to see the conversion
How Millimeters to Inches Conversion Works
A millimeter (mm) is a metric unit of length equal to one thousandth of a meter, and one millimeter equals exactly 0.03937 inches. The conversion factor comes from the internationally agreed definition that 1 inch = 25.4 millimeters exactly. This definition was established by the 1959 International Yard and Pound Agreement and is maintained by standards organizations worldwide.
According to the National Institute of Standards and Technology (NIST), the inch is legally defined in terms of the meter: 1 inch = 0.0254 meters = 25.4 mm. This makes the mm-to-inch conversion exact, not an approximation. The millimeter is the preferred unit for precision measurement in engineering, manufacturing, and science worldwide, while inches remain standard in US construction, woodworking, and everyday measurement.
This conversion is critical in manufacturing (where parts may be specified in either system), 3D printing (nozzle and layer sizes in mm, print dimensions sometimes in inches), electronics (component sizing), and international trade. Over 90% of the world's manufactured goods use metric specifications, according to the BIPM. Our Length Converter handles all length units for more complex conversions.
The Millimeters to Inches Formula
The conversion formula is exact:
Inches = Millimeters / 25.4
To convert in the other direction:
Millimeters = Inches x 25.4
The factor 25.4 is exact by international definition. One inch = 25.4 mm, one foot = 304.8 mm, one yard = 914.4 mm.
Worked example: A bolt is 16 mm in diameter. Converting to inches: 16 / 25.4 = 0.6299 inches. The closest standard US bolt size is 5/8 inch (0.625 inches or 15.875 mm). The 16 mm bolt is slightly larger than 5/8 inch -- they are not interchangeable, which is why mixing metric and imperial fasteners can cause problems.
Key Terms You Should Know
- Millimeter (mm) -- A metric unit equal to 1/1,000 of a meter or 1/10 of a centimeter. Used worldwide for precision measurement in engineering, manufacturing, and science. There are 25.4 mm in one inch.
- Inch (in) -- An imperial unit equal to exactly 25.4 mm. Used primarily in the US, UK, and Canada for everyday and construction measurements. There are 12 inches in a foot and 36 in a yard.
- Thou (mil) -- Equal to 1/1,000 of an inch or 0.0254 mm. Used in engineering for very small measurements, such as film thickness, wire diameter, and surface finish. Not to be confused with the millimeter.
- Decimal Inches -- Inches expressed as decimals (e.g., 0.375") rather than fractions (3/8"). Preferred in precision machining and engineering drawings.
- Fractional Inches -- Inches expressed as fractions in powers of 2: 1/2, 1/4, 1/8, 1/16, 1/32, 1/64. Common in US woodworking, plumbing, and hardware sizing.
Millimeters to Inches Reference Table
Common mm-to-inch conversions with fractional equivalents. Exact values per NIST definitions.
| Millimeters | Decimal Inches | Nearest Fraction |
|---|---|---|
| 1 mm | 0.0394" | ~1/32" |
| 2 mm | 0.0787" | ~5/64" |
| 3 mm | 0.1181" | ~1/8" |
| 5 mm | 0.1969" | ~13/64" |
| 6 mm | 0.2362" | ~15/64" |
| 8 mm | 0.3150" | ~5/16" |
| 10 mm | 0.3937" | ~25/64" |
| 12 mm | 0.4724" | ~15/32" |
| 15 mm | 0.5906" | ~19/32" |
| 20 mm | 0.7874" | ~25/32" |
| 25 mm | 0.9843" | ~63/64" |
| 25.4 mm | 1.0000" | 1" (exact) |
| 50 mm | 1.9685" | ~1-31/32" |
| 100 mm | 3.9370" | ~3-15/16" |
Practical Examples
Wrench and socket sizes: A European car has 13 mm bolts. The closest imperial equivalent is 33/64" (0.515625") or 1/2" (0.5"). Since 13 mm = 0.5118 inches, a 1/2" socket is too small and will slip. Always use the correct metric socket for metric bolts. Common metric-to-imperial near-equivalents: 10 mm = ~3/8", 13 mm = ~1/2", 17 mm = ~11/16", 19 mm = ~3/4".
3D printing: A standard 3D printer nozzle is 0.4 mm (0.0157 inches). Common layer heights are 0.1 mm (0.004") for fine detail and 0.3 mm (0.012") for faster prints. Filament diameter is typically 1.75 mm (0.0689") or 2.85 mm (0.1122"). Understanding these dimensions in both units helps when reading US and international printing guides. Our Inches to mm Converter handles the reverse calculation.
