A #31 drill measures 0.1200 inch (3.048 mm). The closest fractional drill is 1/8" (0.1250") and the closest metric drill is 3.0 mm (0.1181").
What are the #31 drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| #31 | number | 0.1200 | 3.048 | 0 |
| 3.0 mm | metric | 0.1181 | 3.000 | -0.0019 |
| 3.1 mm | metric | 0.1220 | 3.100 | +0.0020 |
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 #31 drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 6,366 | 0.0024 |
| 360 free-machining brass | 4,775 | 0.0022 |
| 1018 cold rolled steel | 2,546 | 0.0018 |
| 304 stainless steel | 1,273 | 0.0012 |
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 #31 drill in inches?
0.1200 inch, which is 3.048 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, 3.0 mm is within 0.0019 inch.
What tap uses a #31 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.