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Drill Bit Size Converter
Convert any drill bit between fractional inch, number (#1–#80), letter (A–Z) and metric sizes. You get the decimal-inch and mm diameter, the nearest bit in every other series and a chart of the sizes around it.
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I have a
In mm or decimal inches, depending on the mode above
Enter your values to see the result.
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How to use the Drill Bit Size Converter
- Choose what you have: fractional, number, letter, metric or decimal inch.
- Pick the size from the list or type the diameter.
- Press Convert to get the decimal inch and mm diameter.
- Read the nearest sizes in the other series and the difference in thou.
- Use the nearby-sizes table to choose a slightly larger or smaller bit.
Common drill sizes and their equivalents
| Drill | Decimal in | mm | Nearest other sizes | Common use |
|---|---|---|---|---|
| #43 | 0.0890 | 2.261 | 2.25 mm, 3/32" (0.0938) | Tap drill for 4-40 |
| #36 | 0.1065 | 2.705 | 2.7 mm, 7/64" (0.1094) | Tap drill for 6-32 |
| #29 | 0.1360 | 3.454 | 3.5 mm, 9/64" (0.1406) | Tap drill for 8-32 |
| #25 | 0.1495 | 3.797 | 3.8 mm, 5/32" (0.1563) | Tap drill for 10-24 |
| #21 | 0.1590 | 4.039 | 4.0 mm, 5/32" (0.1563) | Tap drill for 10-32 |
| #7 | 0.2010 | 5.105 | 5.1 mm, 13/64" (0.2031) | Tap drill for 1/4-20 |
| F | 0.2570 | 6.528 | 6.5 mm, 1/4" (0.2500) | Tap drill for 5/16-18; 1/4" clearance |
| 5/16" | 0.3125 | 7.938 | 7.9 mm, letter O (0.316) | Tap drill for 3/8-16 |
| U | 0.3680 | 9.347 | 9.3 mm, 3/8" (0.3750) | Tap drill for 7/16-14 |
| 27/64" | 0.4219 | 10.716 | 10.7 mm | Tap drill for 1/2-13 |
Tap drill sizes give roughly 75% thread engagement. For other threads use a tap drill size calculator.
How the four drill series fit together
- Fractional bits step in 1/64" (0.0156 in, about 0.4 mm). They are the standard in US hardware stores.
- Number bits (#80 to #1) cover 0.0135–0.228 in in small, uneven steps, filling the gaps between fractions for small precise holes.
- Letter bits (A to Z) cover 0.234–0.413 in, continuing where number drills stop.
- Metric bits usually step by 0.1 mm up to 10 mm and by 0.5 mm above that.
To convert by hand: decimal inch = numerator ÷ denominator (for example 7/32 = 0.21875 in) and mm = inches × 25.4. A thousandth of an inch ("thou" or "mil") is 0.0254 mm.
Frequently asked questions
What size is a #7 drill bit?
A #7 drill is 0.2010 in (5.105 mm). It is the standard tap drill for a 1/4-20 UNC thread, and the nearest fractional size is 13/64" (0.2031 in).
What is the metric equivalent of a 1/4 inch drill bit?
1/4" is 0.2500 in, which is exactly 6.35 mm. The closest metric bits are 6.3 mm and 6.4 mm; letter E is also exactly 0.250 in.
Why are there number and letter drill sizes?
Number and letter drills fill the gaps between 64ths of an inch so machinists can drill precise holes for taps, rivets and pins. Number drills come from wire gauges and run from #80 (0.0135 in) up to #1 (0.2280 in); letter drills continue from A (0.234 in) to Z (0.413 in).
Do bigger drill numbers mean bigger bits?
No. Number drills get smaller as the number rises: #1 is 0.228 in and #80 is 0.0135 in. Letter drills go the other way, from A (smallest) to Z (largest).
What metric drill is closest to 3/8 inch?
3/8" is 0.375 in or 9.525 mm. The nearest metric bit is 9.5 mm (0.374 in), only about 0.025 mm smaller, which is close enough for most clearance holes.
How close is close enough when substituting a drill?
For wood and clearance holes a few thousandths of an inch rarely matter. For tapped holes, press fits and reamed holes, stay with the specified size or check thread engagement, because a 0.002–0.005 in change can matter.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.
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