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Inches to Millimeters Converter

Convert inches to millimeters and millimeters to inches instantly. Bidirectional length converter with formula and common values.

in

Millimeters

88.9

mm

3.5 in = 88.9 mm

mm = in × 25.4

Inches to Millimeters: 1 in equals 25.4 mm. To convert inches to millimeters, multiply by 25.4 (mm = in × 25.4). For example, 10 in = 254 mm.

How to Convert Inches to Millimeters

To convert from inches to millimeters, multiply the value by 25.4. The conversion is linear, meaning doubling the input doubles the output.

Conversion Formula

  • Inches to Millimeters: mm = in × 25.4
  • Millimeters to Inches: in = mm ÷ 25.4

Inches to Millimeters Conversion Chart

Inches (in)Millimeters (mm)
0.12.54
0.256.35
0.512.7
125.4
250.8
376.2
5127
10254
20508
25635
501270
1002540
2506350
100025400

Understanding the Units

What is an Inche?

The inch is a US customary and imperial unit equal to exactly 25.4 millimeters by the 1959 international yard and pound agreement.

From Old English ynce, derived from Latin uncia ("twelfth"), originally a twelfth of a Roman foot.

Common contexts: construction, screen sizes, apparel.

What is a Millimeter?

A millimeter is one thousandth of a meter (1 mm = 0.001 m).

Common contexts: engineering, manufacturing, medicine.

Real-World Reference Points

Item Inches (in) Millimeters (mm)
Quarter coin diameter0.95524.257
Smartphone screen6.1154.94
Standard ruler12304.8

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FAQ

How many millimeters are in an inch?

One inch equals exactly 25.4 millimeters.

How do I convert inches to millimeters?

Multiply the number of inches by 25.4. For example, 3.5 in × 25.4 = 88.9 mm.

Why is the conversion factor exactly 25.4?

Since 1959, the international inch is defined as exactly 25.4 millimeters by agreement among English-speaking nations.

How many inches are in a millimeter?

One millimeter equals approximately 0.03937 inches.

How many millimeters are in 1 inch?

1 in equals 25.4 mm.

How do you convert inches to millimeters?

Multiply the inch value by 25.4. Formula: mm = in × 25.4.

How many millimeters are in 5 inches?

5 in = 127 mm.

How do you convert millimeters back to inches?

Use the inverse formula: in = mm ÷ 25.4. For example, 1 mm = 0.03937 in.

What is the difference between Inch and Millimeter?

The inch is a US customary and imperial unit equal to exactly 25.4 millimeters by the 1959 international yard and pound agreement. A millimeter is one thousandth of a meter (1 mm = 0.001 m).