18 mm equals 0.7087 inches, or about 0.71 inch. It is also exactly 1.8 cm and a little less than 3/4 inch.
A simple way to picture 18 mm is to look at a U.S. dime. A dime is 17.91 mm across, so it is almost the same size.
How Many Inches Is 18mm?
The exact conversion is:
18 mm = 0.7086614173 inches
For everyday use, you can round this to:
18 mm ≈ 0.71 inches
On an inch ruler, 18 mm falls between 11/16 inch and 3/4 inch.
It is much closer to 0.7 inch than to a full inch.
What Is 18mm as a Fraction of an Inch?
There is no common inch fraction that equals 18 mm exactly.
The closest useful fraction in 1/64-inch steps is:
45/64 inch = 17.8594 mm
That is only about 0.14 mm smaller than 18 mm.
Here are a few nearby fractions:
| Fraction | Size in millimeters | Comparison with 18 mm |
|---|---|---|
| 11/16 in | 17.4625 mm | 0.5375 mm smaller |
| 45/64 in | 17.8594 mm | 0.1406 mm smaller |
| 23/32 in | 18.2563 mm | 0.2563 mm larger |
| 3/4 in | 19.05 mm | 1.05 mm larger |
For a quick estimate, you can think of 18 mm as a little more than 11/16 inch and a little less than 3/4 inch.
How Big Is 18mm?
Eighteen millimeters is a small measurement.
It equals 1.8 cm, so on a metric ruler you can find it by going from zero to the 18 mm mark.
One of the easiest comparisons is a U.S. dime. A dime has a standard diameter of 17.91 mm, which makes it only 0.09 mm smaller than 18 mm.
A U.S. penny is slightly larger at 19.05 mm across.
That means 18 mm sits almost exactly between the size of a dime and a penny, but much closer to the dime.
Things That Are About 18mm in Size
1. U.S. Dime
A U.S. dime has a standard diameter of 17.91 mm.
That is only 0.09 mm smaller than 18 mm, so a dime is one of the best everyday ways to picture this measurement.
An 18 mm circle would look almost the same width as a dime.
2. U.S. Penny
A U.S. penny measures 19.05 mm in diameter.
That makes it 1.05 mm wider than 18 mm.
If you picture a penny and make it slightly smaller, you are close to 18 mm.
3. CR2032 Coin Battery
A CR2032 coin-cell battery is about 20 mm in diameter.
That makes it roughly 2 mm wider than 18 mm.
Battery dimensions can vary slightly by manufacturer, so this is a close comparison rather than an exact match.
4. 18mm Watch Strap
Many watches use an 18 mm strap width.
The measurement normally refers to the width of the strap where it fits between the watch lugs.
In this case, 18 mm is a standard nominal product size rather than an approximation.
5. 18mm Drill Bit
Metric drill bits are available with a nominal diameter of 18 mm.
This gives a direct example of how wide 18 mm looks in a tool.
The actual manufactured size may have tiny tolerances, but the stated diameter is 18 mm.
6. M18 Bolt Thread
An M18 bolt has a nominal major thread diameter of 18 mm.
The 18 mm figure refers to the threaded part of the bolt, not the width of the bolt head.
This is a common place to see 18 mm used in engineering and mechanical work.
7. 45/64-Inch Measurement
45/64 inch equals 17.8594 mm.
That makes it only about 0.14 mm smaller than 18 mm.
If you are using a ruler marked in sixty-fourths of an inch, this is one of the closest marks to 18 mm.
8. 11/16 Inch
11/16 inch equals 17.4625 mm.
That is about 0.54 mm smaller than 18 mm.
On a ruler marked in sixteenths, 18 mm sits just past the 11/16-inch line.
9. 3/4 Inch
3/4 inch equals 19.05 mm.
That makes it 1.05 mm larger than 18 mm.
So 18 mm is slightly less than three-quarters of an inch.
10. Table-Tennis Ball Diameter
A standard table-tennis ball is 40 mm in diameter.
Eighteen millimeters is about 45% of that diameter.
This is not a close size match, but it can help show how small 18 mm is compared with a familiar round object.
18mm in Other Units
Here are the most useful conversions for 18 mm:
| Unit | 18 mm equals |
|---|---|
| Inches | 0.7086614 in |
| Inches rounded | 0.71 in |
| Centimeters | 1.8 cm |
| Meters | 0.018 m |
| Micrometers | 18,000 µm |
For most everyday uses, 0.71 inch or 1.8 cm is accurate enough.
How to Convert 18mm to Inches
There are exactly 25.4 millimeters in one inch.
To convert millimeters to inches, divide the number of millimeters by 25.4.
For 18 mm:
18 ÷ 25.4 = 0.7086614 inches
So:
18 mm = 0.7087 inches
If you only need a simple everyday answer, you can round this to 0.71 inch.
For technical work, machining, or precise measurements, use more decimal places.
15–25mm to Inches Conversion Table
This table shows how 18 mm compares with nearby measurements.
| Millimeters | Inches |
|---|---|
| 15 mm | 0.5906 in |
| 16 mm | 0.6299 in |
| 17 mm | 0.6693 in |
| 18 mm | 0.7087 in |
| 19 mm | 0.7480 in |
| 20 mm | 0.7874 in |
| 21 mm | 0.8268 in |
| 22 mm | 0.8661 in |
| 23 mm | 0.9055 in |
| 24 mm | 0.9449 in |
| 25 mm | 0.9843 in |
This makes it easy to see that 18 mm is slightly above 0.7 inch and still well below 1 inch.
Final Thoughts
18 mm equals 0.7087 inches, or about 0.71 inch.
The easiest way to picture it is a U.S. dime, which is 17.91 mm across and almost the same size.
Common Questions
Is 18mm the Same as 3/4 Inch?
No. 3/4 inch equals 19.05 mm, so it is 1.05 mm larger than 18 mm.
Is 18mm About the Size of a Dime?
Yes. A U.S. dime is 17.91 mm in diameter, making it only 0.09 mm smaller than 18 mm.
That makes a dime one of the best real-life comparisons.
What Fraction of an Inch Is Closest to 18mm?
The closest fraction in 1/64-inch steps is about 45/64 inch.
45/64 inch equals 17.8594 mm, so it is very close but not exact.
Is 18mm Bigger Than 11/16 Inch?
Yes. 11/16 inch equals 17.4625 mm.
Eighteen millimeters is about 0.54 mm larger.
How Many Centimeters Is 18mm?
18 mm equals exactly 1.8 cm.
Since 10 millimeters equal 1 centimeter, you divide 18 by 10.
Where Might I See an 18mm Measurement?
You may see 18 mm used for watch straps, drill bits, metric bolts, machine parts, woodworking, and other metric measurements.
It is also a useful size to compare with small coins and hardware.
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