NPT Thread Chart Tap Drill Size Calculator for recommended drill and tapped hole sizes according to ASME B1.20.1 "Pipe Threads, General Purpose (Inch)" standard.

The drill size shown by the calculator is the diameter of the standard drill which is the closest to the minimum minor diameters of the pipe thread.

When nonferrous metals are to be drilled and tapped, it may be needed to use a drill with larger or smaller diameter to have a hole which is appropriate for an acceptable pipe thread with the required thread height.

NPT tap calculator is intended to help only the occasional user of drills.

NPT Tap Drill Chart and Calculator::

Nominal Pipe Size
Pipe Thread Type
Use of Reamer

Notes: If the use of reamer selection is "yes" for taper pipe thread, then ream the hole before tapping with a reamer having a taper of 3/4 inch per foot.


Value Parameter
Nominal Pipe Size --- ---
Outer Diameter of Pipe --- inch
Recommended Inch Drill Size --- ---
Drill Size Decimal Equivalent --- inch


Drill bit: A cutting tool used in drilling machines to drill a hole.

NPT: National Pipe Thread Taper

NPSC: National Standard Straight Coupling Pipe Thread

Tap: A mechanical tool used to make a standard thread through a hole. Different type of taps exist such as straight fluted taps, spiral pointed taps, spiral pointed only taps, spiral fluted taps, fast spiral fluted taps, thread forming taps, pulley taps, nut taps, and pipe taps.

Tap drill size: The required diameter of hole before the tapping operation of the hole.


  • ANSI/ASME B1.20.1 - 1983 Pipe Threads, General Purpose (Inch)