3/8 inch = exactly 9.525 mm. For everyday use, you can round this to 9.5 mm.
That means 3/8 inch is just under 1 centimeter. This article shows the exact conversion, where 3/8 inch sits on a ruler, and some real objects that are close to this size.
How Many Millimeters Is 3/8 Inch?
Three-eighths of an inch equals 9.525 millimeters.
The calculation is simple:
3/8 = 0.375 inch
0.375 × 25.4 = 9.525 mm
Since 1 inch equals exactly 25.4 mm, the conversion is exact.
For normal reading, 9.5 mm is usually close enough. For drilling, machining, bearings, or other work where fit matters, use the exact 9.525 mm value.
How Big Is 3/8 Inch?
Three-eighths of an inch is 0.9525 cm, so it is slightly smaller than 1 centimeter.
The difference is:
10 mm − 9.525 mm = 0.475 mm
So 3/8 inch is less than half a millimeter shorter than 1 cm.
On a ruler divided into eighths, 3/8 inch is the third 1/8-inch mark after zero.
It can also be written as:
3/8 = 6/16 inch
So on a ruler divided into sixteenths, it is the sixth 1/16-inch mark.
It also sits directly between 1/4 inch and 1/2 inch.
Things Close to 3/8 Inch in Size
Some of these examples are exact nominal sizes, while others are only close. Product dimensions can also vary slightly.
1. 3/8-Inch Drill Bit
A 3/8-inch drill bit has a nominal diameter of 9.525 mm.
This is one of the best exact examples of the size. If you have a 3/8-inch drill bit, the width of the cutting part shows what 3/8 inch looks like.
2. 3/8-Inch Steel Bearing Ball
A steel bearing ball sold as 3/8 inch has a nominal diameter of 9.525 mm.
This gives another clear physical example of the measurement. A ball this size is just under 1 cm across.
3. 3/8-Inch Wooden Dowel
Wooden dowels are commonly sold in a 3/8-inch size.
That means they are designed to be about 9.5 mm in diameter. Actual wooden dowels can vary slightly because of manufacturing tolerances and the material itself.
4. 3/8-Inch Tubing
Tubing is often listed by its outside diameter.
A tube marked 3/8-inch OD has a nominal outside diameter of 9.525 mm. You may see tubing around this size in water systems, appliances, and small plumbing jobs.
5. 3/8-Inch Socket Drive
A common ratchet drive size is 3/8 inch.
The square drive is about 9.5 mm across. The 3/8-inch label refers to the square connection between the ratchet and socket, not the size of the nut or bolt.
6. AAA Battery
A typical AAA battery is about 10.5 mm in diameter.
That makes it around 0.975 mm wider than 3/8 inch.
So if you picture the thickness of an AAA battery, 3/8 inch is just a little narrower.
7. 10 mm Smoothie Straw
Some smoothie straws have a diameter of around 10 mm.
That is only 0.475 mm wider than 3/8 inch, so it is a close visual comparison. Straw sizes vary by product, so this is not an exact match.
8. Large Drinking Straw
Large drinking straws can be around 8 to 10 mm wide.
Some are therefore very close to 3/8 inch in diameter. Others are smaller, so the exact size depends on the straw.
9. Wooden Pencil
A typical wooden pencil is often around 7 mm across, although sizes vary.
That makes it roughly 2.5 mm narrower than 3/8 inch.
A pencil is therefore a useful smaller comparison if you are trying to picture the difference.
10. Micro-USB Connector
Some Micro-USB connector housings are around 8 mm wide, depending on the design.
That makes them slightly narrower than 3/8 inch. Connector dimensions vary, so this should only be used as a rough comparison.
3/8 Inch in Other Units
Here are the most useful conversions for 3/8 inch.
| Unit | 3/8 Inch Equals |
|---|---|
| Millimeters | 9.525 mm |
| Centimeters | 0.9525 cm |
| Decimal inches | 0.375 in |
| Feet | 0.03125 ft |
The easiest values to remember are 9.525 mm exactly and about 9.5 mm when rounded.
3/8 Inch Compared With Nearby Sizes
Three-eighths of an inch falls between 5/16 inch and 7/16 inch.
| Inches | Millimeters |
|---|---|
| 1/4″ | 6.35 mm |
| 5/16″ | 7.9375 mm |
| 3/8″ | 9.525 mm |
| 7/16″ | 11.1125 mm |
| 1/2″ | 12.7 mm |
A 9.5 mm size is extremely close to 3/8 inch. It is only 0.025 mm smaller.
A 10 mm size is also close, but it is 0.475 mm larger.
These differences may not matter for a quick visual comparison, but they can matter for tools, holes, bearings, and other parts that need a precise fit.
Where Is 3/8 Inch Commonly Used?
The 3/8-inch size appears often in tools, hardware, woodworking, plumbing, and mechanical work.
You may see it on drill bits, bolts, nuts, washers, tubing, wooden dowels, bearing balls, and ratchet drives.
The meaning of 3/8 inch depends on the item.
For example, a 3/8-inch bolt usually refers to the nominal diameter of its threaded shaft. It does not mean the bolt head is 3/8 inch wide.
A 3/8-inch socket drive refers to the square drive connection. It does not mean the socket itself fits a 3/8-inch nut.
Final Thoughts
3/8 inch equals exactly 9.525 mm, or about 9.5 mm when rounded.
The easiest way to picture it is as a size just under 1 centimeter, or slightly narrower than the diameter of a typical AAA battery.
Common Questions
Is 3/8 inch the same as 10 mm?
No. 3/8 inch is 9.525 mm, while 10 mm is slightly larger.
The difference is 0.475 mm.
Is 3/8 inch the same as 9.5 mm?
Not exactly. 3/8 inch is 9.525 mm, so 9.5 mm is 0.025 mm smaller.
They are very close in size, but they are not mathematically equal.
What is 3/8 inch as a decimal?
3/8 inch = 0.375 inch.
You get this by dividing 3 by 8.
What is 3/8 inch in centimeters?
3/8 inch = 0.9525 cm.
That is why it is often described as being just under 1 centimeter.
What metric size is closest to 3/8 inch?
9.5 mm is a very close common metric size.
The exact conversion is 9.525 mm.
Where is 3/8 inch on a ruler?
It is the third 1/8-inch mark after zero.
On a ruler divided into sixteenths, 3/8 inch equals 6/16 inch, so it is the sixth 1/16-inch mark.
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