Millimeter
Definition: A millimeter (mm) measures length in the world’s standard system (SI). It equals 1/1000 of a meter. That’s the distance light travels in 1/299,792,458,000 of a second.
History/origin: The “milli-” part means one-thousandth. It comes from many metric prefixes and applies to the meter here. The meter’s definition has changed over time. Now, it uses the speed of light over a set time. But the link between meter and millimeter stays the same. Before, the meter came from a metal bar prototype. In 2019, experts updated it based on a new second definition.
Current use: People mainly use millimeter in the United States, Canada, and the United Kingdom. Japan and some other countries use it for electronic parts, like screen sizes.
Inch
Definition: An inch (symbol: in) measures length in the imperial and US customary systems. Since 1959, it equals exactly 25.4 millimeters. A foot has 12 inches, and a yard has 36 inches.
History/origin: The word “inch” comes from the Latin “uncia,” meaning one-twelfth of a Roman foot.
Old standards varied, but today’s inch links to the international yard. Early on, people used three dry, round barley grains laid end-to-end as one inch. Another idea tied it to human thumbs—averaging widths from small, medium, and large ones.
Current use: The inch is common in the United States, Canada, and the United Kingdom. Japan and other places use it for electronic parts, like screen sizes.
Millimeter to Inch Conversion Table
| Millimeter [mm] | Inch [in] |
| 0.01 mm | 0.0003937008 in |
| 0.1 mm | 0.0039370079 in |
| 1 mm | 0.0393700787 in |
| 2 mm | 0.0787401575 in |
| 3 mm | 0.1181102362 in |
| 5 mm | 0.1968503937 in |
| 10 mm | 0.3937007874 in |
| 20 mm | 0.7874015748 in |
| 50 mm | 1.968503937 in |
| 100 mm | 3.937007874 in |
| 1000 mm | 39.3700787402 in |
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1 mm = 0.0393700787 in
1 in = 25.4 mm
Example: convert 15 mm to in:
15 mm = 15 × 0.0393700787 in = 0.5905511811 in
1 inch is 25.4 mm. Think of it like the width of your thumb.
5 mm is about 0.2 inches. That’s super small, like the thickness of a pencil line.
1/8 inch is about 3.2 mm. It’s like the edge of a dime.
“1 by 2 inch” means 1 inch wide and 2 inches long. That’s 25.4 mm by 50.8 mm. Like a small gum stick.
2 inches is 50.8 mm. About the length of a standard crayon.
4 mm is about 0.16 inches. Smaller than a pea.
No, 20 mm is 0.79 inches—close but smaller than 1 inch.
No, 32 mm is 1.26 inches—bigger than 1 inch. (You asked this twice!)
No, 30 mm is 1.18 inches—a bit more than 1 inch. (Close, but not exact!)
No, 12 mm is 0.47 inches—almost half an inch (half is 12.7 mm).
No, 8 mm is 0.31 inches. A quarter inch is 6.35 mm, so 8 mm is a little bigger.
Almost! 25 mm is 0.98 inches—very close to 1 inch.
No, 22 mm is 0.87 inches. Half an inch is 12.7 mm.
No, 4 mm is just 0.16 inches. Half an inch is much bigger (12.7 mm).
No way! 2 mm is only 0.08 inches—tiny compared to 1 inch.
No, as above—it’s 1.18 inches.
Very small! Like the thickness of a paperclip wire.
Picture a small green pea or the tip of a ballpoint pen.
3 inches is 76.2 mm. About the height of a credit card stacked twice.
3/4 inch is 19.05 mm. Like the width of a AA battery.
Half an inch (0.5 inch) is 12.7 mm.
1/2 inch is 1.27 cm (since 10 mm = 1 cm).
4 mm is thin—like two stacked credit cards or a guitar pick.
2 mm is 0.08 inches. Super thin, like a single sheet of paper.
Small—like the head of a matchstick.
1 mm is tiny—the width of a grain of sand or a pinhead.
Imagine the thickness of a fingernail clipping or a single strand of fine hair.
Yes! A small pea is about 5 mm across.
A standard pencil eraser tip or a Tic Tac candy.
Half an inch is 12.7 mm.
A small pea is about 5-8 mm across.
Hold a pencil—5 mm is the width of the metal band around the eraser.
Rulers have lines: big lines are cm, tiny lines between are mm (10 per cm). Start from the 0 mark and count.
Your pinky fingernail is about 5-6 mm wide. Thumb tip is 20-25 mm. Use it to guess, then check with a ruler.
Yes! Download a free app like “Ruler” or “Measure” (on iPhone). Place your phone screen on the object—it uses the camera.
A US quarter coin is 24.26 mm across—almost 1 inch.
Divide mm by 25.4. Example: 50 mm ÷ 25.4 = about 2 inches. Use a calculator for exact.
Multiply inches by 25. Easy rule: 1 inch ≈ 25 mm, 2 inches ≈ 50 mm. Add 0.4 mm for precision.
4 mm is larger—twice as big!