Tap and Die Drill Chart
A tap cuts threads inside a hole; a die cuts threads on the outside of a rod or bolt. This chart pairs them for every common thread — inch UNC/UNF, ISO metric and NPT pipe — with the drill size each tap needs and the die marking you will find on the tool itself.
Inch tap & die chart (UNC / UNF)
| Thread | TPI | Die marking | Tap drill | Decimal |
|---|---|---|---|---|
| #4-40 | 40 | 4-40 | #43 | 0.0890 in |
| #6-32 | 32 | 6-32 | #36 | 0.1065 in |
| #8-32 | 32 | 8-32 | #29 | 0.1360 in |
| #10-32 | 32 | 10-32 | #21 | 0.1590 in |
| #10-24 | 24 | 10-24 | #25 | 0.1495 in |
| 1/4-20 | 20 | 1/4-20 | #7 | 0.2010 in |
| 1/4-28 | 28 | 1/4-28 | #3 | 0.2130 in |
| 5/16-18 | 18 | 5/16-18 | F | 0.2570 in |
| 3/8-16 | 16 | 3/8-16 | 5/16" | 0.3125 in |
| 1/2-13 | 13 | 1/2-13 | 27/64" | 0.4219 in |
| 5/8-11 | 11 | 5/8-11 | 17/32" | 0.5313 in |
Metric tap & die chart (ISO)
| Thread | Pitch mm | Die marking | Tap drill |
|---|---|---|---|
| M3×0.5 | 0.5 | M3×0.5 | 2.5 mm |
| M4×0.7 | 0.7 | M4×0.7 | 3.3 mm |
| M5×0.8 | 0.8 | M5×0.8 | 4.2 mm |
| M6×1.0 | 1.0 | M6×1.0 | 5.0 mm |
| M8×1.25 | 1.25 | M8×1.25 | 6.8 mm |
| M10×1.5 | 1.5 | M10×1.5 | 8.5 mm |
NPT pipe tap & die chart
| Thread | TPI | Die marking | Tap drill | Decimal |
|---|---|---|---|---|
| 1/8 NPT | 27 | 1/8-27 | 21/64" | 0.3281 in |
| 1/4 NPT | 18 | 1/4-18 | 7/16" | 0.4375 in |
| 3/8 NPT | 18 | 3/8-18 | 37/64" | 0.5781 in |
| 1/2 NPT | 14 | 1/2-14 | 23/32" | 0.7188 in |
Pipe dies cut the same taper the tap does — the die "walks" tighter as it advances and seals the same way. NPT dies are marked with the size and TPI, e.g. "1/2-14 NPT".
How to read this chart
- Drill column: the hole to drill before running the tap. Full worked formula and close/free-fit clearance holes on each thread's own page — links in the tables.
- Die marking: the size stamped on the die. For threading a rod, the rod must start at (or a hair under) the thread's major diameter — a 1/4-20 die cuts on 1/4 in rod, no bigger.
- Dies do not need a drill: they remove material from the outside. The "drill" column applies only to the tap side.
Which stock size for the die side
rod Ø for threading = the thread's major diameter 1/4-20 die → 1/4 in rod · M6 die → 6.0 mm rod · 1/2-13 die → 1/2 in rod oversize rod = broken die or jammed start · undersize = shallow threads
Chamfer the rod end before starting the die, use cutting oil, and reverse a quarter turn every few threads to clear chips — same rhythm as tapping.