Inches to MM Converter

The Inches to MM Converter turns any measurement in inches into millimeters using the exact factor 1 inch = 25.4 mm. Unlike a plain box that only accepts decimals, this converter understands fractional inches — enter 3/8, a mixed number like 3 1/2, or a decimal such as 2.75 — and shows the result on an aligned inch-and-millimeter ruler so you can see exactly where your measurement falls.

How to Convert Inches to Millimeters

Converting inches to millimeters is a single multiplication, because the inch is defined as exactly 25.4 millimeters:

1. Turn any fraction into a decimal. For example, 3/8 inch becomes 0.375 inch, and a mixed number like 3 1/2 becomes 3.5 inch.
2. Multiply by 25.4. So 3.5 × 25.4 = 88.9 mm.
3. Round if you like. Choose how many decimals you need; because 25.4 is exact, the result is exact for any terminating decimal inch value.

Common Fractional Inches to MM

These are the fractional-inch values you will meet most often on tape measures, drill bits, and fasteners.

Whole Inches to MM

Why 1 Inch Equals Exactly 25.4 mm

Before 1959 different countries used slightly different inches. In 1959 the United States, United Kingdom, and other Commonwealth nations signed the international yard and pound agreement, fixing the international inch at exactly 25.4 millimeters. Because this is a definition rather than a measurement, converting inches to millimeters introduces no rounding error — every terminating decimal inch value has an exact millimeter equivalent.

Reading the Dual Ruler

The ruler in the result shows two aligned scales: inches on top (with 1/16-inch tick marks for small measurements) and millimeters on the bottom. Because both scales share the same horizontal axis, a single position is simultaneously an inch reading and its millimeter value. The colored marker points to your measurement on both scales at once, which is a quick way to build an intuition for how the two units relate.

Inches, Thou, and Micrometers

For very small measurements, engineers often use the thou (also called a mil), which is one thousandth of an inch. One thou equals 0.0254 mm or 25.4 micrometers (µm). This converter reports the thou and micrometer equivalents alongside millimeters so you can work at any scale.

How to Use This Converter

1. Enter the inches: Type a decimal (2.75), a fraction (3/8), or a mixed number (3 1/2). You can also paste Unicode fractions like ½ or ⅜.
2. Choose precision: Pick how many decimal places you want in the millimeter result.
3. Click Convert to mm: The tool multiplies by 25.4 and shows the result.
4. Review the result: Read the millimeter value, see it on the aligned ruler, and check the cm, meters, thou, and nearest-fraction equivalents plus a step-by-step breakdown.

Frequently Asked Questions

How many millimeters are in an inch?

There are exactly 25.4 millimeters in one inch. This is an exact definition of the international inch adopted in 1959, not a rounded approximation, so 1 in = 25.4 mm, 2 in = 50.8 mm, and so on.

How do I convert fractional inches to millimeters?

First turn the fraction into a decimal, then multiply by 25.4. For example, 3/8 inch is 0.375 inch, and 0.375 × 25.4 = 9.525 mm. For a mixed number like 3 1/2 inches, add the whole number and the fraction first: 3 + 0.5 = 3.5, then 3.5 × 25.4 = 88.9 mm.

What is 1/2 inch in mm?

One half inch equals 12.7 mm, because 0.5 × 25.4 = 12.7. Similarly 1/4 inch is 6.35 mm, 1/8 inch is 3.175 mm, and 3/4 inch is 19.05 mm.

What is the formula to convert inches to mm?

The formula is millimeters = inches × 25.4. To go the other way, from millimeters back to inches, divide by 25.4: inches = mm ÷ 25.4.

Can I enter a mixed number like 3 1/2 inches?

Yes. This converter accepts mixed numbers written with a space (3 1/2) or a hyphen (3-1/2), plain fractions (3/8), decimals (2.75), and Unicode fractions such as ½, ¼, and ⅜.

What is a thou or mil?

A thou, also called a mil, is one thousandth of an inch (0.001 inch), which equals 0.0254 mm or 25.4 micrometers. It is widely used in engineering and manufacturing for small tolerances.

Additional Resources

• Inch - Wikipedia
• Millimetre - Wikipedia
• International Yard and Pound Agreement - Wikipedia