Form Tap Drill Chart: Roll Tap Drill Sizes for Inch and Metric
A form tap needs a bigger hole than a cut tap, because it pushes metal into the thread instead of cutting it away. M6 takes 5.5 mm and 1/4-20 takes #1. As a rule, a 65% form thread needs the major diameter minus 0.442 times the pitch. We lined up the Balax, Jarvis and Haas charts: of the 41 threads at least two of them list, they agree on ten.
Form tap drill chart
The first drill column is the one most of these charts print. Where they all differ, it is the printed drill closest to the 65% hole from the Jarvis formula. The last three columns show what each chart prints and the form thread that drill leaves. A decimal means the chart names no close drill. Use your tap maker's chart if you have one.
| Thread | Cut tap drill | 65% form hole | Form tap drill | Balax | Jarvis | Haas |
|---|---|---|---|---|---|---|
| #0-80 | 3/64 in | 0.0545 in | #54 | #54 59% 1.4 mm 57% | #54 59% | not listed |
| #2-56 | #50 | 0.0781 in | 5/64 in | #47 62% 2.0 mm 60% | 5/64 in 65% | 5/64 in 65% |
| #4-40 | #43 | 0.1009 in | #39 | #39 74% | #39 74% | #39 74% |
| #5-40 | #38 | 0.1139 in | 2.9 mm | #33 71% 2.9 mm 64% | #33 71% | 2.9 mm 64% |
| #6-32 | #36 | 0.1242 in | 3.1 mm | 3.1 mm 75% | 0.1243 in 64% | 3.1 mm 75% |
| #6-40 | #33 | 0.1270 in | 3.2 mm | 1/8 in 76% 3.2 mm 71% | 3.2 mm 71% | 3.2 mm 71% |
| #8-32 | #29 | 0.1502 in | 3.8 mm | #25 68% 3.8 mm 68% | #25 68% | 3.8 mm 68% |
| #10-24 | #25 | 0.1716 in | 11/64 in | 11/64 in 64% | 11/64 in 64% | 4.3 mm 73% |
| #10-32 | #21 | 0.1762 in | #16 | #16 61% 0.176 in 66% | 0.1763 in 64% | 4.4 mm 79% |
| #12-24 | #16 | 0.1976 in | 5.0 mm | #9 71% 5.0 mm 68% | 5.0 mm 68% | 5.0 mm 68% |
| 1/4-20 | #7 | 0.2279 in | #1 | 5.75 mm 69% | #1 65% | 5.8 mm 64% |
| 5/16-18 | letter F | 0.2879 in | 7.3 mm | 7.25 mm 72% | 7.3 mm 66% | 7.3 mm 66% |
| 3/8-16 | 5/16 in | 0.3474 in | letter S | letter S 64% 11/32 in 74% | letter S 64% | 8.8 mm 67% |
| 7/16-14 | letter U | 0.4059 in | 13/32 in | letter Y 69% | 13/32 in 64% | 10.3 mm 66% |
| 1/2-13 | 27/64 in | 0.4660 in | 11.8 mm | 0.463 in 71% | 0.4660 in 65% | 11.8 mm 68% |
| 9/16-12 | 31/64 in | 0.5257 in | 13.3 mm | 0.521 in 73% | 0.5257 in 65% | 13.3 mm 69% |
| 5/8-11 | 17/32 in | 0.5848 in | 14.8 mm | 37/64 in 76% | 0.5848 in 65% | 14.8 mm 68% |
| 3/4-10 | 21/32 in | 0.7058 in | 45/64 in | 45/64 in 69% | 45/64 in 69% | 17.9 mm 67% |
| 7/8-9 | 49/64 in | 0.8259 in | 21.0 mm | 0.823 in 69% | not listed | 21.0 mm 64% |
| 1/4-28 | #3 | 0.2342 in | letter A | letter A 66% | letter A 66% | 5.9 mm 73% |
