NPT Tap Drill Size Chart for Straight Pilot Holes

Use the table below when drilling a straight pilot hole before cutting an internal NPT thread with a conventional cutting tap. These are without-reamer drill recommendations; a taper-reamed hole may require a different drill size and controlled reaming depth. Greenfield and Regal publish the same straight-hole recommendations through 2-inch NPT; Regal extends its chart through 3 inches (Greenfield Industries; Regal Cutting Tools).

The metric column is a conversion, not a recommendation to substitute any nearby metric bit. If the exact inch or letter drill is unavailable, check the actual diameter of the proposed substitute and the tap maker’s guidance rather than simply rounding up.

The NPT size is not the hole diameter

“1/2–14 NPT” means a nominal 1/2-inch pipe thread with 14 threads per inch—not a 1/2-inch pilot hole. Pipe sizes are based on nominal inside diameter or flow size, which is why a 1/2-inch NPT tap uses a much larger 45/64-inch drill (CPC).

NPT is also not the same as NPS or BSP. ASME B1.20.1 covers NPT and several separate straight-pipe thread forms, including NPSC, NPSM and NPSL (ASME). BSP uses a different 55-degree thread form, while NPT uses 60 degrees; even nominal sizes sharing the same pitch should not be treated as interchangeable (CPC).

Straight drilling versus taper reaming

The chart is the simplest route for repair and low-volume work: drill a cylindrical hole, chamfer the mouth lightly, then let the tapered tap cut the thread form. NPT’s diameter changes by 1/16 inch per inch of length, so tapping depth determines the resulting thread diameter (GWS Tool Group).

Do not combine a without-reamer drill size with an uncontrolled taper-reaming step. An oversized reamed hole leaves insufficient thread height; an undersized one increases tap load. Taper reaming is therefore a separate process with its own drill, reamer dimensions and depth control—not an automatic improvement (Cutting Tool Engineering).

A practical drilling and tapping sequence

1. Confirm the designation. Read the tap marking in full, such as 1/4–18 NPT. Do not choose the drill from a measured fitting diameter alone.
2. Secure and align the work. Clamp the part rigidly and drill square to the intended port face. Larger NPT sizes require substantial torque and are generally better suited to a rigid machine setup than a handheld drill.
3. Drill the pilot hole. Use the charted diameter and enough depth for the tap’s lead, required effective thread and chip clearance. Blind-hole depth depends on the specific tap geometry, so obtain it from the tap drawing rather than this diameter chart.
4. Chamfer and deburr. A small entrance chamfer helps the tap start without leaving a raised burr around the port.
5. Tap under control. Keep the tap square, use cutting fluid compatible with the work material, and manage chips according to the tap maker’s instructions. Taper pipe tapping becomes progressively tighter as the tool advances.
6. Stop by size, not by guesswork. Tapping deeper makes the finished tapered thread larger. For repeatable or pressure-containing work, check with the specified NPT plug gage. An L1 plug gage in a properly threaded hole should register at its step or notch within the stated tolerance; trial-fitting one random fitting is not equivalent to inspecting the thread (Cutting Tool Engineering).

Treat the drill diameter as the starting geometry, not proof of a finished leak-tight port. Tap condition, alignment, material, tapping depth and gaging still determine whether the completed thread is usable.