Tap Drill Size Chart

UNEF Tap Drill Chart

Tap drill sizes for every UNEF (Unified National Extra Fine) thread in this chart. UNEF has more threads per inch than UNF at the same diameter, giving the shallowest of the three standard inch threads and the largest tap drill. Sort any column, filter by size, and click a row for that size's full page.

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11 sizes

UNEF tap drill sizes: size, tap drill, decimal inch, millimetres, percent of thread, hard-material drill, form-tap drill, basic minor diameter and threads per inch.
#12-32 #13 0.185 4.70 76.4
1/4-32 7/32″ 0.2188 5.56 76.9
5/16-32 9/32″ 0.2812 7.14 77.1
3/8-32 11/32″ 0.3438 8.73 76.9
7/16-28 Y 0.404 10.26 72.2
1/2-28 15/32″ 0.4688 11.91 67.2
9/16-24 33/64″ 0.5156 13.10 86.6
5/8-24 37/64″ 0.5781 14.68 86.6
3/4-20 45/64″ 0.7031 17.86 72.2
7/8-20 53/64″ 0.8281 21.03 72.2
1-20 61/64″ 0.9531 24.21 72.2

Tap a column heading to sort. Every row links to that size's full page.

Three pitches at the same diameter

Inch threads come in series, and several diameters appear in all three. At a quarter inch you can have 1/4-20 (coarse, #7), 1/4-28 (fine, #3) or 1/4-32 (extra fine, 7/32″). Same bolt diameter, three different taps, three different holes, and none of them interchangeable.

Why the drills are so close to the major diameter

Thread height scales with pitch. Pack more threads into an inch and each one gets shorter, so the hole needs to start closer to the finished major diameter. On the extra-fine sizes the tap is only cutting a few thousandths of an inch deep, which is why the drill sizes on this chart look almost like clearance holes next to the coarse equivalents.

The practical consequence: measure your drill. A drill a few thousandths over nominal, which you would never notice on a coarse thread, can take a real bite out of the engagement here. If you have a tenths indicator or good calipers, check the bit before you use it.

Finding the taps

UNEF taps and dies are a special order in most places rather than something on the shelf. If you are designing a part and a fine thread will do, the UNF series is far easier to buy for, and the fasteners cost less. Save UNEF for when the wall really is too thin for anything else.

Frequently asked questions

What is a UNEF thread?
UNEF stands for Unified National Extra Fine. It is a third inch series with even more threads per inch than UNF: 1/4-32 against 1/4-28 and 1/4-20. This chart lists all 11 UNEF sizes with their tap drills.
What is UNEF used for?
Very thin walls, short thread engagement, and instruments or fittings that need fine adjustment. You meet it on tube fittings, bearing retainers, camera and optical hardware, and aerospace parts. It is not a general-purpose series.
What drill do I use for 1/4-32?
A 7/32″ drill, 0.2188 in, which leaves 76.9% of thread.
How do I tell UNEF from UNF?
Count the threads with a pitch gauge. At 1/4 inch, UNC is 20 threads per inch, UNF is 28 and UNEF is 32. They look similar enough that guessing by eye goes wrong, and all three take different drills: #7, #3 and 7/32″ respectively.

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