Tap Drill Size Chart

Form Tap Drill Chart

Form tap (roll tap) drill sizes for all 124 threads in this chart, inch and metric. A form tap displaces metal instead of cutting it, so every one of these holes is larger than the cut-tap drill for the same thread. Both are shown side by side so you can see the difference. Sort any column, filter by size, and click a row for that size's full page.

Prints as a clean chart, no navigation.

124 sizes

Form tap drill sizes: size, form tap drill, decimal inch, millimetres, theoretical 65% hole, the cut-tap drill for comparison, the difference between them, and pitch.
65% hole
M1 x 0.25 0.9 mm 0.0354 0.90 0.89 mm
M1.2 x 0.25 1.1 mm 0.0433 1.10 1.09 mm
M1.4 x 0.3 1.25 mm 0.0492 1.25 1.27 mm
#0-80 #54 0.055 1.40 0.0545
M1.6 x 0.35 1.45 mm 0.0571 1.45 1.45 mm
#1-64 #51 0.067 1.70 0.0661
#1-72 #51 0.067 1.70 0.0669
M2 x 0.4 1.8 mm 0.0709 1.80 1.82 mm
#2-56 5/64″ 0.0781 1.98 0.0781
#2-64 #47 0.0785 1.99 0.0791
M2.5 x 0.45 2.3 mm 0.0906 2.30 2.3 mm
#3-48 #43 0.089 2.26 0.0898
#3-56 #43 0.089 2.26 0.0911
#4-40 #38 0.1015 2.58 0.1009
#4-48 #37 0.104 2.64 0.1028
M3 x 0.5 2.8 mm 0.1102 2.80 2.78 mm
#5-40 #33 0.113 2.87 0.1139
#5-44 #32 0.116 2.95 0.115
M3.5 x 0.6 3.2 mm 0.126 3.20 3.24 mm
#6-32 1/8″ 0.125 3.18 0.1242
#6-40 #30 0.1285 3.26 0.127
M4 x 0.7 3.7 mm 0.1457 3.70 3.69 mm
#8-32 #25 0.1495 3.80 0.1502
#8-36 #24 0.152 3.86 0.1517
#10-24 11/64″ 0.1719 4.37 0.1716
#10-32 #16 0.177 4.50 0.1762
M5 x 0.8 4.6 mm 0.1811 4.60 4.65 mm
#12-24 #8 0.199 5.06 0.1976
#12-28 #7 0.201 5.11 0.2002
#12-32 13/64″ 0.2031 5.16 0.2022
M6 x 1 5.6 mm 0.2205 5.60 5.56 mm
1/4-20 #1 0.228 5.79 0.2279
1/4-28 15/64″ 0.2344 5.95 0.2342
1/4-32 15/64″ 0.2344 5.95 0.2362
M7 x 1 6.6 mm 0.2598 6.60 6.56 mm
5/16-18 L 0.290 7.37 0.2879
5/16-24 M 0.295 7.49 0.2941
5/16-32 19/64″ 0.2969 7.54 0.2987
M8 x 1.25 7.4 mm 0.2913 7.40 7.45 mm
M8 x 1 7.6 mm 0.2992 7.60 7.56 mm
M8 x 0.75 7.7 mm 0.3031 7.70 7.67 mm
3/8-16 S 0.348 8.84 0.3474
3/8-24 T 0.358 9.09 0.3566
3/8-32 23/64″ 0.3594 9.13 0.3612
M10 x 1.5 9.3 mm 0.3661 9.30 9.34 mm
M10 x 1.25 9.4 mm 0.3701 9.40 9.45 mm
M10 x 1 9.6 mm 0.378 9.60 9.56 mm
M10 x 0.75 9.7 mm 0.3819 9.70 9.67 mm
7/16-14 13/32″ 0.4062 10.32 0.4059
7/16-20 Z 0.413 10.49 0.4154
7/16-28 27/64″ 0.4219 10.72 0.4217
M12 x 1.75 11.2 mm 0.4409 11.20 11.23 mm
M12 x 1.5 11.3 mm 0.4449 11.30 11.34 mm
M12 x 1.25 11.4 mm 0.4488 11.40 11.45 mm
M12 x 1 11.6 mm 0.4567 11.60 11.56 mm
1/2-13 15/32″ 0.4688 11.91 0.466
1/2-20 31/64″ 0.4844 12.30 0.4779
