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Inches to MM Converter

How to Convert Inches to Millimeters

To convert inches to millimeters, multiply by 25.4. One inch equals exactly 25.4 millimeters — this is an exact definition, not an approximation. The relationship was established by international agreement in 1959. This conversion is crucial in engineering, manufacturing, hardware sizing, and any field where imperial and metric specifications coexist. Many technical drawings, CNC programs, and 3D printing specifications require mm, while US hardware and construction use inches.

The Inches to MM Formula

The formula is: millimeters = inches × 25.4. For fractional inches, first convert to decimal: 1/4" = 0.25, 3/8" = 0.375, 1/2" = 0.5, 5/8" = 0.625, 3/4" = 0.75. Then multiply by 25.4. Example: 3/8" × 25.4 = 9.525 mm. For mixed fractions like 2-3/8": 2.375 × 25.4 = 60.325 mm. The 'mil' or 'thou' (0.001") is also useful: 1 mil = 0.0254 mm.

Frequently Asked Questions

How many mm is 1 inch?

1 inch equals exactly 25.4 millimeters. This is an exact definition established by international agreement in 1959, not an approximation.

How do I convert inch fractions to mm?

Convert the fraction to decimal first, then multiply by 25.4. Examples: 1/8" = 0.125 × 25.4 = 3.175 mm. 5/16" = 0.3125 × 25.4 = 7.938 mm. 3/4" = 0.75 × 25.4 = 19.05 mm.

Can I use a 10mm wrench on a 3/8" bolt?

A 3/8" bolt is 9.525 mm, so a 10mm wrench is 0.475 mm larger — it may work but can round off bolt heads over time. For precision work, always use the correct size. Metric and imperial wrenches are close but not interchangeable.

What is a mil or thou?

A mil (also called thou) is 1/1000 of an inch = 0.0254 mm = 25.4 micrometers. It's used in manufacturing, PCB design, and thin material measurements. To convert mils to mm, multiply by 0.0254.

How thick is 1mm in inches?

1 mm = 0.03937 inches ≈ 1/25 of an inch, or about 39.4 mils. For context, a credit card is about 0.76 mm (0.030") thick, and a US dime is about 1.35 mm (0.053") thick.

What are standard drill bit sizes in mm?

US drill bits come in fractional inches (1/16" increments), number sizes (#1-80), and letter sizes (A-Z). Common conversions: 1/8" = 3.175 mm, 1/4" = 6.35 mm, 3/8" = 9.525 mm, 1/2" = 12.7 mm. Metric drill bits go in 0.5mm increments: 3.0, 3.5, 4.0, 4.5, 5.0 mm, etc.