The Challenge
Drill bits in the US are sold in three incompatible sizing systems — fractional inches (1/16, 1/8, 1/4), wire gauge numbers (1–80, smaller number = larger bit), and letter sizes (A–Z). The rest of the world sells metric bits in millimeters. A #29 drill bit for a 10-32 tap is 0.136 inches = 3.454mm — there is no intuitive way to know that without a chart. The formula is straightforward: multiply inches by 25.4 to get millimeters. A 1/4-inch bit is 0.250 × 25.4 = 6.35mm. A 1/2-inch bit is 12.7mm. The hard part is fractional bits — 7/32 is 0.21875 inches = 5.556mm. Wire gauge and letter sizes have no formula; they require a lookup table.
Fractional Drill Bit Sizes: Inches to mm Chart
| Fraction | Decimal (in) | Millimeters (mm) | Nearest Metric Bit |
|---|---|---|---|
| 1/64 | 0.01563 | 0.397 mm | 0.4 mm |
| 1/32 | 0.03125 | 0.794 mm | 0.8 mm |
| 3/64 | 0.04688 | 1.191 mm | 1.2 mm |
| 1/16 | 0.06250 | 1.588 mm | 1.6 mm |
| 5/64 | 0.07813 | 1.984 mm | 2.0 mm |
| 3/32 | 0.09375 | 2.381 mm | 2.4 mm |
| 7/64 | 0.10938 | 2.778 mm | 2.8 mm |
| 1/8 | 0.12500 | 3.175 mm | 3.2 mm |
| 9/64 | 0.14063 | 3.572 mm | 3.5 mm |
| 5/32 | 0.15625 | 3.969 mm | 4.0 mm |
| 11/64 | 0.17188 | 4.366 mm | 4.4 mm |
| 3/16 | 0.18750 | 4.763 mm | 4.8 mm |
| 13/64 | 0.20313 | 5.159 mm | 5.2 mm |
| 7/32 | 0.21875 | 5.556 mm | 5.5 mm |
| 15/64 | 0.23438 | 5.953 mm | 6.0 mm |
| 1/4 | 0.25000 | 6.350 mm | 6.5 mm |
| 17/64 | 0.26563 | 6.747 mm | 6.8 mm |
| 9/32 | 0.28125 | 7.144 mm | 7.2 mm |
| 19/64 | 0.29688 | 7.541 mm | 7.5 mm |
| 5/16 | 0.31250 | 7.938 mm | 8.0 mm |
| 21/64 | 0.32813 | 8.334 mm | 8.4 mm |
| 11/32 | 0.34375 | 8.731 mm | 8.8 mm |
| 23/64 | 0.35938 | 9.128 mm | 9.1 mm |
| 3/8 | 0.37500 | 9.525 mm | 9.5 mm |
| 25/64 | 0.39063 | 9.922 mm | 10.0 mm |
| 13/32 | 0.40625 | 10.319 mm | 10.3 mm |
| 27/64 | 0.42188 | 10.716 mm | 10.7 mm |
| 7/16 | 0.43750 | 11.113 mm | 11.1 mm |
| 29/64 | 0.45313 | 11.509 mm | 11.5 mm |
| 15/32 | 0.46875 | 11.906 mm | 12.0 mm |
| 31/64 | 0.48438 | 12.303 mm | 12.3 mm |
| 1/2 | 0.50000 | 12.700 mm | 12.7 mm |
Wire Gauge Drill Bits: Number to mm Conversion
| Wire Gauge # | Inches (in) | Millimeters (mm) | Common Use |
|---|---|---|---|
| #1 | 0.2280 | 5.791 mm | Clearance, 1/4-20 tap |
| #2 | 0.2210 | 5.613 mm | General metalwork |
| #3 | 0.2130 | 5.410 mm | General metalwork |
| #4 | 0.2090 | 5.309 mm | General metalwork |
| #7 | 0.2010 | 5.105 mm | 1/4-20 tap drill |
| #10 | 0.1935 | 4.915 mm | General metalwork |
| #15 | 0.1800 | 4.572 mm | General metalwork |
| #18 | 0.1695 | 4.305 mm | General metalwork |
