Drill bits are sold in two systems: fractional inch sizes (1/8″, 1/4″, 3/8″…) mostly in the US, and metric sizes (3 mm, 6 mm, 10 mm…) almost everywhere else. The two never line up exactly, so a “close” bit can leave a hole that is too tight or too loose. These tables give the exact conversions and the nearest equivalent in the other system, with the difference in millimeters so you can decide if it is close enough.
Fractional Drill Bits to Metric (1/16″ to 1/2″)
The “nearest metric bit” column uses 0.5 mm steps, which is how most metric drill sets are sized. A plus difference means the metric bit is larger; minus means it is smaller.
| Fractional bit | Decimal inches | Exact mm | Nearest metric bit | Difference |
|---|---|---|---|---|
| 1/16″ | 0.0625″ | 1.59 mm | 1.5 mm | −0.09 mm |
| 5/64″ | 0.0781″ | 1.98 mm | 2.0 mm | +0.02 mm |
| 3/32″ | 0.0938″ | 2.38 mm | 2.5 mm | +0.12 mm |
| 7/64″ | 0.1094″ | 2.78 mm | 3.0 mm | +0.22 mm |
| 1/8″ | 0.1250″ | 3.18 mm | 3.0 mm | −0.17 mm |
| 9/64″ | 0.1406″ | 3.57 mm | 3.5 mm | −0.07 mm |
| 5/32″ | 0.1563″ | 3.97 mm | 4.0 mm | +0.03 mm |
| 11/64″ | 0.1719″ | 4.37 mm | 4.5 mm | +0.13 mm |
| 3/16″ | 0.1875″ | 4.76 mm | 5.0 mm | +0.24 mm |
| 13/64″ | 0.2031″ | 5.16 mm | 5.0 mm | −0.16 mm |
| 7/32″ | 0.2188″ | 5.56 mm | 5.5 mm | −0.06 mm |
| 15/64″ | 0.2344″ | 5.95 mm | 6.0 mm | +0.05 mm |
| 1/4″ | 0.2500″ | 6.35 mm | 6.5 mm | +0.15 mm |
| 17/64″ | 0.2656″ | 6.75 mm | 6.5 mm | −0.25 mm |
| 9/32″ | 0.2813″ | 7.14 mm | 7.0 mm | −0.14 mm |
| 19/64″ | 0.2969″ | 7.54 mm | 7.5 mm | −0.04 mm |
| 5/16″ | 0.3125″ | 7.94 mm | 8.0 mm | +0.06 mm |
| 21/64″ | 0.3281″ | 8.33 mm | 8.5 mm | +0.17 mm |
| 11/32″ | 0.3438″ | 8.73 mm | 8.5 mm | −0.23 mm |
| 23/64″ | 0.3594″ | 9.13 mm | 9.0 mm | −0.13 mm |
| 3/8″ | 0.3750″ | 9.53 mm | 9.5 mm | −0.02 mm |
| 25/64″ | 0.3906″ | 9.92 mm | 10.0 mm | +0.08 mm |
| 13/32″ | 0.4063″ | 10.32 mm | 10.5 mm | +0.18 mm |
| 27/64″ | 0.4219″ | 10.72 mm | 10.5 mm | −0.22 mm |
| 7/16″ | 0.4375″ | 11.11 mm | 11.0 mm | −0.11 mm |
| 29/64″ | 0.4531″ | 11.51 mm | 11.5 mm | −0.01 mm |
| 15/32″ | 0.4688″ | 11.91 mm | 12.0 mm | +0.09 mm |
| 31/64″ | 0.4844″ | 12.30 mm | 12.5 mm | +0.20 mm |
| 1/2″ | 0.5000″ | 12.70 mm | 12.5 mm | −0.20 mm |
Metric Drill Bits to Inches (1 mm to 13 mm)
Here the nearest fractional size is to 1/64″, the finest step in a standard fractional drill index.
