Drill bit sizes span four overlapping systems: fractional inches, decimal inches, wire gauge numbers, and metric millimeters. Wire gauge numbers run backward — a higher number means a smaller drill bit. The table above maps all four systems in one place so you can find the closest available size in any system.
How it works
The four systems cover different diameter ranges:
- Wire gauge (1–80): Covers 0.0135"–0.228" (0.34–5.79 mm). Higher number = smaller drill.
- Letter (A–Z): Covers 0.234"–0.413" (5.94–10.49 mm). A is the smallest, Z the largest.
- Fractional: Covers 1/64" to 1" in 1/64" increments (0.4–25.4 mm).
- Metric: Any diameter in mm — the universal system.
Wire gauge and letter sizes fill gaps between common fractional sizes, especially in the range critical for tapping threads and clearance holes.
Practical examples
Example 1 — Tapping a ¼-20 thread. The correct tap drill for ¼-20 UNC at 75% thread is a #7 wire gauge drill = 0.201" = ~5.1 mm. The fractional nearest equivalent is 13/64" (0.203"), which is slightly too large for a clean thread.
Example 2 — Metric screw pilot hole. M3 screw (3 mm diameter, 0.5 mm pitch) needs a 2.5 mm pilot hole. Look up 2.50 in the metric column — no exact fractional equivalent; nearest is 3/32" (2.38 mm, slightly undersized) or wire gauge #41 (2.438 mm).
Example 3 — Finding the closest metric drill for a US fractional. You need 3/8" but only have a metric set. 3/8" = 0.375" = 9.525 mm. The nearest standard metric drills are 9.5 mm (slightly under) and 9.75 mm (slightly over).
Common mistakes
- Thinking a larger wire gauge number means a larger drill. Wire gauge sizes run backward from most systems — #80 is a hair (~0.343 mm) and #1 is nearly a quarter inch (5.8 mm).
- Confusing wire gauge drills with wire AWG. AWG (American Wire Gauge for electrical wire) and drill bit wire gauges are completely different numbering systems.
- Not choosing the right fit. For clearance holes (screw passes through), drill slightly larger than the screw diameter. For pilot holes (screw cuts threads), drill slightly smaller.
International and regional variations
| Country / Region | Preferred system | Notes |
|---|---|---|
| US | Fractional, wire gauge, letter | All three systems in common use; metric also available |
| Europe / International | Metric (mm) | Metric is the primary standard; fractional rarely used |
| UK | Metric + some fractional | Metric dominant; some older workshop stock uses fractional |
| Japan | Metric | Metric only in modern usage |
Quick reference — common sizes by use
| Use | Metric | Fractional | Wire gauge / Letter |
|---|---|---|---|
| #6 wood screw pilot (hardwood) | 4.0 mm | 5/32" | #22 |
| #8 wood screw pilot (hardwood) | 4.0 mm | 5/32" | #22 |
| ¼-20 tap drill (75% thread) | 5.1 mm | 13/64" | #7 |
| M4 tap drill (0.7 pitch) | 3.3 mm | — | #30 |
| ¼" clearance hole | 6.5 mm | 17/64" | — |