Roll tap (form tap) drill chart
Form taps push metal into the thread instead of cutting it, so they need a larger hole than a cutting tap. Sizes below use the form-tap formula at 65% thread with the nearest standard drill. Your tap maker's table comes first — form taps are sensitive to hole size.
| Thread | Formula drill (in) | Nearest drill | Decimal (in) | Cut tap drill (compare) |
|---|---|---|---|---|
| #1-64 | 0.0661 | #51 | 0.0670 | #53 |
| #2-56 | 0.0781 | 5/64 | 0.0781 | #50 |
| #3-48 | 0.0898 | #43 | 0.0890 | #47 |
| #4-40 | 0.1010 | #38 | 0.1015 | #43 |
| #5-40 | 0.1140 | #33 | 0.1130 | #38 |
| #6-32 | 0.1242 | 1/8 | 0.1250 | #36 |
| #8-32 | 0.1502 | #25 | 0.1495 | #29 |
| #10-24 | 0.1716 | 11/64 | 0.1719 | #25 |
| #10-32 | 0.1762 | #16 | 0.1770 | #21 |
| 1/4-20 | 0.2279 | #1 | 0.2280 | #7 |
| 5/16-18 | 0.2879 | L | 0.2900 | F |
| 3/8-16 | 0.3474 | S | 0.3480 | 5/16" |
| 7/16-14 | 0.4059 | 13/32 | 0.4063 | U |
| 1/2-13 | 0.4660 | 15/32 | 0.4688 | 27/64" |
| 5/8-11 | 0.5848 | 37/64 | 0.5781 | 17/32" |
| 3/4-10 | 0.7058 | 45/64 | 0.7031 | 21/32" |
| 7/8-9 | 0.8259 | 53/64 | 0.8281 | 49/64" |
| 1"-8 | 0.9448 | 15/16 | 0.9375 | 7/8" |
| Thread | Formula drill (in) | Nearest drill | Decimal (in) | Cut tap drill (compare) |
|---|---|---|---|---|
| #0-80 | 0.0545 | #54 | 0.0550 | 3/64" |
| #1-72 | 0.0669 | #51 | 0.0670 | #53 |
| #2-64 | 0.0791 | #47 | 0.0785 | #50 |
| #4-48 | 0.1028 | #37 | 0.1040 | #42 |
| #6-40 | 0.1270 | #30 | 0.1285 | #33 |
| #8-36 | 0.1517 | #24 | 0.1520 | #29 |
| #10-32 | 0.1762 | #16 | 0.1770 | #21 |
| 1/4-28 | 0.2342 | 15/64 | 0.2344 | #3 |
| 5/16-24 | 0.2941 | M | 0.2950 | I |
| 3/8-24 | 0.3566 | T | 0.3580 | Q |
| 7/16-20 | 0.4154 | Z | 0.4130 | W |
| 1/2-20 | 0.4779 | 31/64 | 0.4844 | 29/64" |
| 9/16-18 | 0.5379 | 17/32 | 0.5313 | 33/64" |
| 3/4-16 | 0.7224 | 23/32 | 0.7188 | 11/16" |
| Thread | Formula drill (mm) | Nearest drill | Decimal (mm) | Cut tap drill (compare) |
|---|---|---|---|---|
| M3×0.5 | 2.78 | 2.80 mm | 2.80 | 2.5mm |
| M4×0.7 | 3.69 | 3.70 mm | 3.70 | 3.3mm |
| M5×0.8 | 4.65 | 4.65 mm | 4.65 | 4.2mm |
| M6×1.0 | 5.56 | 5.55 mm | 5.55 | 5.0mm |
| M8×1.25 | 7.45 | 7.45 mm | 7.45 | 6.8mm |
| M10×1.5 | 9.34 | 9.35 mm | 9.35 | 8.5mm |
| M12×1.75 | 11.23 | 11.25 mm | 11.25 | 10.2mm |
| M14×2.0 | 13.12 | 13.10 mm | 13.10 | 12.0mm |
| M16×2.0 | 15.12 | 15.10 mm | 15.10 | 14.0mm |
| M20×2.5 | 18.89 | 18.90 mm | 18.90 | 17.5mm |
| M24×3.0 | 22.67 | 22.65 mm | 22.65 | 21.0mm |
Common questions
- What drill for a 1/4-20 roll tap?
- About 0.228 in — a #1 drill (or 5.8 mm) at 65% thread, versus a #7 for a cutting tap.
- What drill for an M6×1.0 form tap?
- About 5.55 mm at 65% thread, versus 5.0 mm for a cutting tap.
- What is the form tap drill formula?
- Drill = major diameter − (0.0068 × % thread ÷ TPI). In metric, drill (mm) = major − 0.0068 × % thread × pitch.
- Why does a roll tap need a bigger hole?
- It makes no chips: displaced metal flows up into the crest. Too small a hole overloads and breaks the tap.