Millimeters to Inches Converter

The converter above is already set to millimeters and inches, 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 25. Decimals and negative numbers are accepted.
2. Check that From shows millimeters and To shows inches. 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 millimeter.
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 millimeter is about 0.0393701 inches, so the formula is inches = millimeters x 0.0393701. Going the other way, one inch is exactly 25.4 millimeters. A millimeter is one thousandth of a meter, roughly the thickness of a US dime. The inch has been defined as exactly 25.4 millimeters since the international yard and pound agreement of 1959.

Worked examples with the same factor:

• 25 millimeters: 25 x 0.0393701 = 0.984252 inches
• 75 millimeters: 75 x 0.0393701 = 2.95276 inches
• 250 millimeters: 250 x 0.0393701 = 9.84252 inches

Remember that 25.4 mm make an inch, so 25 mm is just under an inch and 100 mm is just under 4 inches.

To check the converter by hand, take 75 millimeters and apply the rule once: the result should be 2.95276 inches. If you go back again with the reverse factor 25.4 you land on 75 millimeters, 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.

Drill bits, camera lenses, screws, phone thickness and rainfall totals are quoted in millimeters, while US hardware stores and building plans use inches and fractions of an inch.

Check which unit the number belongs to before you type it. Tape measures are marked in both systems, and a value read from the wrong edge is off by a factor of about 2.5. When a measurement will be cut, keep at least two decimals until the final step.

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

The page keeps no history, so each visit starts again from the defaults, 25 millimeters. 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.