Tap Drill Size Chart

UNC Tap Drill Chart

Tap drill sizes for every UNC (Unified National Coarse) thread, from #1-64 to 1-1/2-6. Each row gives the standard tap drill in inches and millimetres, the percent of thread it leaves, the larger drill to use in tough material, and the form-tap hole. Sort any column, filter by size, and click a row for that size's full page.

Prints as a clean chart, no navigation.

23 sizes

UNC 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.
#1-64 #53 0.0595 1.51 66.5
#2-56 #50 0.070 1.78 69.0
#3-48 #47 0.0785 1.99 75.7
#4-40 #43 0.089 2.26 70.8
#5-40 #38 0.1015 2.58 72.4
#6-32 #36 0.1065 2.71 77.6
#8-32 #29 0.136 3.45 69.0
#10-24 #25 0.1495 3.80 74.8
#12-24 #16 0.177 4.50 72.1
1/4-20 #7 0.201 5.11 75.4
5/16-18 F 0.257 6.53 76.9
3/8-16 5/16″ 0.3125 7.94 77.0
7/16-14 U 0.368 9.35 74.9
1/2-13 27/64″ 0.4219 10.72 78.2
9/16-12 31/64″ 0.4844 12.30 72.1
5/8-11 17/32″ 0.5312 13.49 79.4
3/4-10 21/32″ 0.6562 16.67 72.2
7/8-9 49/64″ 0.7656 19.45 75.8
1-8 7/8″ 0.875 22.23 77.0
1-1/8-7 63/64″ 0.9844 25.00 75.8
1-1/4-7 1-7/64″ 1.1094 28.18 75.8
1-3/8-6 1-7/32″ 1.2188 30.96 72.1
1-1/2-6 1-11/32″ 1.3438 34.13 72.1

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

How to read this chart

Find your thread in the first column, then read across. The tap drill column is the drill the standard charts call for, and the two columns after it give that same drill in decimal inches and millimetres so you can check whatever is stamped on the bit in your hand.

The % thread column is the one worth paying attention to. It says how tall the thread the tap cuts will be compared with a full, theoretical thread. Around 75% is what the standard drills aim for: 1/4-20 with a #7 comes out at 75.4%. A deeper thread than that buys very little extra strength and costs a great deal more torque, which is how taps get broken.

The hard material column is a larger drill that leaves a shallower thread, around 55-65%. Use it in stainless, tool steel, or anything else that fights back. The form tap column is larger again, because a roll tap pushes metal into the thread instead of cutting it away and needs somewhere for that metal to come from.

The sizes people look up most

1/4-20 is the one most people are here for, at #7 (0.201 in). #10-24 takes #25, and 3/8-16 takes 5/16″. Each of those has its own page with the clearance holes, a to-scale drawing of the hole and a calculator you can change the material on.

Where these numbers come from

The tap drills are the published chart sizes. Every percent of thread on this page was recomputed from that size's own major diameter and threads per inch using % = 76.98 × TPI × (major − drill), and the build fails if a stored figure and the recomputed one disagree by more than 0.2. Clearance holes come from ASME B18.2.8 and are on the clearance hole chart.

Frequently asked questions

What is a UNC thread?
UNC stands for Unified National Coarse. It is the standard inch thread series in the United States and has the fewest threads per inch of any inch series at a given diameter. A hardware-store bolt is almost always UNC. This chart lists all 23 UNC sizes with their tap drills.
What drill do I use for a 1/4-20 tap?
A #7 drill, 0.201 in, which leaves 75.4% of thread. 1/4-20 is the most looked-up size on this chart.
Why does the percent of thread change from size to size?
Because the chart drill is a real drill you can buy, not the exact theoretical size. The published drill is the closest stocked size to about 75% of thread, so the figure it actually gives lands anywhere from the high 60s to the low 80s depending on how the drill sizes fall at that pitch.
Should I use the coarse or the fine thread?
Coarse for general work, soft or brittle material, dirty conditions, and anything taken apart often. Fine for thin walls, short thread engagement, fine adjustment and hard material. The fine versions of these sizes are on the UNF chart.
Can I print this chart?
Yes. Use the Print / Save as PDF button at the top. The print stylesheet drops the navigation and gives you the table on its own, which fits on one or two sheets.

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