Inches to Millimeters Converter

The converter above is already set to inches and millimeters, so you only need to enter your number. The result updates as you type, and nothing is sent anywhere.

1. Type your value in the Value box, for example 1. Decimals and negative numbers are accepted.
2. Check that From shows inches and To shows millimeters. Both menus can be changed if you need a different pair.
3. Read the answer in the large result box. The note under it shows the exact factor used for one inch.
4. Press Copy result to put the answer on your clipboard, or Swap units to convert in the opposite direction.
5. Look at In every unit on the right to see the same amount in all the other length units.

One inch is exactly 25.4 millimeters, so the formula is millimeters = inches x 25.4. Going the other way, one millimeter is about 0.0393701 inches. The inch has been defined as exactly 25.4 millimeters since the international yard and pound agreement of 1959. A millimeter is one thousandth of a meter, roughly the thickness of a US dime.

Worked examples with the same factor:

• 1 inch: 1 x 25.4 = 25.4 millimeters
• 3 inches: 3 x 25.4 = 76.2 millimeters
• 10 inches: 10 x 25.4 = 254 millimeters

Multiply by 25 for a rough figure, then add 1.6 percent: 2 inches is about 50 mm, exactly 50.8.

To check the converter by hand, take 3 inches and apply the rule once: the result should be 76.2 millimeters. If you go back again with the reverse factor 0.0393701 you land on 3 inches, which confirms that the two directions agree. Rounding the factor to fewer digits for mental math gives a result that is close but not identical, so use the tool when the last digits matter.

These reference values come from the same factor the converter uses, so they match what you see above. Use the table for common amounts, and the converter for anything else.

Length units come from two families. The metric units (millimeter, centimeter, meter, kilometer) are powers of ten apart, so moving between them only shifts a decimal point. The customary units (inch, foot, yard, mile) descend from older body-based and road measures and were fixed to the meter by international agreement in 1959, which is why every conversion between the two families has an exact factor.

Use the table as a sanity check rather than a substitute for the converter. If your number sits between two rows, the answer must sit between the two results, and if it is ten times larger the answer is ten times larger. That simple scaling rule catches most typing mistakes, such as an extra zero or a misplaced decimal point.

Machinists, 3D printing and engineering drawings switch between decimal inches and millimeters, where a small error can mean a part that does not fit.

The most common slip is converting only part of a mixed measurement: 5 feet 8 inches is 68 inches in total, not 5.8 feet, so turn the feet into inches (or the inches into a fraction of a foot) before adding. Another is rounding too early, which grows into a visible error over long runs.

Related pages: Millimeters to Inches Converter for the reverse direction, and the feet to inches and feet to meters pages. For other length units use the length converter.

The page keeps no history, so each visit starts again from the defaults, 1 inch. Your numbers are processed by JavaScript in your own browser and are never sent to a server, which makes the converter safe for private figures such as medical, financial or work measurements.