Tap Drill Size Chart

1-8 Tap Drill Size

Tap drill

7/8″ 0.875 in22.23 mm

Use a 7/8 in drill (0.875 in, 22.23 mm) for 77.0% thread.

Fine thread 1-12? Use 59/64 in. (0.9219 in, 23.42 mm, 72.1% thread)

The drill size for a 1-8 tap is 7/8 in (0.875 in / 22.23 mm). If you are wondering what size drill bit a 1-8 tap needs, that is the one: it leaves 77.0% 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 1-8

Tap drills and clearance drills for 1-8 (UNC (Unified coarse), 8 TPI). Percent of thread is the figure stored for each drill, which reproduces from the major diameter and TPI of 1-8.
Use Drill Decimal in mm % thread
Standard tap drill Cut tap, general use (standard chart drill) 7/8″ 0.875 22.23 77.0%
Metric substitute Closest metric drill if you have no inch set 22.5 mm 0.8858 22.50 70.3%
Hard material Stainless, tool steel, anything that fights the tap 29/32″ 0.9062 23.02 57.8%
Form (roll) tap Theoretical 0.9447 in - starting point, check your tap maker's chart 15/16″ 0.9375 23.81 —

Clearance holes for a 1-8 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 1-8 per ASME B18.2.8. Clearance depends on the bolt diameter alone, so every 1 thread shares these figures.
Fit Drill Drill in Hole min in Hole max in Hole mm
Close 1-1/32″ 1.0312 1.031 1.039 26.19–26.39
Normal 1-3/32″ 1.0938 1.094 1.106 27.79–28.09
Loose 1-5/32″ 1.1562 1.156 1.181 29.36–30.00

1-8 dimensions

Nominal dimensions for 1-8. 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 1.000 25.40
Basic internal minor diameter 0.8647 21.96
Pitch (1 / TPI) 0.125 3.18
Threads per inch 8 TPI

What the hole looks like

Cross-section of a 1-8 tapped hole, drawn to scale The drilled hole is 7/8″, 0.875 inches or 22.23 millimetres across. The tap cuts outward from that wall to the major diameter of 1.000 inches. A 100 percent thread would reach in to the basic minor diameter of 0.8647 inches, shown as a dashed line; the difference is why this drill gives 77.0% of thread. major 1.000 in drill 7/8″ · 0.875 in dashed = 100% thread at 0.8647 in
1-8 in section, to scale. The accent line is the thread the tap cuts from a 7/8″ hole; the dashed line is where the crest would sit at 100% thread. The gap between them is the 23.0% of thread this drill leaves behind.

Work out a different hole for 1-8

Pre-filled with 1-8. 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

22.5 mm0.8858 in70.3% thread

Theoretical hole 0.8863 in (22.51 mm)

One size down, deeper thread7/8″ 0.875 in · 77.0%

One size up, easier tapping57/64″ 0.8906 in · 67.4%

For comparison, the published chart drill for 1-8 is 7/8″ at 77.0% thread.

Notes on 1-8

Coarse pitch
1-8 is the coarse pitch at 1 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: 1-12, 1-20.
What the standard drill gives you
7/8 in leaves 77.0% 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
29/32 in opens the hole to 57.8% 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: 15/16 in, against a theoretical 0.9447 in. Treat that as a starting point and check your tap maker's chart.
Closest metric size
M24 x 3 is the nearest metric thread by diameter (24 mm against 25.4 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

M24 x 3 21 mm · 0.8268 in · 77.0% 24 mm major against 25.4 mm

Not interchangeable. M24 x 3 is the nearest size by diameter, nothing more. The pitches differ, so a M24 x 3 fastener will not run into a 1-8 hole and forcing one will strip both.

Guides for tapping 1-8

Longer reads on the decisions behind the numbers above.

All guides →

1-8 tap drill: common questions

What size drill bit do I need for a 1-8 tap?
A 7/8 in drill, 0.875 in / 22.23 mm. That leaves 77.0% of thread, which is the standard chart figure for 1-8.
What is the drill size for a 1-8 tap in mm?
The 7/8 in tap drill measures 22.23 mm. If you only have metric drills, 22.5 mm is the closest, giving 70.3% thread.
What drill should I use for 1-8 in stainless steel?
Use 29/32 in instead of 7/8 in. It leaves 57.8% of thread rather than 77.0%, so there is less material for the tap to cut and less chance of snapping it.
Is 1-8 the same as 1-12?
No. Both are 1 in threads, but the pitch differs, so the drills differ: 7/8 in for 1-8 and 59/64 in for 1-12. The taps and the fasteners are not interchangeable.
What is the clearance hole for a 1-8 bolt?
For a normal fit, 1-3/32 in (1.0938 in), with the finished hole between 1.094 in and 1.106 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.