The Challenge
Inches show up in US product specs constantly — and most of the world, and most precision tooling, works in millimeters. A 1/4-inch drill bit is 6.35mm. A 3/8-inch bolt is 9.525mm. A 0.5-inch aluminum sheet is 12.7mm. Woodworkers following American plans need mm equivalents for European router bits. Gunsmiths, machinists, and 3D printer users run into this conversion dozens of times a day. The exact conversion: 1 inch = 25.4mm exactly. Multiply any inch value by 25.4 to get millimeters. Fractional inches convert the same way — convert the fraction to decimal first: 3/8 = 0.375, then 0.375 × 25.4 = 9.525mm.
Inches to Millimeters Conversion Chart
| Inches (in) | Millimeters (mm) | Notes |
|---|---|---|
| 1/64 in | 0.397 mm | — |
| 1/32 in | 0.794 mm | — |
| 1/16 in | 1.588 mm | — |
| 1/8 in | 3.175 mm | — |
| 3/16 in | 4.763 mm | — |
| 1/4 in | 6.350 mm | exact crossover |
| 5/16 in | 7.938 mm | — |
| 3/8 in | 9.525 mm | — |
| 7/16 in | 11.113 mm | — |
| 1/2 in | 12.700 mm | exact crossover |
| 9/16 in | 14.288 mm | — |
| 5/8 in | 15.875 mm | — |
| 3/4 in | 19.050 mm | exact crossover |
| 7/8 in | 22.225 mm | — |
| 1 in | 25.400 mm | exact crossover |
| 1.5 in | 38.100 mm | — |
| 2 in | 50.800 mm | — |
| 2.5 in | 63.500 mm | — |
| 3 in | 76.200 mm | — |
| 4 in | 101.600 mm | — |
| 5 in | 127.000 mm | — |
| 6 in | 152.400 mm | — |
| 8 in | 203.200 mm | — |
| 10 in | 254.000 mm | — |
| 12 in | 304.800 mm | 1 foot |
| 18 in | 457.200 mm | — |
| 24 in | 609.600 mm | 2 feet |
| 36 in | 914.400 mm | 1 yard |
Common Fastener Sizes: Inches to mm
| Inch Size | Shank Diameter (mm) | Nearest Metric Bolt | Interchangeable? |
|---|---|---|---|
| #4 screw | 2.84 mm | M3 | No |
| #6 screw | 3.51 mm | M3.5 | No |
| #8 screw | 4.17 mm | M4 | No |
| #10 screw | 4.83 mm | M5 | No |
| 1/4 in bolt | 6.35 mm | M6 (6.0mm) | No — 0.35mm gap |
| 5/16 in bolt | 7.94 mm | M8 (8.0mm) | No — 0.06mm gap |
| 3/8 in bolt | 9.53 mm | M10 (10.0mm) | No — 0.48mm gap |
| 1/2 in bolt | 12.70 mm | M12 (12.0mm) | No — 0.7mm gap |
| 5/8 in bolt | 15.88 mm | M16 (16.0mm) | No — 0.12mm gap |
| 3/4 in bolt | 19.05 mm | M20 (20.0mm) | No — 0.95mm gap |
Why 25.4 Is Exact and What That Means for Precision Work
The number 25.4 is not a measured approximation — it is a legal definition. Since 1959, the inch has been defined as exactly 25.4 millimeters by international agreement. This means inch-to-mm conversions carry zero inherent rounding error. If you calculate 3/8 inch × 25.4 = 9.525mm, that result is exact to as many decimal places as you need. The only source of error is in your input value. This matters in precision machining: a tolerance of ±0.001 inch is ±0.0254mm exactly. When converting tolerance specifications between unit systems, you are not introducing conversion uncertainty — you are only converting the numbers.
