Drill bit size chart
Drill bits use several sizing systems that do not line up cleanly. Use this chart to convert a fractional or letter size to its nearest metric millimetre, then choose the right bit for your fastener.
| Bit size | Type | Closest metric | Typical use |
|---|---|---|---|
| 1/16 in | Fractional | 1.59 mm | Pilot holes, small screws |
| 1/8 in | Fractional | 3.18 mm | Wood, metal, plastic |
| 3/16 in | Fractional | 4.76 mm | General purpose drilling |
| 1/4 in | Fractional | 6.35 mm | Lag screws, wall anchors |
| 3/8 in | Fractional | 9.53 mm | Bolts, dowels |
| 1/2 in | Fractional | 12.70 mm | Large bolts, hinges |
| A | Letter | 5.94 mm | Drill size A (0.234 in) |
| E | Letter | 6.35 mm | Drill size E (0.250 in) |
| M | Letter | 7.49 mm | Drill size M (0.295 in) |
| #1 | Wire gauge | 5.79 mm | Drill size #1 (0.228 in) |
| #7 | Wire gauge | 5.11 mm | Drill size #7 (0.201 in) |
| #21 | Wire gauge | 4.04 mm | Drill size #21 (0.159 in) |
| 3 mm | Metric | 3.00 mm | Metric fasteners |
| 5 mm | Metric | 5.00 mm | Metric fasteners |
| 6 mm | Metric | 6.00 mm | Metric fasteners |
| 8 mm | Metric | 8.00 mm | Metric fasteners |
| 10 mm | Metric | 10.00 mm | Metric fasteners |
| 12 mm | Metric | 12.00 mm | Metric fasteners |
Notes
- Letter and number sizes follow the ANSI standard; metric sizes follow ISO millimetres.
- Closest metric is rounded to two decimals — for precise work, measure the bit directly.
- Always drill a test hole in scrap material before the final piece.
Frequently asked questions
- What is a letter drill size?
- Letter sizes (A–Z) are a US system from about 0.234 in to 0.413 in, sitting between fractional and number sizes for finer steps.
- What is a number drill size?
- Wire-gauge or number sizes (#1–#80) run from about 0.0135 in to 0.228 in and are common for tapping and small holes.
- How do I convert inches to mm?
- Multiply the inch value by 25.4. For example 1/4 in = 6.35 mm and 1/2 in = 12.7 mm.
- Which bit should I use for a wood screw?
- Pick a bit slightly smaller than the screw shaft; pilot holes stop splitting and let the threads bite cleanly.