1/2-28 31/64″ 0.4844 12.30 0.4842
M14 x 2 13.1 mm 0.5157 13.10 13.12 mm
M14 x 1.5 13.3 mm 0.5236 13.30 13.34 mm
M14 x 1.25 13.4 mm 0.5276 13.40 13.45 mm
9/16-12 17/32″ 0.5312 13.49 0.5257
9/16-18 17/32″ 0.5312 13.49 0.5379
9/16-24 35/64″ 0.5469 13.89 0.5441
5/8-11 37/64″ 0.5781 14.68 0.5848
5/8-18 19/32″ 0.5938 15.08 0.6004
5/8-24 39/64″ 0.6094 15.48 0.6066
M16 x 2 15.1 mm 0.5945 15.10 15.12 mm
M16 x 1.5 15.3 mm 0.6024 15.30 15.34 mm
M16 x 1 15.6 mm 0.6142 15.60 15.56 mm
M18 x 2.5 16.9 mm 0.6654 16.90 16.9 mm
M18 x 2 17.1 mm 0.6732 17.10 17.12 mm
M18 x 1.5 17.3 mm 0.6811 17.30 17.34 mm
M18 x 1 17.6 mm 0.6929 17.60 17.56 mm
3/4-10 45/64″ 0.7031 17.86 0.7058
3/4-16 23/32″ 0.7188 18.26 0.7224
3/4-20 47/64″ 0.7344 18.65 0.7279
M20 x 2.5 18.9 mm 0.7441 18.90 18.9 mm
M20 x 2 19.1 mm 0.752 19.10 19.12 mm
M20 x 1.5 19.3 mm 0.7598 19.30 19.34 mm
M20 x 1 19.6 mm 0.7717 19.60 19.56 mm
M22 x 2.5 21 mm 0.8268 21.00 20.9 mm
M22 x 2 21 mm 0.8268 21.00 21.12 mm
M22 x 1.5 21.5 mm 0.8465 21.50 21.34 mm
7/8-9 53/64″ 0.8281 21.03 0.8259
7/8-14 27/32″ 0.8438 21.43 0.8434
7/8-20 55/64″ 0.8594 21.83 0.8529
M24 x 3 22.5 mm 0.8858 22.50 22.67 mm
M24 x 2 23 mm 0.9055 23.00 23.12 mm
M24 x 1.5 23.5 mm 0.9252 23.50 23.34 mm
1-8 15/16″ 0.9375 23.81 0.9447
1-12 31/32″ 0.9688 24.61 0.9632
1-20 63/64″ 0.9844 25.00 0.9779
M27 x 3 25.5 mm 1.0039 25.50 25.67 mm
M27 x 2 26 mm 1.0236 26.00 26.12 mm
M27 x 1.5 26.5 mm 1.0433 26.50 26.34 mm
1-1/8-7 1-1/16″ 1.0625 26.99 1.0619
1-1/8-12 1-3/32″ 1.0938 27.78 1.0882
M30 x 3.5 28.5 mm 1.122 28.50 28.45 mm
M30 x 2 29 mm 1.1417 29.00 29.12 mm
M30 x 1.5 29.5 mm 1.1614 29.50 29.34 mm
1-1/4-7 1-3/16″ 1.1875 30.16 1.1869
1-1/4-12 1-7/32″ 1.2188 30.96 1.2132
M33 x 3.5 31.5 mm 1.2402 31.50 31.45 mm
M33 x 2 32 mm 1.2598 32.00 32.12 mm
1-3/8-6 1-19/64″ 1.2969 32.94 1.3013
1-3/8-12 1-11/32″ 1.3438 34.13 1.3382
M36 x 4 34 mm 1.3386 34.00 34.23 mm
M36 x 3 34.5 mm 1.3583 34.50 34.67 mm
M36 x 2 35 mm 1.378 35.00 35.12 mm
1-1/2-6 1-27/64″ 1.4219 36.12 1.4263
1-1/2-12 1-15/32″ 1.4688 37.31 1.4632
M39 x 4 37 mm 1.4567 37.00 37.23 mm
M42 x 4.5 40 mm 1.5748 40.00 40.01 mm
M42 x 3 40.5 mm 1.5945 40.50 40.67 mm
M42 x 2 41 mm 1.6142 41.00 41.12 mm
M45 x 4.5 43 mm 1.6929 43.00 43.01 mm
M48 x 5 46 mm 1.811 46.00 45.79 mm
M48 x 3 46.5 mm 1.8307 46.50 46.67 mm
M48 x 2 47 mm 1.8504 47.00 47.12 mm
M52 x 5 50 mm 1.9685 50.00 49.79 mm
M56 x 5.5 53.5 mm 2.1063 53.50 53.57 mm
M60 x 5.5 57.5 mm 2.2638 57.50 57.57 mm
M64 x 6 60 mm 2.3622 60.00 61.35 mm