| #21 | 0.1590 | 4.039 mm | 10-24 tap drill |
| #25 | 0.1495 | 3.797 mm | General metalwork |
| #29 | 0.1360 | 3.454 mm | 10-32 tap drill |
| #30 | 0.1285 | 3.264 mm | 8-32 tap drill |
| #33 | 0.1130 | 2.870 mm | 6-32 tap drill |
| #36 | 0.1065 | 2.705 mm | General metalwork |
| #40 | 0.0980 | 2.489 mm | 4-40 tap drill |
| #43 | 0.0890 | 2.261 mm | General metalwork |
| #50 | 0.0700 | 1.778 mm | 2-56 tap drill |
| #55 | 0.0520 | 1.321 mm | Electronics, PCB |
| #60 | 0.0400 | 1.016 mm | Electronics, PCB |
| #65 | 0.0350 | 0.889 mm | Fine electronics |
| #70 | 0.0280 | 0.711 mm | Fine electronics |
| #75 | 0.0210 | 0.533 mm | Watchmaking |
| #80 | 0.0135 | 0.343 mm | Smallest standard wire gauge |
Letter Gauge Drill Bits: A–Z in Inches and mm
| Letter | Inches (in) | Millimeters (mm) | Common Use |
|---|---|---|---|
| A | 0.2340 | 5.944 mm | 1/4-28 UNF tap drill |
| B | 0.2380 | 6.045 mm | General |
| C | 0.2420 | 6.147 mm | General |
| D | 0.2460 | 6.248 mm | General |
| E | 0.2500 | 6.350 mm | = 1/4 inch |
| F | 0.2570 | 6.528 mm | 5/16-18 tap drill |
| G | 0.2610 | 6.629 mm | General |
| H | 0.2660 | 6.756 mm | General |
| I | 0.2720 | 6.909 mm | General |
| J | 0.2770 | 7.036 mm | General |
| K | 0.2810 | 7.137 mm | General |
| L | 0.2900 | 7.366 mm | 5/16-24 tap drill |
| M | 0.2950 | 7.493 mm | General |
| N | 0.3020 | 7.671 mm | General |
| O | 0.3160 | 8.026 mm | General |
| P | 0.3230 | 8.204 mm | General |
| Q | 0.3320 | 8.433 mm | 3/8-16 tap drill |
| R | 0.3390 | 8.611 mm | General |
| S | 0.3480 | 8.839 mm | General |
| T | 0.3580 | 9.093 mm | General |
| U | 0.3680 | 9.347 mm | General |
| V | 0.3770 | 9.576 mm | General |
| W | 0.3860 | 9.804 mm | General |
| X | 0.3970 | 10.084 mm | General |
| Y | 0.4040 | 10.262 mm | General |
| Z | 0.4130 | 10.490 mm | General |
Three Drill Sizing Systems and Why the US Uses All of Them
Fractional inch sizes (1/16 through 1/2 and beyond) cover general woodworking and construction — they map directly to common fastener sizes. Wire gauge numbers emerged from the metalworking and machining industry where tight increments matter more than round fractions; the gauge system predates decimal measurement and the numbering direction (larger number = smaller drill) is a historical artifact. Letter sizes fill the gap in the 6–10.5mm range where fractional increments are too coarse for precision tapping operations. A machinist working on US-spec parts needs all three systems. The metric system sidesteps this entirely — metric bits run in clean 0.1mm or 0.5mm steps with no lookup tables required.
