What is a letter E drill in decimal and mm?

0.2500 in, or 6.350 mm; nearest fractional drill: 1/4" (0.2500 in, +0.0000 in); nearest metric drill: 6.3 mm (0.2480 in, -0.0020 in).

What RPM for a letter E drill in steel?

In low-carbon steel (1018) an HSS letter E drill (0.2500 in) runs about 1,380 RPM at 90 SFM; a carbide drill about 4,580 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a letter E drill?

About 0.0065 in/rev in mild steel, 0.0060 in/rev in aluminum and 0.0043 in/rev in 304 stainless. At 1,380 RPM that is 8.97 IPM in steel (IPM = RPM × IPR).

What RPM for a letter E drill in aluminum?

3,820 RPM with HSS (250 SFM) or up to 9,170 RPM with carbide (600 SFM), feeding about 0.0060 in/rev.

What size is a letter E drill?

0.2500 in / 6.350 mm. The nearest other drills are 1/4" (0.2500 in), 6.3 mm (0.2480 in), 6.4 mm (0.2520 in).

Letter E drill size

A letter E drill is 0.2500 in (6.350 mm).

Decimal inch: 0.2500
Millimeters: 6.350
Nearest 1/64: 1/4" (+0.0000)
Type: letter drill

Nearest equivalents in other systems

Drills closest to Letter E
System	Drill	Inch	mm	Difference
Fractional	1/4"	0.2500	6.350	+0.0000 in
Metric	6.3 mm	0.2480	6.300	-0.0020 in

Speeds and feeds: in mild steel an HSS drill starts at about 1,380 RPM and 0.0065 in/rev (8.97 IPM); in 6061 aluminum about 3,820 RPM.

Letter E drill speeds and feeds by material

Jobber-length twist drill, D = 0.2500 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

Letter E drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	3,820	600	9,170	0.0060	22.9	55.0
Brass / free-machining	150	2,290	400	6,110	0.0053	12.1	32.4
Low-carbon steel (1018)	90	1,380	300	4,580	0.0065	8.97	29.8
Alloy steel (4140, annealed)	70	1,070	250	3,820	0.0065	6.96	24.8
Stainless 304	40	610	150	2,290	0.0043	2.62	9.85
Gray cast iron	80	1,220	250	3,820	0.0045	5.49	17.2
Titanium 6Al-4V	25	380	100	1,530	0.0030	1.14	4.59
Plastics (acetal)	300	4,580	500	7,640	0.0040	18.3	30.6

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,380 RPM × 0.165 mm/rev = 228 mm/min.

Worked example: letter E drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.2500 ≈ 1,380 RPM.
Feed for a 0.250 in drill in mild steel ≈ 0.0065 in/rev.
IPM = 1,380 × 0.0065 = 8.97 IPM.

Peck cycle by hole depth

Letter E drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 0.750 in (19 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 1.250 in (31.8 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 2.000 in (50.8 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 letter E drill

No standard UNC, UNF or metric tap in our tables calls for a letter E drill. Use it for clearance, pilot or reamer prep holes.

Nearest drills

1/4": 0.2500 in / 6.350 mm (0.0000 in)
6.3 mm: 0.2480 in / 6.300 mm (-0.0020 in)
6.4 mm: 0.2520 in / 6.400 mm (+0.0020 in)
Letter D: 0.2460 in / 6.248 mm (-0.0040 in)

Drill speed calculator, prefilled for letter E Drill finder: sizes near letter E Letter drill chart Decimal equivalent chart

Common questions

What is a letter E drill in decimal and mm?: 0.2500 in, or 6.350 mm; nearest fractional drill: 1/4" (0.2500 in, +0.0000 in); nearest metric drill: 6.3 mm (0.2480 in, -0.0020 in).
What RPM for a letter E drill in steel?: In low-carbon steel (1018) an HSS letter E drill (0.2500 in) runs about 1,380 RPM at 90 SFM; a carbide drill about 4,580 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a letter E drill?: About 0.0065 in/rev in mild steel, 0.0060 in/rev in aluminum and 0.0043 in/rev in 304 stainless. At 1,380 RPM that is 8.97 IPM in steel (IPM = RPM × IPR).
What RPM for a letter E drill in aluminum?: 3,820 RPM with HSS (250 SFM) or up to 9,170 RPM with carbide (600 SFM), feeding about 0.0060 in/rev.
What size is a letter E drill?: 0.2500 in / 6.350 mm. The nearest other drills are 1/4" (0.2500 in), 6.3 mm (0.2480 in), 6.4 mm (0.2520 in).

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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