A #40 drill measures 0.0980 inch (2.489 mm). The closest fractional drill is 3/32" (0.0938") and the closest metric drill is 2.5 mm (0.0984").
What are the #40 drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| #40 | number | 0.0980 | 2.489 | 0 |
| #41 | number | 0.0960 | 2.438 | -0.0020 |
| 2.5 mm | metric | 0.0984 | 2.500 | +0.0004 |
| #39 | number | 0.0995 | 2.527 | +0.0015 |
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 #40 drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 7,795 | 0.0020 |
| 360 free-machining brass | 5,847 | 0.0018 |
| 1018 cold rolled steel | 3,118 | 0.0015 |
| 304 stainless steel | 1,559 | 0.0010 |
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 #40 drill in inches?
0.0980 inch, which is 2.489 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, 2.5 mm is within 0.0004 inch.
What tap uses a #40 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.