Tap Drill Size Chart

7/8-20 Tap Drill Size

Tap drill

53/64″ 0.8281 in21.03 mm

Use a 53/64 in drill (0.8281 in, 21.03 mm) for 72.2% thread.

The drill size for a 7/8-20 tap is 53/64 in (0.8281 in / 21.03 mm). If you are wondering what size drill bit a 7/8-20 tap needs, that is the one: it leaves 72.2% 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 7/8-20

Tap drills and clearance drills for 7/8-20 (UNEF (Unified extra fine), 20 TPI). Percent of thread is the figure stored for each drill, which reproduces from the major diameter and TPI of 7/8-20.
Use Drill Decimal in mm % thread
Standard tap drill Cut tap, general use (nearest to 75% (computed)) 53/64″ 0.8281 21.03 72.2%
Metric substitute Closest metric drill if you have no inch set 21 mm 0.8268 21.00 74.2%
Hard material Stainless, tool steel, anything that fights the tap 27/32″ 0.8438 21.43 48.0%
Form (roll) tap Theoretical 0.8529 in - starting point, check your tap maker's chart 55/64″ 0.8594 21.83 —

Clearance holes for a 7/8-20 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 7/8-20 per ASME B18.2.8. Clearance depends on the bolt diameter alone, so every 7/8 thread shares these figures.
Fit Drill Drill in Hole min in Hole max in Hole mm
Close 29/32″ 0.9062 0.906 0.914 23.01–23.22
Normal 15/16″ 0.9375 0.938 0.950 23.83–24.13
Loose 1-1/32″ 1.0312 1.031 1.051 26.19–26.70

7/8-20 dimensions

Nominal dimensions for 7/8-20. 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 0.875 22.23
Basic internal minor diameter 0.8209 20.85
Pitch (1 / TPI) 0.050 1.27
Threads per inch 20 TPI

What the hole looks like

Cross-section of a 7/8-20 tapped hole, drawn to scale The drilled hole is 53/64″, 0.8281 inches or 21.03 millimetres across. The tap cuts outward from that wall to the major diameter of 0.875 inches. A 100 percent thread would reach in to the basic minor diameter of 0.8209 inches, shown as a dashed line; the difference is why this drill gives 72.2% of thread. major 0.875 in drill 53/64″ · 0.8281 in dashed = 100% thread at 0.8209 in
7/8-20 in section, to scale. The accent line is the thread the tap cuts from a 53/64″ hole; the dashed line is where the crest would sit at 100% thread. The gap between them is the 27.8% of thread this drill leaves behind.

Work out a different hole for 7/8-20

Pre-filled with 7/8-20. 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

53/64″0.8281 in21.03 mm72.2% thread

Theoretical hole 0.8295 in (21.07 mm)

One size down, deeper thread21 mm 0.8268 in · 74.2%

One size up, easier tapping27/32″ 0.8438 in · 48.0%

For comparison, the published chart drill for 7/8-20 is 53/64″ at 72.2% thread.

Notes on 7/8-20

Finest pitch at this diameter
7/8-20 is the finest of the standard pitches at 7/8 in. Fine threads suit thin walls and joints that see vibration, but the hole size matters more: the window between a weak thread and a broken tap is narrower than on 7/8-9.
Not the same as 7/8-9
7/8-9 and 7/8-20 share a 7/8 in major diameter but not a pitch, and they take different drills: 49/64 in for 7/8-9, 53/64 in for 7/8-20. The taps and the fasteners are not interchangeable.
What the standard drill gives you
53/64 in leaves 72.2% of thread (nearest to 75% (computed)). That is in the normal 65-80% band most shop charts aim for.
Stainless, tool steel and other tough material
27/32 in opens the hole to 48.0% 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: 55/64 in, against a theoretical 0.8529 in. Treat that as a starting point and check your tap maker's chart.
Closest metric size
M22 x 2.5 is the nearest metric thread by diameter (22 mm against 22.23 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

M22 x 2.5 19.5 mm · 0.7677 in · 77.0% 22 mm major against 22.23 mm

Not interchangeable. M22 x 2.5 is the nearest size by diameter, nothing more. The pitches differ, so a M22 x 2.5 fastener will not run into a 7/8-20 hole and forcing one will strip both.

Guides for tapping 7/8-20

Longer reads on the decisions behind the numbers above.

All guides →

7/8-20 tap drill: common questions

What size drill bit do I need for a 7/8-20 tap?
A 53/64 in drill, 0.8281 in / 21.03 mm. That leaves 72.2% of thread, which is the standard chart figure for 7/8-20.
What is the drill size for a 7/8-20 tap in mm?
The 53/64 in tap drill measures 21.03 mm. If you only have metric drills, 21 mm is the closest, giving 74.2% thread.
What drill should I use for 7/8-20 in stainless steel?
Use 27/32 in instead of 53/64 in. It leaves 48.0% of thread rather than 72.2%, so there is less material for the tap to cut and less chance of snapping it.
Is 7/8-20 the same as 7/8-9?
No. Both are 7/8 in threads, but the pitch differs, so the drills differ: 53/64 in for 7/8-20 and 49/64 in for 7/8-9. The taps and the fasteners are not interchangeable.
What is the clearance hole for a 7/8-20 bolt?
For a normal fit, 15/16 in (0.9375 in), with the finished hole between 0.938 in and 0.950 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.