Tap Drill Size Chart

5/16-18 Tap Drill Size

Tap drill

F 0.257 in6.53 mm

Use a F drill (0.257 in, 6.53 mm) for 76.9% thread.

Fine thread 5/16-24? Use I. (0.272 in, 6.91 mm, 74.8% thread)

The drill size for a 5/16-18 tap is F (0.257 in / 6.53 mm). If you are wondering what size drill bit a 5/16-18 tap needs, that is the one: it leaves 76.9% of thread, which is the figure standard tap and drill charts are built around. Below are the other holes you might want for this size - a larger drill for tough material, the form-tap hole, and the clearance holes for the bolt that goes in.

Every drill for 5/16-18

Tap drills and clearance drills for 5/16-18 (UNC (Unified coarse), 18 TPI). Percent of thread is the figure stored for each drill, which reproduces from the major diameter and TPI of 5/16-18.
Use Drill Decimal in mm % thread
Standard tap drill Cut tap, general use (standard chart drill) F 0.257 6.53 76.9%
Metric substitute Closest metric drill if you have no inch set 6.6 mm 0.2598 6.60 73.0%
Hard material Stainless, tool steel, anything that fights the tap I 0.272 6.91 56.1%
Form (roll) tap Theoretical 0.2879 in - starting point, check your tap maker's chart L 0.290 7.37 —

Clearance holes for a 5/16-18 bolt

A clearance hole is not tapped. It is the hole in the part the bolt passes through, so it is always larger than the thread.

Clearance holes for 5/16-18 per ASME B18.2.8. Clearance depends on the bolt diameter alone, so every 5/16 thread shares these figures.
Fit Drill Drill in Hole min in Hole max in Hole mm
Close 21/64″ 0.3281 0.328 0.334 8.33–8.48
Normal 11/32″ 0.3438 0.344 0.354 8.74–8.99
Loose 23/64″ 0.3594 0.359 0.373 9.12–9.47

5/16-18 dimensions

Nominal dimensions for 5/16-18. The basic internal minor diameter is the hole a 100% thread would start from, which is why no tap drill is ever that small.
Dimension Inch mm
Major diameter 0.3125 7.94
Basic internal minor diameter 0.2524 6.41
Pitch (1 / TPI) 0.0556 1.41
Threads per inch 18 TPI

What the hole looks like

Cross-section of a 5/16-18 tapped hole, drawn to scale The drilled hole is F, 0.257 inches or 6.53 millimetres across. The tap cuts outward from that wall to the major diameter of 0.3125 inches. A 100 percent thread would reach in to the basic minor diameter of 0.2524 inches, shown as a dashed line; the difference is why this drill gives 76.9% of thread. major 0.3125 in drill F · 0.257 in dashed = 100% thread at 0.2524 in
5/16-18 in section, to scale. The accent line is the thread the tap cuts from a F hole; the dashed line is where the crest would sit at 100% thread. The gap between them is the 23.1% of thread this drill leaves behind.

Work out a different hole for 5/16-18

Pre-filled with 5/16-18. Change the material, the tap type, or which drills you actually own, and it picks the closest real drill and tells you the percent of thread you end up with. The tables above are the published figures and do not depend on this.

The general-purpose target

Tap type
Drills you have

Best drill you have, cut tap, 70% target

G0.261 in6.63 mm71.4% thread

Theoretical hole 0.262 in (6.65 mm)

One size down, deeper thread6.6 mm 0.2598 in · 73.0%

One size up, easier tapping6.7 mm 0.2638 in · 67.5%

For comparison, the published chart drill for 5/16-18 is F at 76.9% thread.

Notes on 5/16-18

Coarse pitch
5/16-18 is the coarse pitch at 5/16 in. It is the usual choice for general work, and it tolerates a slightly off-size hole better than the finer pitches do. Finer options at the same diameter: 5/16-24, 5/16-32.
What the standard drill gives you
F leaves 76.9% of thread (standard chart drill). That is in the normal 65-80% band most shop charts aim for.
Stainless, tool steel and other tough material
I opens the hole to 56.1% engagement. Less material for the tap to cut means less torque and less heat, at the cost of some thread depth.
Using a form (roll) tap
A form tap displaces metal instead of cutting it, so it needs a larger hole: L, against a theoretical 0.2879 in. Treat that as a starting point and check your tap maker's chart.
Closest metric size
M8 x 1.25 is the nearest metric thread by diameter (8 mm against 7.94 mm). It is a planning reference only. The two are not interchangeable: the pitches differ, and a bolt from one will not run into a hole tapped for the other.

Related sizes

Same diameter, different pitch

Next size up and down

Closest metric size

M8 x 1.25 6.8 mm · 0.2677 in · 73.9% 8 mm major against 7.94 mm

Not interchangeable. M8 x 1.25 is the nearest size by diameter, nothing more. The pitches differ, so a M8 x 1.25 fastener will not run into a 5/16-18 hole and forcing one will strip both.

Guides for tapping 5/16-18

Longer reads on the decisions behind the numbers above.

All guides →

5/16-18 tap drill: common questions

What size drill bit do I need for a 5/16-18 tap?
A F drill, 0.257 in / 6.53 mm. That leaves 76.9% of thread, which is the standard chart figure for 5/16-18.
What is the drill size for a 5/16-18 tap in mm?
The F tap drill measures 6.53 mm. If you only have metric drills, 6.6 mm is the closest, giving 73.0% thread.
What drill should I use for 5/16-18 in stainless steel?
Use I instead of F. It leaves 56.1% of thread rather than 76.9%, so there is less material for the tap to cut and less chance of snapping it.
Is 5/16-18 the same as 5/16-24?
No. Both are 5/16 in threads, but the pitch differs, so the drills differ: F for 5/16-18 and I for 5/16-24. The taps and the fasteners are not interchangeable.
What is the clearance hole for a 5/16-18 bolt?
For a normal fit, 11/32 in (0.3438 in), with the finished hole between 0.344 in and 0.354 in per ASME B18.2.8. A clearance hole is not tapped; the bolt passes straight through it.
A 4 × 6 in card with the drills for this size.