Choosing the Right Drill Size for a Tapped Hole
- Identify the thread specification — for example 1/4-20 UNC means 1/4-inch diameter, 20 threads per inch
- Look up the tap drill size in a thread chart — 1/4-20 requires a #7 bit (0.201in = 5.105mm)
- Convert to mm if using a metric drill set: 5.105mm — nearest standard metric is 5.1mm
- Drill the hole, then tap — undersized holes break taps, oversized holes strip threads under load
Tap Drill Tolerance: Where Getting It Wrong Is Expensive
- Going 0.1mm oversize on a tap drill reduces thread engagement by approximately 5–10% — acceptable for light loads
- Going 0.2mm oversize in aluminum or plastic often strips under the first full torque load
- A broken tap inside a hole is difficult to remove — always drill the exact specified size in hard materials
- For blind holes (holes that don't go all the way through), undersizing is worse than oversizing — chips pack and snap the tap
Step-by-Step Workflow
Enter your drill bit size in decimal inches — or use the chart below for fractional, wire gauge, or letter sizes
Millimeter equivalent appears instantly
Match to nearest available metric bit — tolerances within 0.05mm are generally acceptable for clearance holes
Specifications
- Formula
- mm = inches × 25.4
- 1/16 inch
- 1.588 mm
- 1/8 inch
- 3.175 mm
- 1/4 inch
- 6.350 mm
- 1/2 inch
- 12.700 mm
- Wire gauge #1
- 5.791 mm (largest wire gauge)
- Wire gauge #80
- 0.343 mm (smallest wire gauge)
Best Practices
- Convert fractions to decimals first: 7/32 = 7 ÷ 32 = 0.21875, then × 25.4 = 5.556mm
- Wire gauge numbering is counterintuitive — #1 is the largest (5.791mm), #80 is the smallest (0.343mm)
- Letter sizes run A (smallest, 6.045mm) to Z (largest, 10.490mm) — they overlap with fractional sizes in the 6–10.5mm range
- For tap drill sizes: the correct bit leaves enough material for thread cutting — #29 for 10-32, #7 for 1/4-20
- Metric bits are available in 0.1mm increments — if your conversion lands between sizes, go smaller for tapped holes, larger for clearance holes
Frequently Asked Questions
How do I convert fractional drill bit sizes to mm?
Divide the numerator by the denominator to get decimal inches, then multiply by 25.4. Examples: 3/8 = 0.375 × 25.4 = 9.525mm. 7/64 = 0.109375 × 25.4 = 2.778mm. 1/2 = 0.500 × 25.4 = 12.700mm. Most metric sets don't include 9.525mm — the nearest available size is usually 9.5mm, which is 0.025mm smaller.
What is a #10 wire gauge drill bit in mm?
#10 wire gauge drill bit is 0.1935 inches = 4.915mm. Wire gauge drill bits run from #1 (5.791mm) to #80 (0.343mm) — the number increases as the bit gets smaller. This system is common in US aerospace and electronics work but has no metric equivalent; you look up the number in a table.
What drill bit size do I need for a 10-32 tap?
A 10-32 thread requires a #29 drill bit, which is 0.136 inches = 3.454mm. The nearest standard metric bit is 3.5mm, which is slightly oversize — acceptable for most materials but will produce slightly looser threads. For precise tapping in aluminum or steel, use the exact #29 if available.
What is a 1/4-inch drill bit in mm?
1/4 inch = 0.250 inches × 25.4 = 6.350mm. Standard metric sets include 6.0mm and 6.5mm but not 6.35mm. For a clearance hole (bolt passes through freely), use 6.5mm. For a tighter fit or if the hole is a locating feature, use 6.0mm and note the 0.35mm undersize.
How do letter gauge drill bits compare to mm?
Letter sizes run A through Z: A = 0.234in = 5.944mm up to Z = 0.413in = 10.490mm. They fill the gap between fractional sizes in the 6–10.5mm range. Letter A (5.944mm) is used as the tap drill for 1/4-28 UNF threads. Letter drills are common in US machine shops but are not a standard outside North America.
Can I use a metric drill bit instead of a fractional inch bit?
Yes, within tolerance. The question is how much size difference matters for your application. Clearance holes (bolt slides through) tolerate 0.1–0.3mm oversize without issue. Tapped holes (thread is cut) need the drill within 0.05mm of spec or thread engagement drops. Press-fit holes need the drill within 0.01–0.02mm — use the correct size or ream to final dimension.
What are the most common drill bit sizes in inches and their mm equivalents?
The most-used fractional sizes: 1/16=1.588mm, 5/64=1.984mm, 3/32=2.381mm, 1/8=3.175mm, 5/32=3.969mm, 3/16=4.763mm, 7/32=5.556mm, 1/4=6.350mm, 5/16=7.938mm, 3/8=9.525mm, 7/16=11.113mm, 1/2=12.700mm. These cover the majority of woodworking and light metalworking applications.