Conversion · exact first, nominal second
Convert the diameter.
Then find a real bit.
Inches and millimetres convert exactly at 25.4 mm per inch. Drill designations are discrete, so the nearest named bit is shown separately with a signed diameter difference.
Decimal inches
The question behind most conversions
Close is not the same.
Metric and fractional drills are separate series that never line up. A 6 mm bit is not a 1/4 in bit: 1/4 in is 6.350 mm, so it cuts 0.350 mm wider. The rows below pair each common metric size with the fraction usually assumed to match it, then name the fraction that is actually closest.
| Metric size | Assumed fraction | Difference | Closest fraction available |
|---|---|---|---|
| 3 mm · 0.1181 in | 1/8 in · 3.175 mm | +0.175 mm · +0.0069 in | 1/8 in · +0.175 mm |
| 4 mm · 0.1575 in | 5/32 in · 3.969 mm | −0.031 mm · −0.0012 in | 5/32 in · −0.031 mm |
| 5 mm · 0.1969 in | 3/16 in · 4.762 mm | −0.238 mm · −0.0094 in | 13/64 in · +0.159 mm |
| 6 mm · 0.2362 in | 1/4 in · 6.350 mm | +0.350 mm · +0.0138 in | 15/64 in · −0.047 mm |
| 8 mm · 0.3150 in | 5/16 in · 7.938 mm | −0.063 mm · −0.0025 in | 5/16 in · −0.063 mm |
| 10 mm · 0.3937 in | 3/8 in · 9.525 mm | −0.475 mm · −0.0187 in | 25/64 in · −0.078 mm |
| 12 mm · 0.4724 in | 1/2 in · 12.700 mm | +0.700 mm · +0.0276 in | 15/32 in · −0.094 mm |
| 13 mm · 0.5118 in | 1/2 in · 12.700 mm | −0.300 mm · −0.0118 in | 33/64 in · +0.097 mm |
A negative difference means the fraction is smaller than the metric size. “Closest” is nominal diameter only—it says nothing about fit, tolerance, or whether the hole will do its job.
Do not round away the application
A close conversion does not prove interchangeability. Tap drill, reamer allowance, press fit, clearance, coating, material, and machine capability can all make a small diameter difference consequential.