8 mm is about 0.315 inches, or about 5/16 inch (slightly more). Divide millimeters by 25.4, because 1 inch is defined as exactly 25.4 mm.
8 mm ≈ 5/16″
Slightly more than 5/16″ (by 0.06 mm) · ≈ 0.315 in
- Inches
- ≈ 0.315 in
- Nearest 1/16″
- 5/16″ 8 mm is 0.06 mm longer
- Nearest 1/32″
- 5/16″ 8 mm is 0.06 mm longer
- Nearest 1/64″
- 5/16″ 8 mm is 0.06 mm longer
How big is 8 mm?
8 mm = 8 ÷ 25.4 in ≈ 0.31496 in. On a tape measure that is 20/64 in, which reduces to 5/16 in (0.3125 in), and 8 mm is longer than that by about 0.06 mm, less than the thickness of a sheet of paper. In other metric units, 8 mm is exactly 0.8 cm and exactly 0.008 m.
Compared with other common fractions: 1/4 in is 6.35 mm, so 8 mm is about 1.65 mm more; 3/8 in is 9.525 mm, so 8 mm is about 1.5 mm less.
Things that are about 8 mm
These are nominal sizes. Real parts vary a little with the maker, so check the part when fit matters.
An M8 bolt or screw
The “M8” label means a thread diameter of 8 mm. M8 is a very common metric size for furniture, bikes and machinery.
An 8 mm socket or wrench
This is the metric size for M5 bolt heads, usually. The nearest inch size is 5/16 in (7.94 mm), which is slightly smaller, so an inch socket can slip on an 8 mm head. Use the exact size when you can.
An 8 mm drill bit
A standard metric bit size. The closest fraction bit, 5/16 in, drills a hole about 0.06 mm smaller.
8 mm film
Standard 8 and Super 8 film are both 8 mm wide, which gave the format its name.
8 mm panel board
Plywood, MDF and similar boards are sold in 8 mm thickness. Compared with inch sizes, 8 mm is a little more than 5/16 in (7.94 mm).
An 8 mm bead
8 mm is a common round bead size in jewelry and crafts, a little under 1/3 of an inch across.
Nearby millimeter sizes
Decimals are rounded to 3 places. In the fraction columns, + means the value is slightly longer than the fraction, − slightly shorter, and = exact.
| Millimeters (mm) | Inches (decimal) | Nearest 1/16″ | Nearest 1/32″ |
|---|---|---|---|
| 1 mm | 0.039 | 1/16″ − | 1/32″ + |
| 2 mm | 0.079 | 1/16″ + | 3/32″ − |
| 3 mm | 0.118 | 1/8″ − | 1/8″ − |
| 4 mm | 0.157 | 3/16″ − | 5/32″ + |
| 5 mm | 0.197 | 3/16″ + | 3/16″ + |
| 6 mm | 0.236 | 1/4″ − | 1/4″ − |
| 7 mm | 0.276 | 1/4″ + | 9/32″ − |
| 8 mm | 0.315 | 5/16″ + | 5/16″ + |
| 9 mm | 0.354 | 3/8″ − | 11/32″ + |
| 10 mm | 0.394 | 3/8″ + | 13/32″ − |
| 11 mm | 0.433 | 7/16″ − | 7/16″ − |
| 12 mm | 0.472 | 1/2″ − | 15/32″ + |
| 13 mm | 0.512 | 1/2″ + | 1/2″ + |
| 14 mm | 0.551 | 9/16″ − | 9/16″ − |
| 15 mm | 0.591 | 9/16″ + | 19/32″ − |
| 16 mm | 0.63 | 5/8″ + | 5/8″ + |
| 17 mm | 0.669 | 11/16″ − | 21/32″ + |
| 18 mm | 0.709 | 11/16″ + | 23/32″ − |
| 19 mm | 0.748 | 3/4″ − | 3/4″ − |
| 20 mm | 0.787 | 13/16″ − | 25/32″ + |
Frequently asked questions
How many inches is 8 mm?
8 mm is about 0.315 inches (0.31496 in), or about 5/16 inch, slightly more.
Is 8 mm the same as 5/16 inch?
Almost. 5/16 inch is 7.9375 mm, so 8 mm is about 0.06 mm longer. For most everyday uses they are treated as the same size.
How many cm is 8 mm?
8 mm is exactly 0.8 cm, because 1 cm is 10 mm.
Is 8 mm the same as 3/8 inch?
No. 3/8 inch is 9.525 mm, which is about 1.5 mm larger than 8 mm.
Can I use a 5/16 inch socket on an 8 mm bolt head?
It may fit, but it is slightly small and can slip or damage the head. Use an 8 mm socket when you have one.
How do I convert mm to inches?
Divide the millimeters by 25.4. For 8 mm, 8 ÷ 25.4 ≈ 0.315 inches.