What is a 5/16" drill in decimal and mm?

0.3125 in, or 7.938 mm; nearest number / letter drill: Letter O (0.3160 in, +0.0035 in); nearest metric drill: 7.9 mm (0.3110 in, -0.0015 in).

What RPM for a 5/16" drill in steel?

In low-carbon steel (1018) an HSS 5/16" drill (0.3125 in) runs about 1,100 RPM at 90 SFM; a carbide drill about 3,670 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a 5/16" drill?

About 0.0080 in/rev in mild steel, 0.0073 in/rev in aluminum and 0.0053 in/rev in 304 stainless. At 1,100 RPM that is 8.80 IPM in steel (IPM = RPM × IPR).

What RPM for a 5/16" drill in aluminum?

3,060 RPM with HSS (250 SFM) or up to 7,330 RPM with carbide (600 SFM), feeding about 0.0073 in/rev.

5/16" drill size

A 5/16" fractional drill is 0.3125 in (7.938 mm). It is the cut tap, 75% thread drill for 3/8-16.

Decimal inch: 0.3125
Millimeters: 7.938
Nearest 1/64: 5/16" (+0.0000)
Type: fractional drill

Nearest equivalents in other systems

Drills closest to 5/16"
System	Drill	Inch	mm	Difference
Number / letter	Letter O	0.3160	8.026	+0.0035 in
Metric	7.9 mm	0.3110	7.900	-0.0015 in

Speeds and feeds: in mild steel an HSS drill starts at about 1,100 RPM and 0.0080 in/rev (8.80 IPM); in 6061 aluminum about 3,060 RPM.

5/16" drill speeds and feeds by material

Jobber-length twist drill, D = 0.3125 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

5/16" drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	3,060	600	7,330	0.0073	22.3	53.5
Brass / free-machining	150	1,830	400	4,890	0.0058	10.6	28.4
Low-carbon steel (1018)	90	1,100	300	3,670	0.0080	8.80	29.4
Alloy steel (4140, annealed)	70	860	250	3,060	0.0085	7.31	26.0
Stainless 304	40	490	150	1,830	0.0053	2.60	9.70
Gray cast iron	80	980	250	3,060	0.0055	5.39	16.8
Titanium 6Al-4V	25	310	100	1,220	0.0036	1.12	4.39
Plastics (acetal)	300	3,670	500	6,110	0.0045	16.5	27.5

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,100 RPM × 0.203 mm/rev = 224 mm/min.

Worked example: 5/16" drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.3125 ≈ 1,100 RPM.
Feed for a 0.313 in drill in mild steel ≈ 0.0080 in/rev.
IPM = 1,100 × 0.0080 = 8.80 IPM.

Peck cycle by hole depth

5/16" drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 0.938 in (23.8 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 1.563 in (39.7 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 2.500 in (63.5 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 5/16" drill

3/8-16 — cut tap, 75% thread · tap speeds and feeds

Nearest drills

7.9 mm: 0.3110 in / 7.900 mm (-0.0015 in)
8.0 mm: 0.3150 in / 8.000 mm (+0.0025 in)
Letter O: 0.3160 in / 8.026 mm (+0.0035 in)
7.8 mm: 0.3071 in / 7.800 mm (-0.0054 in)

Drill speed calculator, prefilled for 5/16"Drill finder: sizes near 5/16"Fractional drill chart Decimal equivalent chart

Common questions

What is a 5/16" drill in decimal and mm?: 0.3125 in, or 7.938 mm; nearest number / letter drill: Letter O (0.3160 in, +0.0035 in); nearest metric drill: 7.9 mm (0.3110 in, -0.0015 in).
What RPM for a 5/16" drill in steel?: In low-carbon steel (1018) an HSS 5/16" drill (0.3125 in) runs about 1,100 RPM at 90 SFM; a carbide drill about 3,670 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a 5/16" drill?: About 0.0080 in/rev in mild steel, 0.0073 in/rev in aluminum and 0.0053 in/rev in 304 stainless. At 1,100 RPM that is 8.80 IPM in steel (IPM = RPM × IPR).
What RPM for a 5/16" drill in aluminum?: 3,060 RPM with HSS (250 SFM) or up to 7,330 RPM with carbide (600 SFM), feeding about 0.0073 in/rev.
What tap uses a 5/16" drill?: 3/8-16 (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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