Millimeters to Inches
Convert Millimeters to Inches with Machinist Precision
Enter 0.01 to 10,000 mm. Up to 2 decimal places.
Common sizes:
Precision Conversion
How Millimeters to Inches Conversion Works
Converting millimeters to inches is essential for precision work where tiny measurements matter. Unlike larger unit conversions where rounding errors are acceptable, millimeter conversions require exactness because 1 mm equals only 0.03937 inches. A 0.5 mm error in machining becomes 0.0197 inches, which can mean the difference between a part fitting perfectly or not fitting at all.
The Exact Conversion Formula
Inches = Millimeters ÷ 25.4
One inch equals exactly 25.4 millimeters. This relationship was standardized internationally in 1959 and remains the basis for all precision conversions between these units.
Our calculator divides your millimeter measurement by 25.4 to give you the exact decimal inch equivalent. What makes it particularly valuable for technical work is the fractional inch display. It automatically finds the closest standard fraction (1/64″, 1/32″, 1/16″, etc.) used in machining, carpentry, and engineering drawings. This dual display means you get both the exact decimal value for CAD work and the practical fraction for manual measurement.
🔧 Real-World Example:
You’re machining a part with a 8 mm diameter hole.
Step 1: 8 mm ÷ 25.4 = 0.31496 inches.
Step 2: Check fraction: 0.315 is closest to 5/16″ (0.3125).
Step 3: Tolerance check: Difference is 0.00146 inches (0.037 mm).
Decision: Use 5/16″ drill bit (7.9375 mm) or ream to exact 8 mm if tolerance is critical.
Why This Precision Conversion Matters
Millimeter-to-inch conversion isn’t just academic; it’s practical daily work for professionals and hobbyists across multiple fields. Specific applications include:
- Machining & Manufacturing: CNC machines often use metric programming internationally, while American machine shops work in inches. Converting toolpaths accurately prevents costly scrap parts.
- 3D Printing & Modeling: STL files from international designers use millimeters. American 3D printer settings often use inches. Accurate conversion ensures prints match intended dimensions.
- Jewelry Making: Gemstone sizes are measured in millimeters internationally. Mountings and settings in the US use inches. A 6 mm stone needs a 0.236″ setting.
- Electronics & PCBs: Component spacing on circuit boards often uses millimeter grids, while American datasheets may provide inch specifications.
- Medical & Dental: Implant sizes and surgical instruments are often metric globally but need inch equivalents for American medical documentation.
- International Trade: Product specifications from European or Asian suppliers list metric dimensions that must convert accurately for American markets.
Common Precision Conversion Questions
What is 10 mm in inches exactly?
10 mm equals 0.39370 inches. The closest standard fraction is 25/64″ (0.390625) with a difference of 0.003075 inches. For most precision work, this is close enough. For exact work, use the decimal value directly.
How do I convert mm to fractional inches manually?
Divide mm by 25.4 to get decimal inches. Multiply the decimal part by 64 (for 64ths), round to nearest whole number. Example: 8 mm = 0.31496″. Decimal part 0.31496 × 64 = 20.157, rounds to 20/64″ = 5/16″. Our calculator automates this with better rounding logic.
What’s the smallest mm measurement that matters?
In precision machining, 0.01 mm (0.0004 inches) is significant. This is roughly 1/4 the thickness of a human hair. Our calculator shows 4 decimal places because in high-precision work, 0.0001″ (0.00254 mm) differences affect fit and function.
When should I use fractions vs decimals?
Use fractions for manual measurement (rulers, calipers), machining (drill bits, taps), and woodworking. Use decimals for CAD/CAM programming, 3D printing settings, scientific calculations, and when using digital measuring tools that display decimals.
Precision Reference: Millimeters to Inches Table
This table shows exact conversions for common millimeter measurements used in technical work. Decimal inches show 4 places for precision, fractions show nearest standard equivalent.
| Millimeters (mm) | Decimal Inches | Fractional Inch | Common Use |
|---|---|---|---|
| 0.5 mm | 0.0197″ | 1/64″ (0.0156) | Paper thickness, fine tolerance |
| 1 mm | 0.0394″ | 1/32″ (0.0313) | Small hardware, credit card thickness |
| 3 mm | 0.1181″ | 1/8″ (0.1250) | Small drill bit, jewelry findings |
| 6 mm | 0.2362″ | 15/64″ (0.2344) | Pencil diameter, small bolt |
| 8 mm | 0.3150″ | 5/16″ (0.3125) | Common metric bolt, jewelry |
| 10 mm | 0.3937″ | 25/64″ (0.3906) | Medium bolt, socket size |
| 12.7 mm | 0.5000″ | 1/2″ exactly | Half-inch equivalent |
| 19.05 mm | 0.7500″ | 3/4″ exactly | Three-quarter inch |
| 25.4 mm | 1.0000″ | 1″ exactly | One inch standard |
| 50.8 mm | 2.0000″ | 2″ exactly | Two inches |
| 100 mm | 3.9370″ | 3 15/16″ (3.9375) | Small ruler length |
| 304.8 mm | 12.000″ | 12″ exactly | One foot in millimeters |
Tip for Machinists and Engineers
When converting critical dimensions, always work with the original metric measurement in your CAD/CAM software and let it handle conversions internally. If you must convert manually, use our calculator’s 4-decimal-place values for programming. For manual machining, use the fractional equivalent but verify with calipers. Remember that drill bits and taps in fractional inches are nominal sizes; actual cutting diameters can vary by 0.001-0.003 inches. When tolerance is under ±0.005 inches (0.127 mm), specify the decimal inch value rather than fraction to avoid interpretation errors.
Millimeters to Inches Precision Conversion
Original: 25.40 millimeters
Decimal Inches: 1.0000 inches
Fractional Equivalent: 1 inch (exactly)
Tolerance: ±0.0004 inches (±0.01 mm)
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