Jewelry sizing: A wedding band is 6 mm wide. In inches: 6 / 25.4 = 0.236 inches, or approximately 15/64 inch. Ring sizes also differ: a US size 7 ring has an inner diameter of 17.3 mm (0.681 inches). Our Ring Size Converter helps translate between international ring sizing systems.
Tips for Converting Millimeters to Inches
- Divide by 25.4 for exact results: The factor 25.4 mm per inch is exact by definition. For quick mental math, dividing by 25 gives a reasonable estimate (about 1.6% high). For example, 50 mm / 25 = 2 inches (exact: 1.969 inches).
- Memorize key equivalents: 25.4 mm = 1 inch. 12.7 mm = 1/2 inch. 6.35 mm = 1/4 inch. 3.175 mm = 1/8 inch. These anchors help you estimate quickly without a calculator.
- Do not interchange metric and imperial fasteners: A 1/2-inch bolt (12.7 mm) and a 13 mm bolt differ by 0.3 mm. Using the wrong wrench will strip bolt heads. Always match the measurement system of the fastener.
- For woodworking, round to the nearest 1/16" or 1/32": Convert mm to decimal inches, then find the nearest fraction. Most US woodworking tools measure in 1/16" or 1/32" increments.
- Use our Length Converter for multi-unit conversions: When working with mm, cm, inches, feet, and meters simultaneously, the comprehensive converter saves time and prevents errors.
Common Metric vs. Imperial Fastener Sizes
One of the most practical applications of mm-to-inch conversion is selecting the right tools for fasteners. The table below shows common bolt/wrench sizes in both systems and highlights why they are not interchangeable.
| Metric (mm) | Inches (decimal) | Nearest Imperial Size | Difference |
|---|---|---|---|
| 8 mm | 0.315" | 5/16" (0.3125") | 0.08% |
| 10 mm | 0.394" | 3/8" (0.375") | 5.0% |
| 13 mm | 0.512" | 1/2" (0.500") | 2.4% |
| 17 mm | 0.669" | 11/16" (0.6875") | 2.7% |
| 19 mm | 0.748" | 3/4" (0.750") | 0.3% |
| 22 mm | 0.866" | 7/8" (0.875") | 1.0% |
Frequently Asked Questions
How many inches are in a millimeter?
One millimeter equals exactly 0.03937007874 inches, or approximately 1/25.4 of an inch. This is because 1 inch is defined as exactly 25.4 millimeters by the 1959 International Yard and Pound Agreement. To convert mm to inches, simply divide by 25.4. For example, 10 mm / 25.4 = 0.3937 inches, which is very close to 3/8 inch (0.375 inches).
How do I convert millimeters to inches?
Divide the millimeter value by 25.4 to get inches. For example, 50 mm / 25.4 = 1.9685 inches, or approximately 2 inches. For a quick estimate, divide mm by 25 for a rough inch value (this is about 1.6% higher than exact). The factor 25.4 mm per inch is exact by international definition, established by NIST and used globally in engineering and manufacturing.
What is 10mm in inches?
10 millimeters equals 0.3937 inches, which is very close to 3/8 inch (0.375 inches) or just over 25/64 inch (0.390625 inches). In fractional terms, 10 mm is most commonly approximated as 3/8 inch for general purposes, though the exact conversion is slightly larger. This conversion is frequently needed for drill bit sizes, wrench sizes, and jewelry measurements.
What common items are measured in both mm and inches?
Many products use both measurement systems. Wrench and socket sizes come in both metric (mm) and imperial (inch) sets. Drill bits are manufactured in both systems. Jewelry is often measured in mm (ring width, gemstone diameter). Screen sizes are in inches diagonally but bezel thickness in mm. Paper sizes use mm internationally (A4 is 210 x 297 mm) but inches in the US (Letter is 8.5 x 11 inches). Bullet calibers use both: 9mm vs .45 caliber (11.43 mm).
How do I convert mm to fractional inches?
First divide mm by 25.4 to get decimal inches, then convert to the nearest fraction. Common fractions are in 1/16, 1/32, or 1/64 increments. For example, 6 mm = 0.2362 inches. The nearest 1/16 is 4/16 = 1/4 inch (0.25). The nearest 1/32 is 8/32 = 1/4 inch. The nearest 1/64 is 15/64 inch (0.234375). Engineers often use decimal inches for precision, while carpenters prefer fractional inches.
Why do metric and imperial measurements coexist in manufacturing?
Both systems coexist because the US manufacturing sector uses imperial units domestically while exporting to metric markets. According to NIST, the US is one of only three countries that have not fully adopted the metric system. Automotive manufacturing is largely metric globally (bolt sizes in mm), while US construction uses inches and feet. This dual system creates ongoing demand for mm-to-inch conversion, particularly in aerospace, automotive repair, and international trade.