Metric to Imperial Conversion Table
This table converts metric drill bit sizes (mm) to their decimal inch and nearest fractional inch equivalents. Conversion factor: 1mm = 0.03937 inches.
0.5–5.0 mm
| Metric (mm) |
Decimal Inch |
Nearest Fraction |
| 0.5 | 0.0197" | 1/64" |
| 1.0 | 0.0394" | 1/32" |
| 1.5 | 0.0591" | 1/16" |
| 2.0 | 0.0787" | 5/64" |
| 2.5 | 0.0984" | 3/32" |
| 3.0 | 0.1181" | 1/8" |
| 3.5 | 0.1378" | 9/64" |
| 4.0 | 0.1575" | 5/32" |
| 4.5 | 0.1772" | 11/64" |
| 5.0 | 0.1969" | 13/64" |
5.5–10.0 mm
| Metric (mm) |
Decimal Inch |
Nearest Fraction |
| 5.5 | 0.2165" | 7/32" |
| 6.0 | 0.2362" | 15/64" |
| 6.5 | 0.2559" | 1/4" |
| 7.0 | 0.2756" | 9/32" |
| 7.5 | 0.2953" | 19/64" |
| 8.0 | 0.3150" | 5/16" |
| 8.5 | 0.3346" | 21/64" |
| 9.0 | 0.3543" | 23/64" |
| 9.5 | 0.3740" | 3/8" |
| 10.0 | 0.3937" | 25/64" |
10.5–15.0 mm
| Metric (mm) |
Decimal Inch |
Nearest Fraction |
| 10.5 | 0.4134" | 13/32" |
| 11.0 | 0.4331" | 7/16" |
| 11.5 | 0.4528" | 29/64" |
| 12.0 | 0.4724" | 15/32" |
| 12.5 | 0.4921" | 1/2" |
| 13.0 | 0.5118" | 33/64" |
| 13.5 | 0.5315" | 17/32" |
| 14.0 | 0.5512" | 35/64" |
| 14.5 | 0.5709" | 9/16" |
| 15.0 | 0.5906" | 19/32" |
15.5–20.0 mm
| Metric (mm) |
Decimal Inch |
Nearest Fraction |
| 15.5 | 0.6102" | 39/64" |
| 16.0 | 0.6299" | 5/8" |
| 16.5 | 0.6496" | 21/32" |
| 17.0 | 0.6693" | 43/64" |
| 17.5 | 0.6890" | 11/16" |
| 18.0 | 0.7087" | 45/64" |
| 18.5 | 0.7283" | 47/64" |
| 19.0 | 0.7480" | 3/4" |
| 19.5 | 0.7677" | 49/64" |
| 20.0 | 0.7874" | 25/32" |
20.5–25.0 mm
| Metric (mm) |
Decimal Inch |
Nearest Fraction |
| 20.5 | 0.8071" | 13/16" |
| 21.0 | 0.8268" | 53/64" |
| 21.5 | 0.8465" | 27/32" |
| 22.0 | 0.8661" | 55/64" |
| 22.5 | 0.8858" | 57/64" |
| 23.0 | 0.9055" | 29/32" |
| 23.5 | 0.9252" | 59/64" |
| 24.0 | 0.9449" | 15/16" |
| 24.5 | 0.9646" | 31/32" |
| 25.0 | 0.9843" | 63/64" |
Number Drill Sizes (#1-#60)
Number drill bits are designated from #1 (largest) to #80 (smallest). Below are the most commonly used sizes #1 through #60.
#1–#20
| Number |
Diameter (inch) |
Diameter (mm) |
| #1 | 0.2280" | 5.791 |
| #2 | 0.2210" | 5.613 |
| #3 | 0.2130" | 5.410 |
| #4 | 0.2090" | 5.309 |
| #5 | 0.2055" | 5.220 |
| #6 | 0.2040" | 5.182 |
| #7 | 0.2010" | 5.105 |
| #8 | 0.1990" | 5.055 |
| #9 | 0.1960" | 4.978 |
| #10 | 0.1935" | 4.915 |
| #11 | 0.1910" | 4.851 |
| #12 | 0.1890" | 4.801 |
| #13 | 0.1850" | 4.699 |
| #14 | 0.1820" | 4.623 |
| #15 | 0.1800" | 4.572 |
| #16 | 0.1770" | 4.496 |
| #17 | 0.1730" | 4.394 |
| #18 | 0.1695" | 4.305 |
| #19 | 0.1660" | 4.216 |
| #20 | 0.1610" | 4.089 |
#21–#40
| Number |
Diameter (inch) |
Diameter (mm) |
| #21 | 0.1590" | 4.039 |
| #22 | 0.1570" | 3.988 |
| #23 | 0.1540" | 3.912 |
| #24 | 0.1520" | 3.861 |
| #25 | 0.1495" | 3.797 |
| #26 | 0.1470" | 3.734 |
| #27 | 0.1440" | 3.658 |
| #28 | 0.1405" | 3.569 |
| #29 | 0.1360" | 3.454 |
| #30 | 0.1285" | 3.264 |
| #31 | 0.1200" | 3.048 |
| #32 | 0.1160" | 2.946 |
| #33 | 0.1130" | 2.870 |
| #34 | 0.1110" | 2.819 |
| #35 | 0.1100" | 2.794 |
| #36 | 0.1065" | 2.705 |
| #37 | 0.1040" | 2.642 |
| #38 | 0.1015" | 2.578 |
| #39 | 0.0995" | 2.527 |
| #40 | 0.0980" | 2.489 |
#41–#60
| Number |
Diameter (inch) |
Diameter (mm) |
| #41 | 0.0960" | 2.438 |
| #42 | 0.0935" | 2.375 |
| #43 | 0.0890" | 2.261 |
| #44 | 0.0860" | 2.184 |
| #45 | 0.0820" | 2.083 |
| #46 | 0.0810" | 2.057 |
| #47 | 0.0785" | 1.994 |
| #48 | 0.0760" | 1.930 |
| #49 | 0.0730" | 1.854 |
| #50 | 0.0700" | 1.778 |
| #51 | 0.0670" | 1.702 |
| #52 | 0.0635" | 1.613 |
| #53 | 0.0595" | 1.511 |
| #54 | 0.0550" | 1.397 |
| #55 | 0.0520" | 1.321 |
| #56 | 0.0465" | 1.181 |
| #57 | 0.0430" | 1.092 |
| #58 | 0.0420" | 1.067 |
| #59 | 0.0410" | 1.041 |
| #60 | 0.0400" | 1.016 |
Letter Drill Sizes (A-Z)
Letter drill bits range from A (smallest) to Z (largest), filling gaps between number drills and fractional inch sizes.
