Drill Bit Size Converter
Fraction, metric, letter, number.
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About Drill Bit Size Converter
Convert drill bit sizes between fractional inch, decimal inch, metric millimetres, number gauge (#1–#80) and letter (A–Z) sizes. Pick a standard size to see all of its equivalents, or type a measurement to find the nearest standard drill — plus a handy tap-drill reference chart.
Values follow ASME B94.11M and BS 328. It is completely free with no sign-up, and it runs entirely in your browser.
Frequently asked questions
- What size is a #7 drill bit?
- A #7 drill bit is 0.201 inch, or about 5.11 mm. It's the standard tap drill for a 1/4-20 UNC thread — you'll find that and other taps in the tap-drill chart in the tool.
- How do I convert a millimetre measurement to a drill bit size?
- Choose 'Metric (mm)', type your measurement, and the tool shows the nearest standard drill in every system. Number, letter and fractional bits rarely equal a round millimetre, so it gives the closest available size rather than a false exact match.
- What drill bit do I need for a 1/4-20 tap?
- A #7 (0.201 inch) drill. The tool includes a tap-drill reference covering common imperial (UNC/UNF) and metric coarse taps.
- Do number and letter drill sizes equal metric sizes?
- Almost never exactly — number (#1–#80) and letter (A–Z) sizes are a North-American convention that falls between millimetre sizes, which is why the tool shows the nearest standard drill instead of claiming an exact equivalent.
- Which standard are the sizes based on?
- The fractional, number and letter sizes follow ASME B94.11M, and the metric list follows BS 328 / ISO. Millimetre and inch values are related by the exact factor 25.4.