#3 Drill Bit Size and Diameter
A #3 drill bit measures 0.2130 in
5.41 mm · number-size drill · used as the 1/4-28 tap drill
The #3 is a numbered drill bit in the American gauge series, measuring 0.2130 inches (5.41 mm). It sits between a 13/64 in bit (0.2031 in) and a 7/32 in bit (0.2188 in). It is best known as the standard tap drill for a 1/4-28 UNF fine thread.
At a glance
| Drill size | #3 |
|---|---|
| Diameter | 0.2130 in |
| Millimetres | 5.41 mm |
| Series | Number drill (gauge) 1–97 |
| Closest fractional | 7/32 in (0.2188 in, slightly larger) |
What the #3 is used for
1/4-28 tap drill: drillØ = major − (pitch × 0.75) = 0.2500 − (0.0357 × 0.75) = 0.2232 in → nearest standard bit = #3 (0.2130 in)
A #3 hole before a 1/4-28 tap gives the standard ~75% thread engagement. Do not confuse it with the 1/4-20 coarse thread, which uses the smaller #7 bit.
Related sizes
Other number drill bits
Where the #3 sits in the gauge series — larger number means smaller bit: