Drill Size Calculator
Type any size, like 0.309, 7.8 mm or 5/16, and get the nearest drill bit in every series (fractional, number, letter and metric) plus the closest inch fraction.
| Series | Nearest bit | Size | Difference |
|---|
| Rounded to | Fraction | Decimal | Difference |
|---|
Examples
| Size | Number or letter bit | Fractional bit | Metric bit |
|---|---|---|---|
| 0.201 in | #7 (exact) | 13/64 (0.2031) | 5.1 mm |
| 0.257 in | F (exact) | 1/4 (0.250) or 17/64 (0.2656) | 6.5 mm |
| 0.309 in | N (0.302) or O (0.316) | 5/16 (0.3125) | 7.8 mm |
| 0.346 in | S (0.348) | 11/32 (0.3438) | 8.8 mm |
| 5 mm | #9 (0.196) | 13/64 (0.2031) | 5.0 mm |
| 6.8 mm | H (0.266) | 17/64 (0.2656) | 6.8 mm |
| 8.5 mm | Q (0.332) | 21/64 (0.3281) | 8.5 mm |
How the drill series fit together
- Fractional bits go up in 1/64 inch steps (0.0156 in, about 0.4 mm): 1/16, 5/64, 3/32 …
- Number bits run backwards, from #80 (0.0135 in) up to #1 (0.228 in), and fill the gaps between fractions.
- Letter bits continue from A (0.234 in) to Z (0.413 in).
- Metric bits come in 0.1 mm steps in full sets, 0.5 mm in basic ones.
Full charts: number and letter drill sizes, drill bit sizes in fractions and mm and tap drill sizes. Measuring a bit or a bolt? The mm ruler shows it at actual size.
Frequently asked questions
How do I find the closest drill bit to a size?
Convert the size to decimal inches (mm ÷ 25.4), then compare it with each drill series. Fractional bits come in 1/64 inch steps (0.0156 in), so there is always one within 0.008 in. Number and letter bits fill the gaps, and metric bits come in 0.1 mm steps. The calculator above checks every series at once.
What is 0.854 inch to the nearest 1/16?
7/8 inch (0.875). In sixteenths, 0.854 × 16 = 13.66, which rounds to 14/16 = 7/8. To the nearest 1/32 it's 27/32 (0.8438), and to the nearest 1/64, 55/64 (0.8594).
What drill bit is closest to 5/16?
5/16 inch is 0.3125 in (7.94 mm). A letter O drill is 0.316 in and an 8 mm bit is 0.315 in, both slightly larger; a 7.9 mm bit is 0.311 in, slightly smaller.
Should I round a drill size up or down?
For a clearance hole, where a bolt or pin must pass through, round up. For a tap drill, a dowel or a press fit, use the exact size or the next size smaller and test on scrap first. For wood screws, the pilot hole should match the screw's core, not its outside diameter.
Identifying a bolt rather than a hole? Try the thread identifier.