Tap Drill Size Chart

1-12 Tap Drill Size

Tap drill

59/64″ 0.9219 in23.42 mm

Use a 59/64 in drill (0.9219 in, 23.42 mm) for 72.1% thread.

Fine thread 1-20? Use 61/64 in. (0.9531 in, 24.21 mm, 72.2% thread)

Published charts disagree on this size. Some published charts list 15/16″ for 1-12 as the general-purpose drill. That gives only about 57.7% thread, so we use 59/64″ (72.1%). 15/16″ is listed below as the hard-material drill, where a shallower thread is a deliberate choice.

The drill size for a 1-12 tap is 59/64 in (0.9219 in / 23.42 mm). If you are wondering what size drill bit a 1-12 tap needs, that is the one: it leaves 72.1% 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 1-12

Tap drills and clearance drills for 1-12 (UNF (Unified fine), 12 TPI). Percent of thread is the figure stored for each drill, which reproduces from the major diameter and TPI of 1-12.
Use Drill Decimal in mm % thread
Standard tap drill Cut tap, general use (standard chart drill) 59/64″ 0.9219 23.42 72.1%
Metric substitute Closest metric drill if you have no inch set 23.5 mm 0.9252 23.50 69.1%
Hard material Stainless, tool steel, anything that fights the tap 15/16″ 0.9375 23.81 57.7%
Form (roll) tap Theoretical 0.9632 in - starting point, check your tap maker's chart 31/32″ 0.9688 24.61 —

Clearance holes for a 1-12 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 1-12 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

1-12 dimensions

Nominal dimensions for 1-12. 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 0.9098 23.11
Pitch (1 / TPI) 0.0833 2.12
Threads per inch 12 TPI

What the hole looks like

Cross-section of a 1-12 tapped hole, drawn to scale The drilled hole is 59/64″, 0.9219 inches or 23.42 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 0.9098 inches, shown as a dashed line; the difference is why this drill gives 72.1% of thread. major 1.000 in drill 59/64″ · 0.9219 in dashed = 100% thread at 0.9098 in
1-12 in section, to scale. The accent line is the thread the tap cuts from a 59/64″ hole; the dashed line is where the crest would sit at 100% thread. The gap between them is the 27.9% of thread this drill leaves behind.

Work out a different hole for 1-12

Pre-filled with 1-12. 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

23.5 mm0.9252 in69.1% thread

Theoretical hole 0.9242 in (23.48 mm)

One size down, deeper thread59/64″ 0.9219 in · 72.1%

One size up, easier tapping15/16″ 0.9375 in · 57.7%

For comparison, the published chart drill for 1-12 is 59/64″ at 72.1% thread.

Notes on 1-12

Middle pitch at this diameter
At 1 in the data holds 3 pitches: 1-8, 1-12, 1-20. 1-12 sits between 1-8 and 1-20.
Not the same as 1-8
1-8 and 1-12 share a 1 in major diameter but not a pitch, and they take different drills: 7/8 in for 1-8, 59/64 in for 1-12. The taps and the fasteners are not interchangeable.
What the standard drill gives you
59/64 in leaves 72.1% 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
15/16 in opens the hole to 57.7% 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: 31/32 in, against a theoretical 0.9632 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 1-12 hole and forcing one will strip both.

Guides for tapping 1-12

Longer reads on the decisions behind the numbers above.

All guides →

1-12 tap drill: common questions

What size drill bit do I need for a 1-12 tap?
A 59/64 in drill, 0.9219 in / 23.42 mm. That leaves 72.1% of thread, which is the standard chart figure for 1-12.
What is the drill size for a 1-12 tap in mm?
The 59/64 in tap drill measures 23.42 mm. If you only have metric drills, 23.5 mm is the closest, giving 69.1% thread.
What drill should I use for 1-12 in stainless steel?
Use 15/16 in instead of 59/64 in. It leaves 57.7% of thread rather than 72.1%, so there is less material for the tap to cut and less chance of snapping it.
Is 1-12 the same as 1-20?
No. Both are 1 in threads, but the pitch differs, so the drills differ: 59/64 in for 1-12 and 61/64 in for 1-20. The taps and the fasteners are not interchangeable.
What is the clearance hole for a 1-12 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.