National Pipe Thread Taper tap drill sizes range from 0.242 inches to 3.243 inches, but pipe thread tap drill sizes vary widely depending on the type of thread to be cut. Some common pipe thread tap drill sizes are 0.438, 0.562, and 1.156, 1.50 and 2.22 inches.
Recommended pipe thread tap drill sizes differ depending on the type of thread to be cut, and whether the thread is to be straight or tapered. Tapered pipe threads are used to create a seal that is fluid-tight. Commonly used tap drill sizes for tapered threads range between 0.125 and 2.22 inches in diameter. Sizes outside of these dimensions are less common because alternative means of joining are generally used with larger pipe diameters.
Tap drill sizes for straight pipe threads are the same as or slightly larger than drill sizes for tapered threads. Some common sizes for straight pipe threads are 0.453, 0.734, 1.19, 1.75 and 2.22 inches in diameter.
U.S. standards for pipe thread and tap drill sizes are determined by American Standard Pipe Thread specifications. Tapered threads used on pipes and fittings are defined by National Pipe Thread Taper standards. Straight threads, such as those found on bolts, are defined by National Straight Pipe specifications.