Drill Bit Size Chart & Converter
Fraction, metric, letter, number.
Fraction, metric, letter, number.
How to use Drill Bit Size Chart & Converter
- Select a standard size, or type a measurement to find the nearest drill
- See fractional, decimal, metric, number, and letter equivalents
- Check the tap-drill reference chart for matching thread sizes
About Drill Bit Size Chart & Converter
Convert drill bit sizes between fractional inch, decimal inch, metric millimetres, number gauge (#1–#80) and letter (A–Z) sizes. Pick any standard size and the tool lays out all of its equivalents at once, so you can turn a 1/8" bit into millimetres, read the decimal inch value of a #30, or find the metric match for a letter Q without hunting through a printed chart.
Working the other way, type a measurement in whichever system you have and the converter finds the nearest standard drill. Because number, letter and fractional bits rarely land on a round millimetre, it shows the closest available size rather than pretending an exact equivalent exists — useful when a plan calls for a metric hole but your index only has imperial bits, or vice versa. A built-in tap-drill reference covers common imperial UNC/UNF threads (like the #7 drill for a 1/4-20 tap) and metric coarse taps, so you can size the correct clearance or pilot hole before you cut.
Fractional, number and letter sizes follow ASME B94.11M and the metric list follows BS 328 / ISO, with inch and millimetre values related by the exact factor of 25.4. That keeps every result consistent whether you're matching a tap, chasing a threaded hole, or just decoding an unlabelled bit from a mixed set. It's completely free with no sign-up, and it runs entirely in your browser — nothing you type is uploaded anywhere.
Read it as a drill bit size chart — fraction, decimal, metric, number and letter sizes lined up together — or convert a single measurement.
Frequently asked questions
- What size is a #7 drill bit?
- A #7 drill bit is 0.201 inch, or about 5.11 mm. It is the standard tap drill for a 1/4-20 UNC thread, which you'll also find in the tap-drill chart built into 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.
- How do I convert a fractional drill bit to millimetres?
- Select the fractional inch size and the converter shows its exact millimetre value alongside the decimal inch, number and letter equivalents. Inch and mm are linked by the exact factor 25.4, so a 1/4" bit is precisely 6.35 mm.
- What size is a letter drill bit like F, Q or Z?
- Letter sizes run from A (smallest, 0.234 inch) up to Z (largest, 0.413 inch) and sit between the number and fractional ranges. Pick the letter and the tool gives its decimal inch, millimetre and nearest fractional equivalents.
- What is the decimal inch equivalent of a drill bit?
- Every size in the tool lists its decimal inch value, so you can read the decimal for a fraction, a number gauge or a letter bit at a glance. This is handy for setting a caliper or comparing bits that use different labelling systems.
- What drill bit do I need for a 1/4-20 tap?
- A #7 (0.201 inch) drill. The tool's tap-drill reference covers common imperial UNC/UNF threads and metric coarse taps so you can pick the right pilot size.
- What drill bit do I use for an M6 tap?
- An M6 x 1.0 coarse thread uses a 5.0 mm tap drill. The metric side of the tap-drill chart lists the standard coarse taps alongside their recommended drill sizes.
- How do the number drill sizes (#1–#80) work?
- Number gauge sizes run backwards: #1 is the largest at 0.228 inch and #80 is the smallest at about 0.0135 inch. The tool converts any number bit to its decimal inch, millimetre and nearest fractional equivalents.
- 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.
- Is the drill bit size converter free and private?
- Yes. It is completely free with no sign-up, and every conversion happens locally in your browser, so nothing you enter is sent to a server or uploaded anywhere.