#31 drill size
A #31 drill is 0.1200 in (3.05 mm). It sits between #32 and #30 in the number series; the nearest metric drill is 3.0 mm.
#31 at a glance
Decimal, metric and the nearest sizes in the other series.
Nearest metric drills
Metric drills step by 0.05 mm below 3 mm and by 0.1 mm from 3 mm. Difference is metric minus inch size.
| Metric drill | Inch | Difference | In inches | Hole |
|---|---|---|---|---|
| 3.0 mm | 0.1181" | −0.048 mm | −0.0019" | Smaller |
| 3.1 mm | 0.1220" | +0.052 mm | +0.0020" | Larger |
Nearby inch drills
The next size down and up in each inch series, so you can pick a substitute from the drawer.
Threads a #31 drill can tap
Threads from the tap tables on this site that this drill can tap, with the thread percentage it leaves (55–85 % shown).
| Thread | Standard drill | Thread with this drill | Suits |
|---|---|---|---|
| M3.5 × 0.6 | 2.9 mm | 58 % | Hard, tough metals |
| #6-40 UNF | #33 | 55 % | Hard, tough metals |
Clearance for screws and bolts
Bolts and screws a #31 hole clears, with the diametral clearance and the gap each side. Metric sizes are compared with ISO 273.
| Fastener | Diameter | Clearance | Per side | ISO 273, metric only |
|---|---|---|---|---|
| #1 screw | 0.0730" | 0.0470" | 0.0235" | – |
| #2 screw | 0.0860" | 0.0340" | 0.0170" | – |
| M2.5 bolt | 2.5 mm | 0.55 mm | 0.27 mm | Normal fit (normal 2.9, loose 3.1 mm) |
| #3 screw | 0.0990" | 0.0210" | 0.0105" | – |
| #4 screw | 0.1120" | 0.0080" | 0.0040" | – |
| M3 bolt | 3 mm | 0.05 mm | 0.02 mm | Tighter than close fit (3.2 mm) |
Inch clearance is the drill size minus the basic major diameter. A drilled hole usually cuts slightly oversize.
Number drill index
Every size in the series, holes drawn to scale.
Common questions
What size is a #31 drill?
A #31 drill is 0.1200 in, which is 3.05 mm (3.0480 mm exactly).
What is the metric equivalent of a #31 drill?
The nearest metric drill is 3.0 mm, 0.048 mm smaller. The other close size is 3.1 mm.
What thread can I tap with a #31 drill?
It leaves about 58 % thread in M3.5 × 0.6 and 55 % in #6-40 UNF.
Which fractional drill is closest to #31?
1/8 in (0.1250 in) is the closest, 0.0050 in larger.
What does a #31 drill clear?
It clears a M3 bolt with 0.05 mm total clearance (0.02 mm per side); for metric bolts: tighter than close fit.