A #55 drill measures 0.0520 inch (1.321 mm). The closest fractional drill is 3/64" (0.0469") and the closest metric drill is 1.3 mm (0.0512").
What are the #55 drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| #55 | number | 0.0520 | 1.321 | 0 |
| 1.3 mm | metric | 0.0512 | 1.300 | -0.0008 |
Drill sets that include this size
Number and letter sizes come in indexed sets; buy the set rather than single drills.
What speed should a #55 drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 14,691 | 0.0010 |
| 360 free-machining brass | 11,018 | 0.0009 |
| 1018 cold rolled steel | 5,876 | 0.0008 |
| 304 stainless steel | 2,938 | 0.0005 |
Starting points from conservative HSS drilling speeds. Small drills often call for more RPM than a benchtop drill press offers; run at the top speed and feed gently. Use cutting oil in steel and stainless, and clamp the work: never hold it by hand.
Cutting fluid for drilling
Frequently asked questions
What is a #55 drill in inches?
0.0520 inch, which is 1.321 mm. Number drills get smaller as the number rises, so the next number up is a smaller drill. If you do not have this exact size, 1.3 mm is within 0.0008 inch.
What tap uses a #55 drill?
No common UNC or UNF thread uses it as its standard 75% tap drill, but it often serves as an alternative drill for a slightly different percentage of thread, or as a clearance or pilot size. The tap drill chart lists every standard pairing.