What is a #8 drill in decimal and mm?

0.1990 in, or 5.055 mm; nearest fractional drill: 13/64" (0.2031 in, +0.0041 in); nearest metric drill: 5.1 mm (0.2008 in, +0.0018 in).

What RPM for a #8 drill in steel?

In low-carbon steel (1018) an HSS #8 drill (0.1990 in) runs about 1,730 RPM at 90 SFM; a carbide drill about 5,760 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a #8 drill?

About 0.0051 in/rev in mild steel, 0.0047 in/rev in aluminum and 0.0033 in/rev in 304 stainless. At 1,730 RPM that is 8.82 IPM in steel (IPM = RPM × IPR).

What RPM for a #8 drill in aluminum?

4,800 RPM with HSS (250 SFM) or up to 11,520 RPM with carbide (600 SFM), feeding about 0.0047 in/rev.

What tap uses a #8 drill?

12-24 (form (roll) tap, 65% thread).

#8 drill size

A #8 number (wire-gauge) drill is 0.1990 in (5.055 mm). It is the form (roll) tap, 65% thread drill for 12-24.

Decimal inch: 0.1990
Millimeters: 5.055
Nearest 1/64: 13/64" (-0.0041)
Type: number (wire-gauge) drill

Nearest equivalents in other systems

Drills closest to #8
System	Drill	Inch	mm	Difference
Fractional	13/64"	0.2031	5.159	+0.0041 in
Metric	5.1 mm	0.2008	5.100	+0.0018 in

Speeds and feeds: in mild steel an HSS drill starts at about 1,730 RPM and 0.0051 in/rev (8.82 IPM); in 6061 aluminum about 4,800 RPM.

#8 drill speeds and feeds by material

Jobber-length twist drill, D = 0.1990 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

#8 drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	4,800	600	11,520	0.0047	22.6	54.1
Brass / free-machining	150	2,880	400	7,680	0.0041	11.8	31.5
Low-carbon steel (1018)	90	1,730	300	5,760	0.0051	8.82	29.4
Alloy steel (4140, annealed)	70	1,340	250	4,800	0.0051	6.83	24.5
Stainless 304	40	770	150	2,880	0.0033	2.54	9.50
Gray cast iron	80	1,540	250	4,800	0.0034	5.24	16.3
Titanium 6Al-4V	25	480	100	1,920	0.0024	1.15	4.61
Plastics (acetal)	300	5,760	500	9,600	0.0044	25.3	42.2

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,730 RPM × 0.130 mm/rev = 224 mm/min.

Worked example: #8 drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.1990 ≈ 1,730 RPM.
Feed for a 0.199 in drill in mild steel ≈ 0.0051 in/rev.
IPM = 1,730 × 0.0051 = 8.82 IPM.

Peck cycle by hole depth

#8 drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 0.597 in (15.2 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 0.995 in (25.3 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 1.592 in (40.4 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 #8 drill

12-24 — form (roll) tap, 65% thread · tap speeds and feeds

Nearest drills

5.1 mm: 0.2008 in / 5.100 mm (+0.0018 in)
#7: 0.2010 in / 5.105 mm (+0.0020 in)
5.0 mm: 0.1968 in / 5.000 mm (-0.0022 in)
#9: 0.1960 in / 4.978 mm (-0.0030 in)

Drill speed calculator, prefilled for #8 Drill finder: sizes near #8 Number drill chart Decimal equivalent chart

Common questions

What is a #8 drill in decimal and mm?: 0.1990 in, or 5.055 mm; nearest fractional drill: 13/64" (0.2031 in, +0.0041 in); nearest metric drill: 5.1 mm (0.2008 in, +0.0018 in).
What RPM for a #8 drill in steel?: In low-carbon steel (1018) an HSS #8 drill (0.1990 in) runs about 1,730 RPM at 90 SFM; a carbide drill about 5,760 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a #8 drill?: About 0.0051 in/rev in mild steel, 0.0047 in/rev in aluminum and 0.0033 in/rev in 304 stainless. At 1,730 RPM that is 8.82 IPM in steel (IPM = RPM × IPR).
What RPM for a #8 drill in aluminum?: 4,800 RPM with HSS (250 SFM) or up to 11,520 RPM with carbide (600 SFM), feeding about 0.0047 in/rev.
What tap uses a #8 drill?: 12-24 (form (roll) tap, 65% thread).

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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