| Letter |
Diameter (inch) |
Diameter (mm) |
| A | 0.2340" | 5.944 |
| B | 0.2380" | 6.045 |
| C | 0.2420" | 6.147 |
| D | 0.2460" | 6.248 |
| E | 0.2500" | 6.350 |
| F | 0.2570" | 6.528 |
| G | 0.2610" | 6.629 |
| H | 0.2660" | 6.756 |
| I | 0.2720" | 6.909 |
| J | 0.2770" | 7.036 |
| K | 0.2810" | 7.137 |
| L | 0.2900" | 7.366 |
| M | 0.2950" | 7.493 |
| N | 0.3020" | 7.671 |
| O | 0.3160" | 8.026 |
| P | 0.3230" | 8.204 |
| Q | 0.3320" | 8.433 |
| R | 0.3390" | 8.611 |
| S | 0.3480" | 8.839 |
| T | 0.3580" | 9.093 |
| U | 0.3680" | 9.347 |
| V | 0.3770" | 9.576 |
| W | 0.3860" | 9.804 |
| X | 0.3970" | 10.084 |
| Y | 0.4040" | 10.262 |
| Z | 0.4130" | 10.490 |
Common Metric-Imperial Equivalents
Quick-reference table of the most frequently needed metric-to-imperial conversions for everyday drilling tasks.
| Metric (mm) |
Exact Decimal (inch) |
Nearest Fractional Inch |
Difference |
| 1 mm | 0.0394" | 1/32" (0.0313") | -0.21 mm |
| 1.5 mm | 0.0591" | 1/16" (0.0625") | +0.09 mm |
| 2 mm | 0.0787" | 5/64" (0.0781") | -0.02 mm |
| 3 mm | 0.1181" | 1/8" (0.1250") | +0.18 mm |
| 4 mm | 0.1575" | 5/32" (0.1563") | -0.03 mm |
| 5 mm | 0.1969" | 13/64" (0.2031") | +0.16 mm |
| 6 mm | 0.2362" | 15/64" (0.2344") | -0.05 mm |
| 6.5 mm | 0.2559" | 1/4" (0.2500") | -0.15 mm |
| 7 mm | 0.2756" | 9/32" (0.2813") | +0.14 mm |
| 8 mm | 0.3150" | 5/16" (0.3125") | -0.06 mm |
| 10 mm | 0.3937" | 25/64" (0.3906") | -0.08 mm |
| 12 mm | 0.4724" | 15/32" (0.4688") | -0.09 mm |
| 12.5 mm | 0.4921" | 1/2" (0.5000") | +0.20 mm |
| 14 mm | 0.5512" | 35/64" (0.5469") | -0.11 mm |
| 16 mm | 0.6299" | 5/8" (0.6250") | -0.12 mm |
| 18 mm | 0.7087" | 45/64" (0.7031") | -0.14 mm |
| 19 mm | 0.7480" | 3/4" (0.7500") | +0.05 mm |
| 20 mm | 0.7874" | 25/32" (0.7813") | -0.16 mm |
| 22 mm | 0.8661" | 55/64" (0.8594") | -0.17 mm |
| 25 mm | 0.9843" | 63/64" (0.9844") | +0.00 mm |
Tips for Choosing Between Metric and Imperial
When to use metric: Use metric drill bits when working with metric fasteners, European or Asian equipment, or whenever the engineering drawings specify metric dimensions. Metric sizes are the global standard in manufacturing.
When to use imperial: Use imperial (inch) drill bits when working with American-made machinery, UNC/UNF threaded fasteners, or when drawings are dimensioned in inches. Imperial sizes are still dominant in US construction and plumbing.
Handling "close enough" sizes: For clearance holes (where the bolt passes through freely), a size difference of 0.1mm or less is generally acceptable. For tapping, reaming, or press-fit work, always use the exact size specified. When in doubt, choose the next size up rather than forcing an undersized bit.
Measuring tips:
- Use digital calipers to verify drill bit diameter before drilling. Printed sizes can wear off.
- Measure across the widest point of the cutting edges (the outer diameter), not the shank.
- If you are unsure whether a hole is metric or imperial, measure it with calipers and check both conversion tables above.
- Store metric and imperial bits separately to avoid mix-ups on the job site.
- A drill gauge (index plate) is a quick way to identify unmarked bits.
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