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

These are starting points, not specifications. Every tap maker publishes its own form-tap hole sizes and they do not agree with each other. Check the chart that came with your tap, and cut a test hole in scrap before you touch a part that matters.

What a form tap actually does

A cutting tap has flutes and sharp edges. It shaves metal out of the hole and the chips go somewhere, which is its own problem in a blind hole. A form tap has no cutting edges at all: it is a hardened, lobed screw that is pressed into the hole and swages the thread into existence. Nothing is removed, so there are no chips.

The metal that makes the thread crests has to come out of the hole wall, and that is the whole reason for the bigger drill. The hole gets smaller as the thread forms, because displaced metal flows inward toward the centre.

How much bigger

The difference column makes it concrete. 1/4-20 goes from #7 to #1, which is 0.027 in larger. M6 x 1 goes from 5 mm to 5.6 mm. That is not a rounding difference, it is several drill sizes, and it is the mistake that breaks form taps on their first hole.

What you get for the trouble

  • No chips. The reason form taps are standard in blind holes and on CNC work, where a packed chip is a broken tap and sometimes a scrapped part.
  • Stronger threads. The metal is cold worked and its grain follows the thread form rather than being cut through.
  • Longer tap life. No cutting edges to go dull.
  • Higher torque. The trade-off. A form tap needs a rigid setup and real lubricant, and it will not forgive a hole drilled to cut-tap size.

Where the theoretical column comes from

The theoretical column is the formula result at 65% of thread, before rounding to a real drill. The drill column is the closest stocked size to it. Where the two are far apart, the size page says so: at the top of the chart the theoretical hole can be larger than any twist drill that exists, and those holes are bored rather than drilled.

Frequently asked questions

Why does a form tap need a bigger drill?
A form tap does not cut metal away, it pushes it. The thread crests are made of material displaced from the hole wall, so that material has to come from somewhere: a larger starting hole. Use a cut-tap drill with a form tap and there is far too much metal to move, which spikes the torque and breaks the tap.
What is the drill size for a 1/4-20 form tap?
#1 (0.228 in), against #7 (0.201 in) for a cutting tap. The theoretical 65% figure is 0.2279 in.
What is the form tap formula?
Drill = major diameter minus 0.0068 times percent of thread times pitch, with lengths in one unit and pitch as 1/TPI for inch threads. It is the same shape as the cut-tap formula, which uses 0.01299 instead. This chart uses 65% throughout.
Which materials can be form tapped?
Ductile ones that flow rather than crack: aluminium, copper, brass, mild steel, and many stainless grades. Brittle, short-chipping materials such as grey cast iron are not suitable, because the metal fractures instead of flowing into the thread.
Are these sizes exact?
No, and no form tap chart is. Every tap maker uses a slightly different constant and recommends its own hole sizes. Treat this chart as a starting point, check it against the chart for the tap you are actually holding, and test in scrap before you commit a real part.

Guides

All guides →

Other charts