| 5/16-24 | letter I | 0.2941 in | letter M | 0.293 in 69% | letter M 62% | 7.4 mm 75% |
| 3/8-24 | letter Q | 0.3566 in | 9.0 mm | 9.0 mm 73% | 0.3566 in 65% | 9.0 mm 73% |
| 7/16-20 | 25/64 in | 0.4154 in | 10.5 mm | letter Z 72% 10.5 mm 71% | 0.4154 in 65% | 10.5 mm 71% |
| 1/2-20 | 29/64 in | 0.4779 in | 12.1 mm | 0.476 in 71% | 0.4779 in 65% | 12.1 mm 69% |
| 9/16-18 | 33/64 in | 0.5379 in | 0.536 in | 0.536 in 70% | 0.5380 in 65% | 13.6 mm 72% |
| 5/8-18 | 37/64 in | 0.6004 in | 15.2 mm | 0.598 in 71% | 0.6004 in 65% | 15.2 mm 70% |
| 3/4-16 | 11/16 in | 0.7224 in | 18.3 mm | 23/32 in 74% | 0.7224 in 65% | 18.3 mm 69% |
| 7/8-14 | 13/16 in | 0.8434 in | 27/32 in | 27/32 in 64% | not listed | 21.4 mm 67% |
| M2 | 1.6 mm | 1.82 mm | 1.85 mm | 1.85 mm 55% | not listed | not listed |
| M2.5 | 2.05 mm | 2.30 mm | 2.3 mm | 2.3 mm 65% | not listed | 2.3 mm 65% |
| M3 | 2.5 mm | 2.78 mm | 7/64 in | #35 61% | 7/64 in 65% | 7/64 in 65% |
| M3.5 | 2.9 mm | 3.23 mm | 3.2 mm | #30 58% | 3.2 mm 74% | 3.2 mm 74% |
| M4 | 3.3 mm | 3.69 mm | 3.7 mm | 3.7 mm 63% | 3.7 mm 63% | 3.7 mm 63% |
| M5 | 4.2 mm | 4.65 mm | #14 | #14 69% | #14 69% | 4.6 mm 74% |
| M6 | 5.0 mm | 5.56 mm | 5.5 mm | 5.5 mm 74% | 7/32 in 65% | 5.5 mm 74% |
| M8 | 6.8 mm | 7.45 mm | 7.4 mm | 7.4 mm 71% | 7.4 mm 71% | 7.4 mm 71% |
| M10 | 8.5 mm | 9.34 mm | 9.3 mm | 9.3 mm 69% | 9.3 mm 69% | 9.3 mm 69% |
| M12 | 10.2 mm | 11.23 mm | 11.2 mm | 7/16 in 75% | 11.2 mm 67% | 11.2 mm 67% |
| M14 | 12.0 mm | 13.12 mm | 13.1 mm | 13.0 mm 74% | 13.1 mm 66% | 13.1 mm 66% |
| M16 | 14.0 mm | 15.12 mm | 15.1 mm | 15.0 mm 74% | 19/32 in 68% | 15.1 mm 66% |
| M18 | 15.5 mm | 16.89 mm | 16.9 mm | not listed | 16.75 mm 74% | 16.9 mm 65% |
| M20 | 17.5 mm | 18.89 mm | 18.9 mm | not listed | 18.75 mm 74% | 18.9 mm 65% |
| M22 | 19.5 mm | 20.89 mm | 20.9 mm | not listed | not listed | 20.9 mm 65% |
| M24 | 21.0 mm | 22.67 mm | 22.6 mm | not listed | not listed | 22.6 mm 69% |
Percentages use the form tap formula below, not the cutting formula. Haas lists no #0-80 or M2. Jarvis stops at 3/4-16 and M20 among these threads, and Balax's metric table stops at M16 in coarse pitches. None of the three covers 1/2-28, 5/8-24 or M30, so they are left out.
Form tap drill formula
Jarvis Cutting Tools, a tap maker, prints the formula: hole = major diameter - 0.0068 x percent of thread x pitch. Its metric version divides by 147.06, which comes to the same thing. At 65%, 0.0068 x 65 is 0.442. Its worked example for M8 x 1.25 gives 7.448 mm, and the nearest drill is 7.4 mm.
