Seldons
inches to mm
1 inch = 25.4 millimeters.
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How do you convert inches to millimeters?
Use this formula. It is exact — these units are defined by international agreement, not measured.
mm = in × 25.4
Example: 10 in × 25.4 = 254 mm.
inches to mm conversion table
Common values, rounded to six significant figures.
| in | mm |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127 mm |
| 6 in | 152.4 mm |
| 8 in | 203.2 mm |
| 10 in | 254 mm |
| 12 in | 304.8 mm |
| 15 in | 381 mm |
| 18 in | 457.2 mm |
| 24 in | 609.6 mm |
| 30 in | 762 mm |
| 36 in | 914.4 mm |
| 48 in | 1,219.2 mm |
| 60 in | 1,524 mm |
| 72 in | 1,828.8 mm |
Spec sheets written in inches have to be built on metric machines, which is where this conversion usually happens — fastener lengths, tubing, plate thickness, connector pitches. The relationship is exact rather than measured: since the 1959 international yard and pound agreement an inch is defined as 25.4 mm, so you can carry as many decimals as your tolerance needs without introducing error.
Values worth remembering
These come up often enough that knowing them saves a lookup.
- 12 in = 304.8 mmone foot
- 8.5 in = 215.9 mmUS Letter width
- 11 in = 279.4 mmUS Letter height
Questions people ask
How many millimeters is 1 inch?
1 inch is 25.4 millimeters.
How many inches is 1 millimeter?
1 millimeter is 0.0393701 inches.
How do you convert inches to millimeters in your head?
Use × 25.4. That lands within a percent or so of the exact figure, which is close enough for packing, shipping estimates and shopping — use the calculator when the number has to be right.
Is the inch to millimeter conversion exact?
Yes. The relationship is exact by international agreement, so 25.4 is a definition rather than a measurement. Rounding in the answer comes from display, not from the conversion.