1/8 NPT Tap Drill Size and Pipe Thread Bit
Use a 21/64 drill bit
1/8-27 NPT · 0.3281 in / 8.33 mm · pipe taper
1/8 NPT is a common tapered pipe thread for pneumatic and plumbing fittings. The drill is 21/64 in, sized to the pipe taper.
At a glance
| Tap drill | 21/64 |
|---|---|
| Inches | 0.3281 in |
| Millimetres | 8.33 mm |
| Threads per inch | 27 |
Why this size
NPT is a taper pipe thread. The drill is sized to the pipe minor diameter at the gauge plane. Tap drill: 21/64 (0.3281 in / 8.33 mm).
NPT threads taper, so the drill is not set by the percent-engagement rule. 21/64 in is the correct tap drill for 1/8 NPT.
Related sizes
Compare with adjacent threads – coarse, fine and the opposite measurement system: