Drill Bit Size Converter
Convert drill bit sizes between fractions, decimal inches, millimeters, number sizes, and letter sizes. Enter the size you have and get practical nearest matches for the sizes you can buy.
Convert a drill bit size
Use a fraction like 1/4, a metric size like 6.5 mm, a number size like #7, or a letter size like F.
A 1/4 inch drill bit is 0.250 inches or 6.35 mm. The closest 0.5 mm metric size is 6.5 mm.
How to use drill bit conversions
- Enter the size from your plan, fastener chart, tap chart, or drill index.
- Use the decimal and millimeter results when comparing exact sizes.
- Use the closest fraction, number, letter, or rounded metric size when choosing a bit from a set.
- For tapping, press-fit parts, and metalwork, confirm the required clearance before drilling the final hole.
Quick drill bit size chart
| Size | Decimal in | Millimeters | Type |
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Drill bit conversion FAQ
What does a number drill bit size mean?
Number drill bits run from #80, which is very small, up to #1, which is larger. The numbers are a standard drill index system rather than a simple inch or millimeter measurement.
What does a letter drill bit size mean?
Letter drill bits run from A through Z. They cover sizes above common number bits and below many larger fractional sizes, with A at 0.234 inches and Z at 0.413 inches.
Should I round up or down to the nearest drill bit?
It depends on the job. Clearance holes often use a slightly larger bit, while pilot holes, tapping, and tight-fitting hardware can require a specific size. When tolerance matters, use the exact drill size from the project or fastener specification.