A second rule comes from Baer, a tap maker: hole = major diameter - 0.5 x pitch + 0.05 mm. For M8 that gives 7.425 mm and for M6 5.55 mm, both close to the Jarvis figures of 7.448 and 5.558 mm. The gap grows with the pitch: on M30 x 3.5, Baer gives 28.30 mm and the Jarvis formula 28.45 mm.
A form tap's 65% is not a cut tap's 65%
The two formulas use different scales. A cut thread's 100% is a hole 1.299 pitches under the major diameter. In the form tap formula, 100% is a hole only 0.68 pitches under it. Put the #1 drill for a form thread on 1/4-20 through the cutting formula and it reads 34%. That figure tells you nothing useful about a formed thread, so the thread percentage chart and the calculator on this site do not apply to form taps.
Where the charts differ
On 1/4-20, Balax prints 5.75 mm, Jarvis prints #1 and Haas prints 5.8 mm. By the formula those drills leave 69%, 65% and 64%. On M6, Balax and Haas print 5.5 mm (74%) and Jarvis prints 7/32 in (65%).
The widest gaps here are 0.0067 in on 5/8-11 and 0.15 mm on M18. Haas prints its own "actual % thread" column, and those figures agree with the form tap formula to within about half a point. Across its whole chart they run from 62.6% on #12-28 to 78.9% on #10-32, so it does not hold one target. Balax aims for about 65% and also prints the holes for 75% and 55%, along with the tap limit (H or D number) for each class of fit. If a drawing names a thread class, read the Balax table, because it pairs each hole with a tap. Balax also says to use a drill that makes a 75% hole where the thread will be gauged to 2B or 3B, and that a 55% hole leaves the minor diameter too large for those classes. So the 65% drills in this chart are a starting point for general work, and 2B or 3B work needs the 75% hole.
Check the hole before you tap
Jarvis says to check the hole after drilling, that reaming may be necessary, and that an undersize hole may break the tap. Where the thread percentage is critical, it says the hole varies by application and may take some experimenting. Haas calls its chart a starting point and says the hole may need adjusting for tolerance, material and hardness. Balax warns not to expect a drill to cut the size marked on it, and AmesWeb lists typical oversize amounts. This chart uses nominal sizes.
Forming only suits malleable metals such as mild steel and aluminum (Wikipedia). For cut taps, use the tap drill chart.
FAQ
What size drill do I use for a form tap?
5.5 mm for M6, 7.4 mm for M8 and #1 for 1/4-20, the drills most of these charts print. For other sizes, take the major diameter minus 0.442 times the pitch, which is a 65% form thread, and use a drill close to that.
Can I use a normal tap drill chart for a form tap?
No. A form tap moves metal instead of cutting it, so it needs a bigger hole. By the form tap formula, the 5.0 mm hole for a cut M6 tap works out at 147%. Jarvis advises against going over 75% and warns that an undersize hole can break the tap.
What thread percentage should a form tap make?
Start at 65%, which Jarvis recommends as a starting point, and do not go over 75%. Balax says a properly sized hole gives 65 to 75%, and to use a drill that makes a 75% hole where the thread will be gauged to 2B or 3B. Haas's drills run from about 63% to 79%.
Why do form tap charts disagree?
They round to different drills, and Haas prints metric drills on most inch threads. The biggest gap between charts on an inch thread here is 0.0067 in, on 5/8-11. On metric threads it is 0.15 mm, on M18. Your tap maker's chart should win for its own tap.
Should the hole for a form tap be reamed?
Haas says the recommended practice is to drill and ream. Jarvis says to check the hole after drilling, because reaming may be necessary.
Cite or reuse this data
Please quote these figures with a link to this page: TapDrillSize, "Form tap drill chart", https://tapdrillsize.com/form-tap-drill-chart/, published November 6, 2026. Every row is in the CSV download. If a number is wrong, write to contact@tapdrillsize.com and we will correct it.
Drills from Balax, Jarvis Cutting Tools, Haas and Balax's metric table. Read on 30 October 2026.