What is a 25/64" drill in decimal and mm?

0.3906 in, or 9.922 mm; nearest number / letter drill: Letter W (0.3860 in, -0.0046 in); nearest metric drill: 9.9 mm (0.3898 in, -0.0009 in).

What RPM for a 25/64" drill in steel?

In low-carbon steel (1018) an HSS 25/64" drill (0.3906 in) runs about 880 RPM at 90 SFM; a carbide drill about 2,930 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a 25/64" drill?

About 0.0096 in/rev in mild steel, 0.0087 in/rev in aluminum and 0.0065 in/rev in 304 stainless. At 880 RPM that is 8.45 IPM in steel (IPM = RPM × IPR).

What RPM for a 25/64" drill in aluminum?

2,440 RPM with HSS (250 SFM) or up to 5,870 RPM with carbide (600 SFM), feeding about 0.0087 in/rev.

What tap uses a 25/64" drill?

7/16-20 (cut tap, 75% thread); 7/16-14 (cut tap, 50% thread); 3/8-16 (Helicoil / STI insert (3/8-16×1D)).

25/64" drill size

A 25/64" fractional drill is 0.3906 in (9.922 mm). It is the cut tap, 75% thread drill for 7/16-20.

Decimal inch: 0.3906
Millimeters: 9.922
Nearest 1/64: 25/64" (+0.0000)
Type: fractional drill

Nearest equivalents in other systems

Drills closest to 25/64"
System	Drill	Inch	mm	Difference
Number / letter	Letter W	0.3860	9.804	-0.0046 in
Metric	9.9 mm	0.3898	9.900	-0.0009 in

Speeds and feeds: in mild steel an HSS drill starts at about 880 RPM and 0.0096 in/rev (8.45 IPM); in 6061 aluminum about 2,440 RPM.

25/64" drill speeds and feeds by material

Jobber-length twist drill, D = 0.3906 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

25/64" drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	2,440	600	5,870	0.0087	21.2	51.1
Brass / free-machining	150	1,470	400	3,910	0.0065	9.56	25.4
Low-carbon steel (1018)	90	880	300	2,930	0.0096	8.45	28.1
Alloy steel (4140, annealed)	70	680	250	2,440	0.0106	7.21	25.9
Stainless 304	40	390	150	1,470	0.0065	2.53	9.56
Gray cast iron	80	780	250	2,440	0.0068	5.30	16.6
Titanium 6Al-4V	25	240	100	980	0.0043	1.03	4.21
Plastics (acetal)	300	2,930	500	4,890	0.0051	14.9	24.9

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 880 RPM × 0.244 mm/rev = 215 mm/min.

Worked example: 25/64" drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.3906 ≈ 880 RPM.
Feed for a 0.391 in drill in mild steel ≈ 0.0096 in/rev.
IPM = 880 × 0.0096 = 8.45 IPM.

Peck cycle by hole depth

25/64" drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 1.172 in (29.8 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 1.953 in (49.6 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 3.125 in (79.4 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 25/64" drill

7/16-20 — cut tap, 75% thread · tap speeds and feeds
7/16-14 — cut tap, 50% thread · tap speeds and feeds
3/8-16 — Helicoil / STI insert (3/8-16×1D) · tap speeds and feeds

Nearest drills

9.9 mm: 0.3898 in / 9.900 mm (-0.0009 in)
10.0 mm: 0.3937 in / 10.000 mm (+0.0031 in)
Letter W: 0.3860 in / 9.804 mm (-0.0046 in)
9.8 mm: 0.3858 in / 9.800 mm (-0.0048 in)

Drill speed calculator, prefilled for 25/64"Drill finder: sizes near 25/64"Fractional drill chart Decimal equivalent chart

Common questions

What is a 25/64" drill in decimal and mm?: 0.3906 in, or 9.922 mm; nearest number / letter drill: Letter W (0.3860 in, -0.0046 in); nearest metric drill: 9.9 mm (0.3898 in, -0.0009 in).
What RPM for a 25/64" drill in steel?: In low-carbon steel (1018) an HSS 25/64" drill (0.3906 in) runs about 880 RPM at 90 SFM; a carbide drill about 2,930 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a 25/64" drill?: About 0.0096 in/rev in mild steel, 0.0087 in/rev in aluminum and 0.0065 in/rev in 304 stainless. At 880 RPM that is 8.45 IPM in steel (IPM = RPM × IPR).
What RPM for a 25/64" drill in aluminum?: 2,440 RPM with HSS (250 SFM) or up to 5,870 RPM with carbide (600 SFM), feeding about 0.0087 in/rev.
What tap uses a 25/64" drill?: 7/16-20 (cut tap, 75% thread); 7/16-14 (cut tap, 50% thread); 3/8-16 (Helicoil / STI insert (3/8-16×1D)).

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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