| Metric bit | Decimal inches | Nearest fraction (1/64″) | Difference |
|---|---|---|---|
| 1.0 mm | 0.0394″ | 3/64″ | +0.19 mm |
| 1.5 mm | 0.0591″ | 1/16″ | +0.09 mm |
| 2.0 mm | 0.0787″ | 5/64″ | −0.02 mm |
| 2.5 mm | 0.0984″ | 3/32″ | −0.12 mm |
| 3.0 mm | 0.1181″ | 1/8″ | +0.17 mm |
| 3.5 mm | 0.1378″ | 9/64″ | +0.07 mm |
| 4.0 mm | 0.1575″ | 5/32″ | −0.03 mm |
| 4.5 mm | 0.1772″ | 11/64″ | −0.13 mm |
| 5.0 mm | 0.1969″ | 13/64″ | +0.16 mm |
| 5.5 mm | 0.2165″ | 7/32″ | +0.06 mm |
| 6.0 mm | 0.2362″ | 15/64″ | −0.05 mm |
| 6.5 mm | 0.2559″ | 1/4″ | −0.15 mm |
| 7.0 mm | 0.2756″ | 9/32″ | +0.14 mm |
| 7.5 mm | 0.2953″ | 19/64″ | +0.04 mm |
| 8.0 mm | 0.3150″ | 5/16″ | −0.06 mm |
| 8.5 mm | 0.3346″ | 21/64″ | −0.17 mm |
| 9.0 mm | 0.3543″ | 23/64″ | +0.13 mm |
| 9.5 mm | 0.3740″ | 3/8″ | +0.02 mm |
| 10.0 mm | 0.3937″ | 25/64″ | −0.08 mm |
| 10.5 mm | 0.4134″ | 13/32″ | −0.18 mm |
| 11.0 mm | 0.4331″ | 7/16″ | +0.11 mm |
| 11.5 mm | 0.4528″ | 29/64″ | +0.01 mm |
| 12.0 mm | 0.4724″ | 15/32″ | −0.09 mm |
| 12.5 mm | 0.4921″ | 31/64″ | −0.20 mm |
| 13.0 mm | 0.5118″ | 33/64″ | +0.10 mm |
How Close Is Close Enough?
It depends on what the hole is for:
- Clearance holes (a bolt or screw passes straight through): a bit that is slightly larger is fine and often preferred. A 6.5 mm bit is a good clearance hole for a 1/4″ bolt.
- Pilot holes for wood screws: small differences rarely matter; stay slightly smaller than the screw’s core so the threads can grip.
- Dowels, pins and press fits: use the exact size. A 6 mm dowel in a 1/4″ (6.35 mm) hole will be loose; a 1/4″ dowel will not go into a 6 mm hole.
- Tapping threads: use the tap drill size specified for that thread — never a “near” size from another system.
Sizes That Are Nearly Identical
A few pairs are within about 0.1 mm and are effectively interchangeable for most jobs: 3/16″ (4.76 mm) ≈ 4.8 mm, 5/16″ (7.94 mm) ≈ 8 mm, and 15/64″ (5.95 mm) ≈ 6 mm. Pairs such as 1/4″ and 6 mm (0.35 mm apart) or 3/8″ and 10 mm (0.475 mm apart) are not interchangeable where fit matters.
Number and Letter Drill Sizes
In the US you will also see number sizes (#1 to #80, getting smaller as the number rises) and letter sizes (A to Z, getting larger). They fill the gaps between fractional sizes and are mainly used for tap drills and precision work. If a plan calls for one, look up its exact decimal size and convert with 1 inch = 25.4 mm.
How to Convert Any Drill Size
- Fraction to mm: divide the top number by the bottom number, then multiply by 25.4. Example: 5/32″ = 0.15625 × 25.4 = 3.97 mm.
- mm to inches: divide by 25.4. Example: 8 mm ÷ 25.4 = 0.3150″, which is just over 5/16″ (0.3125″).
For quick lookups, use our mm to inches converter or the small metric sizes page, which lists common screw and socket sizes too.
Frequently Asked Questions
What size drill bit is 1/4 inch in mm?
1/4″ is exactly 6.35 mm. The nearest common metric bit is 6.5 mm (0.15 mm larger) or 6 mm (0.35 mm smaller).
Is a 10 mm drill bit the same as 3/8 inch?
No. 10 mm is 0.3937″, while 3/8″ is 0.375″ (9.525 mm). The 10 mm bit is about 0.475 mm larger.
What is the metric equivalent of a 1/8 inch drill bit?
1/8″ is 3.175 mm. A 3 mm bit is slightly smaller and 3.2 mm (sold in some metric sets) is almost exact.
Related: How to read a tape measure · Inches to MM chart (printable) · Small metric sizes in inches