How to use the drill bit size chart and converter
- Choose how you know the size: a fraction, decimal inches, millimeters, a number gauge from 1 to 80 or a letter from A to Z.
- Type the size in the box, for example 5/16, 0.3125, 7.5, 8 or H.
- Read the size in decimal inches and millimeters.
- Use the table to find the closest bit in each system, and how far off it is in thousandths of an inch.
Formula
Millimeters = inches x 25.4
Fractional bits come in steps of 1/64 inch
Number bits (#1 to #80) and letter bits (A to Z) are table values in inches
Off by = bit size - your size, in thousandths of an inch
The conversion between inches and millimeters is exact by definition. The number and letter sizes are reference tables, not formulas.
Worked example
A 5/16 inch bit is 0.3125 in, which is 7.9375 mm. The nearest letter bit is O, at 0.316 in, which is 3.5 thousandths larger. A metric 8.0 mm bit is 0.315 in.
Going the other way, #7 is 0.201 in (5.105 mm), letter A is 0.234 in and the smallest number bit, #80, is 0.0135 in. A size of 0.1995 in is closest to #8, which is 0.199 in.
Four ways to name a drill bit
Fractional bits are common in the United States and go up in sixty-fourths of an inch. Number bits cover the small sizes, with a higher number meaning a smaller bit, from #1 at 0.228 in down to #80 at 0.0135 in. Letter bits fill in the range from A (0.234 in) to Z (0.413 in). Metric bits are named by diameter in millimeters. The tool converts between all four and shows the nearest match.
Choosing a bit for a screw or tap
Clearance and pilot holes depend on the screw and the material, and tapping needs the right tap drill for the thread. This tool converts sizes but does not choose them. For the right size, follow the screw maker or a tap drill chart for your thread, and test in scrap first. For dimension changes use the unit converter, and for framing work see the stud calculator.
Reference tables
The number and letter sizes in this tool come from the standard drill size tables, as listed in the Wikipedia article on drill bit sizes, fetched on 2026-10-12. Check the marking on your own bit or a drill gauge, since sets differ and wear changes the size.
Assumptions and limits
- Number and letter sizes are reference table values in inches, from the standard drill size tables.
- Fractional bits are available in 1/64 inch steps from 1/64 to 1 inch. Metric bits are shown to the nearest 0.1 mm, and stores may stock fewer sizes.
- The tool covers 0.0135 inch (#80) to 1 inch.
- A bit that is nearest in size is not always the right bit for the job.
Frequently asked questions
What is the decimal size of a 5/16 drill bit?
It is 0.3125 inch, or 7.9375 millimeters. The nearest letter bit is O at 0.316 inch.
Which is larger, a number 7 or a number 8 drill bit?
A number 7 is larger. For number bits, the higher the number, the smaller the bit.
What is the closest fractional bit to 8 mm?
Eight millimeters is 0.315 inch, closest to 5/16 inch (0.3125 inch), a difference of about 2.5 thousandths.
How do I convert millimeters to inches?
Divide by 25.4. For example 6 mm is 0.2362 inch.