5/16 inch equals exactly 7.9375 millimeters. For everyday use, this is usually rounded to 7.94 mm. It is also very close to 8 mm, although the two sizes are not exactly the same.
This article explains the conversion, shows how big 5/16 inch is, and compares it with common objects and nearby measurements.
How Many Millimeters Is 5/16 Inch?
5/16 inch = 7.9375 mm
To convert 5/16 inch to millimeters, first turn the fraction into a decimal:
5 ÷ 16 = 0.3125 inch
One inch equals exactly 25.4 mm, so:
0.3125 × 25.4 = 7.9375 mm
For most everyday measurements, you can round this to:
5/16 inch ≈ 7.94 mm
For a rough size estimate, it is also reasonable to think of it as almost 8 mm.
How Big Is 5/16 Inch?
5/16 inch is a small measurement that is just under 8 mm across.
In centimeters, it equals:
5/16 inch = 0.79375 cm
That is a little more than three-quarters of a centimeter.
For a size this small, it is easier to picture the measurement as the diameter or thickness of an object rather than its length.
5/16 inch also sits exactly halfway between 1/4 inch and 3/8 inch.
- 1/4 inch = 4/16 inch
- 5/16 inch = 5/16 inch
- 3/8 inch = 6/16 inch
Things Close to 5/16 Inch
The examples below show objects and sizes close to 7.94 mm. Some are exact nominal measurements, while others are only useful visual comparisons.
1. A 5/16-Inch Drill Bit
A 5/16-inch drill bit has a nominal diameter of exactly 0.3125 inch, which equals 7.9375 mm.
This is one of the best ways to picture the measurement because the stated drill-bit size is an exact nominal match.
The actual hole made by a drill can vary slightly because of the drill, material, and working conditions.
2. A 5/16-Inch Bolt
A bolt with a nominal diameter of 5/16 inch is also 7.9375 mm across its major thread diameter.
This makes 5/16-inch bolts another useful exact nominal comparison.
You may see this size written on hardware as 5/16-18 or 5/16-24, where the second number refers to the thread count.
3. An 8 mm Knitting Needle
An 8 mm knitting needle is extremely close to 5/16 inch.
The difference is:
8 mm − 7.9375 mm = 0.0625 mm
So an 8 mm knitting needle is only 0.0625 mm thicker than 5/16 inch.
It is a good visual comparison, but not an exact match.
4. An 8 mm Crochet Hook
An 8 mm crochet hook is also very close in size.
Its listed size is just 0.0625 mm larger than 5/16 inch.
Crochet hooks change shape along their length, so the 8 mm rating should be treated as a tool size rather than saying every part of the hook is exactly 8 mm wide.
5. A 7.9 mm Drill Bit
A 7.9 mm drill bit is slightly smaller than 5/16 inch.
The difference is only:
7.9375 − 7.9 = 0.0375 mm
That makes it one of the closest metric sizes to 5/16 inch.
For precision work, however, the specified drill size should still be used.
6. A Typical Wooden Pencil
Many ordinary wooden pencils are around 7 mm across, although their exact width varies by brand and shape.
Compared with a 7 mm pencil, 5/16 inch is about:
7.9375 − 7 = 0.9375 mm wider
A pencil therefore gives a useful general idea of the size, but it is slightly smaller.
7. Five U.S. Pennies Stacked
A U.S. penny is about 1.52 mm thick.
Five pennies stacked measure:
1.52 × 5 = 7.60 mm
That is 0.3375 mm thinner than 5/16 inch.
The stack is close enough to give a useful visual reference.
8. Six U.S. Dimes Stacked
A U.S. dime is about 1.35 mm thick.
Six dimes stacked measure:
1.35 × 6 = 8.10 mm
That is only 0.1625 mm thicker than 5/16 inch.
This is one of the closest familiar coin-stack comparisons.
9. Four U.S. Quarters Stacked
A U.S. quarter is about 1.75 mm thick.
