What is a #31 drill in decimal and mm?

0.1200 in, or 3.048 mm; nearest fractional drill: 1/8" (0.1250 in, +0.0050 in); nearest metric drill: 3.0 mm (0.1181 in, -0.0019 in).

What RPM for a #31 drill in steel?

In low-carbon steel (1018) an HSS #31 drill (0.1200 in) runs about 2,860 RPM at 90 SFM; a carbide drill about 9,550 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a #31 drill?

About 0.0028 in/rev in mild steel, 0.0027 in/rev in aluminum and 0.0019 in/rev in 304 stainless. At 2,860 RPM that is 8.01 IPM in steel (IPM = RPM × IPR).

What RPM for a #31 drill in aluminum?

7,960 RPM with HSS (250 SFM) or up to 19,100 RPM with carbide (600 SFM), feeding about 0.0027 in/rev.

What tap uses a #31 drill?

6-40 (cut tap, 50% thread); 4-40 (Helicoil / STI insert (#4-40×1D)).

#31 drill size

A #31 number (wire-gauge) drill is 0.1200 in (3.048 mm). It is the cut tap, 50% thread drill for 6-40.

Decimal inch: 0.1200
Millimeters: 3.048
Nearest 1/64: 1/8" (-0.0050)
Type: number (wire-gauge) drill

Nearest equivalents in other systems

Drills closest to #31
System	Drill	Inch	mm	Difference
Fractional	1/8"	0.1250	3.175	+0.0050 in
Metric	3.0 mm	0.1181	3.000	-0.0019 in

Speeds and feeds: in mild steel an HSS drill starts at about 2,860 RPM and 0.0028 in/rev (8.01 IPM); in 6061 aluminum about 7,960 RPM.

#31 drill speeds and feeds by material

Jobber-length twist drill, D = 0.1200 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

#31 drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	7,960	600	19,100	0.0027	21.5	51.6
Brass / free-machining	150	4,770	400	12,730	0.0023	11.0	29.3
Low-carbon steel (1018)	90	2,860	300	9,550	0.0028	8.01	26.7
Alloy steel (4140, annealed)	70	2,230	250	7,960	0.0028	6.24	22.3
Stainless 304	40	1,270	150	4,770	0.0019	2.41	9.06
Gray cast iron	80	2,550	250	7,960	0.0017	4.34	13.5
Titanium 6Al-4V	25	800	100	3,180	0.0015	1.20	4.77
Plastics (acetal)	300	9,550	500	15,920	0.0048	45.8	76.4

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 2,860 RPM × 0.071 mm/rev = 203 mm/min.

Worked example: #31 drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.1200 ≈ 2,860 RPM.
Feed for a 0.120 in drill in mild steel ≈ 0.0028 in/rev.
IPM = 2,860 × 0.0028 = 8.01 IPM.

Peck cycle by hole depth

#31 drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 0.360 in (9.1 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 0.600 in (15.2 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 0.960 in (24.4 mm)	G83 full retract	0.5-1 × D, shrinking with depth	Retract to clear chips; reduce feed ~10-20% past 5×D.

Taps that use a #31 drill

6-40 — cut tap, 50% thread · tap speeds and feeds
4-40 — Helicoil / STI insert (#4-40×1D) · tap speeds and feeds

Nearest drills

3.0 mm: 0.1181 in / 3.000 mm (-0.0019 in)
3.1 mm: 0.1221 in / 3.100 mm (+0.0021 in)
2.95 mm: 0.1161 in / 2.950 mm (-0.0039 in)
#32: 0.1160 in / 2.946 mm (-0.0040 in)

Drill speed calculator, prefilled for #31 Drill finder: sizes near #31 Number drill chart Decimal equivalent chart

Common questions

What is a #31 drill in decimal and mm?: 0.1200 in, or 3.048 mm; nearest fractional drill: 1/8" (0.1250 in, +0.0050 in); nearest metric drill: 3.0 mm (0.1181 in, -0.0019 in).
What RPM for a #31 drill in steel?: In low-carbon steel (1018) an HSS #31 drill (0.1200 in) runs about 2,860 RPM at 90 SFM; a carbide drill about 9,550 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a #31 drill?: About 0.0028 in/rev in mild steel, 0.0027 in/rev in aluminum and 0.0019 in/rev in 304 stainless. At 2,860 RPM that is 8.01 IPM in steel (IPM = RPM × IPR).
What RPM for a #31 drill in aluminum?: 7,960 RPM with HSS (250 SFM) or up to 19,100 RPM with carbide (600 SFM), feeding about 0.0027 in/rev.
What tap uses a #31 drill?: 6-40 (cut tap, 50% thread); 4-40 (Helicoil / STI insert (#4-40×1D)).

Nearby drill sizes

Sources

Machinery's Handbook, 31st ed., Speeds and Feeds for Drilling, Reaming and Threading (Tables 17-23) — HSS and carbide drilling SFM (rounded).
RedLine Tools (Productivity Inc.) catalog p. 340, Cobalt & HSS Drills Speeds & Feeds — feed per rev by drill diameter and material (middle of the catalog range).
ASME B94.11M twist drills: number (#1-#80) and letter (A-Z) drill sizes; drill size reference.

Material feed multipliers (aluminum 1.3×, brass 1.1×, 4140 0.85×, stainless and titanium 0.7× the mild-steel feed) are JobLine estimates. Peck guidance is JobLine shop practice. Use your drill maker's data first.

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