Converting a Fractional Inch Dimension to mm for CNC
- Write the fraction as a division: 7/16 inch = 7 ÷ 16 = 0.4375 inches
- Multiply by 25.4: 0.4375 × 25.4 = 11.1125mm
- Use the full decimal in your CAM software — do not round to 11.1mm unless your tolerance allows it
- For a ±0.005in tolerance: ±0.005 × 25.4 = ±0.127mm — enter this as your tolerance band in metric
Nominal Pipe Size Is Not Actual Diameter
- A 1/2-inch nominal pipe does not have a 12.7mm OD — actual OD is 21.336mm (0.840in)
- A 3/4-inch nominal pipe OD is 26.670mm, not 19.05mm
- Pipe sizing follows a historical convention unrelated to actual dimensions — always look up the pipe schedule, not the nominal size
- This mismatch catches out machinists and plumbers every time they switch between unit systems
Step-by-Step Workflow
Enter your inch value — decimal (0.375) or fractional (3/8)
Millimeter result appears instantly below
Click swap to reverse and convert mm back to inches
Specifications
- Formula
- millimeters = inches × 25.4
- 1 inch equals
- 25.4 mm (exact, defined)
- 1/2 inch
- 12.7 mm
- 1/4 inch
- 6.35 mm
- 1/8 inch
- 3.175 mm
- 12 inches (1 ft)
- 304.8 mm
- Quick estimate
- multiply by 25 (1.6% error)
Best Practices
- Multiply by 25.4 for exact results — 25.4 is defined, not rounded
- Fractional inches: divide numerator by denominator, then multiply by 25.4 — 7/16 = 0.4375 × 25.4 = 11.1125mm
- Sheet metal gauges: 18 gauge steel = 0.0478in = 1.214mm, 16 gauge = 0.0598in = 1.519mm, 14 gauge = 0.0747in = 1.897mm
- 3D printing: 0.1in layer height = 2.54mm, 0.2in = 5.08mm — most slicers accept mm only
- Pipe: 1/2in nominal pipe OD is 21.336mm, 3/4in nominal is 26.670mm — nominal size does not equal actual OD
Frequently Asked Questions
How many millimeters is 1 inch?
1 inch equals exactly 25.4 millimeters. This is a defined value established by the 1959 International Yard and Pound Agreement — it is not a rounded approximation. Every inch-to-mm conversion is a simple multiplication by 25.4.
How do I convert fractional inches to mm?
Convert the fraction to a decimal first, then multiply by 25.4. Examples: 1/4in = 0.25 × 25.4 = 6.35mm. 3/8in = 0.375 × 25.4 = 9.525mm. 5/8in = 0.625 × 25.4 = 15.875mm. 7/8in = 0.875 × 25.4 = 22.225mm. These are all exact values — no rounding in the conversion factor.
What is 1/2 inch in mm?
1/2 inch = 0.5 × 25.4 = 12.7mm exactly. This is one of the cleanest crossover points between the two systems. A 12.7mm drill bit and a 1/2-inch drill bit are identical — unlike most metric/imperial pairs which don't align exactly.
What is 2 inches in mm?
2 inches = 2 × 25.4 = 50.8mm. For reference: 1in=25.4mm, 2in=50.8mm, 3in=76.2mm, 4in=101.6mm, 5in=127mm, 6in=152.4mm, 12in=304.8mm.
How do US sheet metal gauges convert to mm?
Sheet metal gauge does not convert linearly — each gauge has a defined thickness. For steel: 10ga=3.416mm, 12ga=2.657mm, 14ga=1.897mm, 16ga=1.519mm, 18ga=1.214mm, 20ga=0.912mm, 22ga=0.759mm. For aluminum gauges differ: 10ga=2.588mm, 14ga=1.628mm, 18ga=1.024mm. Always check the gauge chart for the specific material.
What is 0.001 inches (1 thou) in mm?
0.001 inches = 0.0254mm = 25.4 micrometers. Machinists call 0.001 inch a 'thou' or 'mil.' In metric shops the equivalent precision unit is 0.01mm (10 micrometers), which equals 0.000394 inches. When communicating tolerances across unit systems, always specify ±mm or ±in explicitly — '±1' is meaningless without the unit.
How do standard wood screw lengths in inches convert to mm?
Common wood screw lengths: 1/2in=12.7mm, 5/8in=15.9mm, 3/4in=19.1mm, 1in=25.4mm, 1-1/4in=31.8mm, 1-1/2in=38.1mm, 2in=50.8mm, 2-1/2in=63.5mm, 3in=76.2mm. Screw diameters use gauge numbers — #8 screw is 0.164in=4.17mm shank diameter, #10 is 0.190in=4.83mm.