Four quarters stacked measure:
1.75 × 4 = 7 mm
The stack is therefore about 0.94 mm thinner than 5/16 inch.
It gives a useful example of something slightly smaller than the target measurement.
10. Five U.S. Quarters Stacked
Five U.S. quarters have a combined thickness of:
1.75 × 5 = 8.75 mm
That is 0.8125 mm thicker than 5/16 inch.
So 5/16 inch falls between the thickness of four and five stacked quarters.
Is 5/16 Inch the Same as 8 MM?
No. 5/16 inch is not exactly the same as 8 mm.
The two measurements are:
- 5/16 inch = 7.9375 mm
- 8 mm = 8.0000 mm
The difference is:
8 − 7.9375 = 0.0625 mm
That difference is very small and may be difficult to see on an ordinary ruler.
For rough size comparisons, saying that 5/16 inch is almost 8 mm is fine. For drill bits, bolts, machining, or other work where exact fit matters, the two measurements should not automatically be treated as interchangeable.
5/16 Inch Compared to Nearby Sizes
5/16 inch falls between several common fractional-inch sizes.
| Fraction | Millimeters |
|---|---|
| 1/4 in | 6.35 mm |
| 9/32 in | 7.14375 mm |
| 5/16 in | 7.9375 mm |
| 11/32 in | 8.73125 mm |
| 3/8 in | 9.525 mm |
Compared with 1/4 inch, 5/16 inch is 1.5875 mm larger.
Compared with 3/8 inch, it is 1.5875 mm smaller.
This is why 5/16 inch sits exactly halfway between 1/4 inch and 3/8 inch.
Where Is 5/16 Inch Commonly Used?
5/16 inch is commonly seen in hardware, construction, repair work, and workshops.
A common example is a 5/16-inch drill bit, where the size refers to the nominal diameter of the bit.
The measurement is also used for bolts and threaded fasteners. For example, a 5/16-18 bolt has a nominal diameter of 5/16 inch and 18 threads per inch. A 5/16-24 bolt uses the same nominal diameter but has 24 threads per inch.
You may also see 5/16 inch used for holes, sockets, mechanical parts, woodworking, and metalworking.
Because these jobs can involve tight fits, it is better to use the specified size instead of replacing 5/16 inch with 8 mm without checking.
Final Thoughts
5/16 inch equals exactly 7.9375 mm, or about 7.94 mm. It is just slightly smaller than 8 mm.
A 5/16-inch drill bit or bolt gives the clearest exact nominal comparison, while an 8 mm knitting needle or crochet hook is an easy way to picture roughly how big the measurement is.
Common Questions
What Is 5/16 Inch in MM Exactly?
5/16 inch equals exactly 7.9375 mm.
For everyday use, it is normally rounded to 7.94 mm.
What Is 5/16 Inch as a Decimal?
5/16 inch = 0.3125 inch.
You get this by dividing 5 by 16.
Is 5/16 the Same as 8 MM?
No. 5/16 inch is 7.9375 mm, while 8 mm is exactly 8 mm.
The difference is only 0.0625 mm.
What Metric Size Is Closest to 5/16 Inch?
8 mm is a very close common metric size.
It is only 0.0625 mm larger than 5/16 inch. A 7.9 mm size is even closer mathematically, at only 0.0375 mm smaller.
Is 5/16 Bigger Than 1/4 Inch?
Yes. 5/16 inch is larger than 1/4 inch.
1/4 inch equals 6.35 mm, while 5/16 inch equals 7.9375 mm.
Is 5/16 Smaller Than 3/8 Inch?
Yes. 5/16 inch is smaller than 3/8 inch.
3/8 inch equals 9.525 mm, making it 1.5875 mm larger than 5/16 inch.
What Is 5/16 Inch in Centimeters?
5/16 inch equals 0.79375 cm.
Rounded to two decimal places, that is about 0.79 cm.
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