What is a letter I drill in decimal and mm?

0.2720 in, or 6.909 mm; nearest fractional drill: 17/64" (0.2656 in, -0.0064 in); nearest metric drill: 6.9 mm (0.2717 in, -0.0003 in).

What RPM for a letter I drill in steel?

In low-carbon steel (1018) an HSS letter I drill (0.2720 in) runs about 1,260 RPM at 90 SFM; a carbide drill about 4,210 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a letter I drill?

About 0.0070 in/rev in mild steel, 0.0064 in/rev in aluminum and 0.0046 in/rev in 304 stainless. At 1,260 RPM that is 8.82 IPM in steel (IPM = RPM × IPR).

What RPM for a letter I drill in aluminum?

3,510 RPM with HSS (250 SFM) or up to 8,430 RPM with carbide (600 SFM), feeding about 0.0064 in/rev.

Letter I drill size

A letter I drill is 0.2720 in (6.909 mm). It is the cut tap, 75% thread drill for 5/16-24.

Decimal inch: 0.2720
Millimeters: 6.909
Nearest 1/64: 17/64" (+0.0064)
Type: letter drill

Nearest equivalents in other systems

Drills closest to Letter I
System	Drill	Inch	mm	Difference
Fractional	17/64"	0.2656	6.747	-0.0064 in
Metric	6.9 mm	0.2717	6.900	-0.0003 in

Speeds and feeds: in mild steel an HSS drill starts at about 1,260 RPM and 0.0070 in/rev (8.82 IPM); in 6061 aluminum about 3,510 RPM.

Letter I drill speeds and feeds by material

Jobber-length twist drill, D = 0.2720 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

Letter I drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	3,510	600	8,430	0.0064	22.5	54.0
Brass / free-machining	150	2,110	400	5,620	0.0054	11.4	30.4
Low-carbon steel (1018)	90	1,260	300	4,210	0.0070	8.82	29.5
Alloy steel (4140, annealed)	70	980	250	3,510	0.0072	7.06	25.3
Stainless 304	40	560	150	2,110	0.0046	2.58	9.71
Gray cast iron	80	1,120	250	3,510	0.0049	5.49	17.2
Titanium 6Al-4V	25	350	100	1,400	0.0032	1.12	4.48
Plastics (acetal)	300	4,210	500	7,020	0.0042	17.7	29.5

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,260 RPM × 0.178 mm/rev = 224 mm/min.

Worked example: letter I drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.2720 ≈ 1,260 RPM.
Feed for a 0.272 in drill in mild steel ≈ 0.0070 in/rev.
IPM = 1,260 × 0.0070 = 8.82 IPM.

Peck cycle by hole depth

Letter I drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 0.816 in (20.7 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 1.360 in (34.5 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 2.176 in (55.3 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 I drill

5/16-24 — cut tap, 75% thread · tap speeds and feeds

Nearest drills

6.9 mm: 0.2717 in / 6.900 mm (-0.0003 in)
7.0 mm: 0.2756 in / 7.000 mm (+0.0036 in)
6.8 mm: 0.2677 in / 6.800 mm (-0.0043 in)
Letter J: 0.2770 in / 7.036 mm (+0.0050 in)

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

Common questions

What is a letter I drill in decimal and mm?: 0.2720 in, or 6.909 mm; nearest fractional drill: 17/64" (0.2656 in, -0.0064 in); nearest metric drill: 6.9 mm (0.2717 in, -0.0003 in).
What RPM for a letter I drill in steel?: In low-carbon steel (1018) an HSS letter I drill (0.2720 in) runs about 1,260 RPM at 90 SFM; a carbide drill about 4,210 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a letter I drill?: About 0.0070 in/rev in mild steel, 0.0064 in/rev in aluminum and 0.0046 in/rev in 304 stainless. At 1,260 RPM that is 8.82 IPM in steel (IPM = RPM × IPR).
What RPM for a letter I drill in aluminum?: 3,510 RPM with HSS (250 SFM) or up to 8,430 RPM with carbide (600 SFM), feeding about 0.0064 in/rev.
What tap uses a letter I drill?: 5/16-24 (cut tap, 75% 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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