Pipe threads are different from machine screw threads in one fundamental way: they are tapered. The 3/4-inch-per-foot taper on NPT threads means the thread gets progressively tighter as you screw it in, creating a mechanical wedge that (with sealant) produces a leak-free joint. Understanding this taper and its implications for thread depth, engagement length, and tap drill selection is essential for anyone cutting or inspecting pipe threads.
\nThis guide covers NPT (National Pipe Taper) per ASME B1.20.1, explains how it differs from NPTF (dryseal) and BSPT (British Standard), and addresses the practical questions that come up when machining pipe thread connections: how deep to drill, how tight to make the joint, and when sealant is required.
The Taper: 3/4 Inch Per Foot
All three common tapered pipe thread standards (NPT, NPTF, BSPT) use the same taper rate: 3/4 inch per foot, which is a 1:16 ratio. This translates to 1 degree 47 minutes (1.7899 degrees) per side, or 3 degrees 34 minutes (3.5799 degrees) included angle. The taper is small enough that the threads look straight at a glance, but it is enough to create the wedging action that makes the joint self-sealing.
\nThe taper means that as a male pipe thread screws into a female fitting, the thread engagement gets progressively tighter. The first few turns are loose (hand-tight zone), and the last few turns require wrench torque as the thread flanks wedge together. This wedging action deforms the thread crests and roots slightly, creating the metal-to-metal contact that (with sealant on NPT) produces a seal.
\nBecause of the taper, the major diameter, pitch diameter, and minor diameter all change along the length of the thread. Thread dimensions are specified at a reference point called the "gauge plane" or "L1 gauge point," which is typically at the end of the hand-tight engagement zone.
Taper rate: 3/4 inch per foot (0.0625 inch per inch)
Taper angle: 1°47' per side (3°34' included)
Ratio: 1:16
Per inch of thread length, the diameter changes by 0.0625 inches (1/16").
Pipe Thread Dimension Calculator
Calculate NPT, NPTF, BSPT, and BSPP pipe thread dimensions. Tap drill sizes, taper rates, effective thread length, and pitch diameters for 1/8 through 4 inch pipe.
NPT vs NPTF vs BSPT: The Practical Differences
NPT (National Pipe Taper): The most common pipe thread in North America. The thread form has slightly truncated crests and roots that leave a small spiral gap between the male and female threads. This gap is the "spiral leak path" that makes NPT threads require sealant (Teflon tape, pipe dope, or anaerobic sealant) to be leak-free. Without sealant, NPT connections will seep.
\nNPTF (National Pipe Taper Fuel / Dryseal): Same taper, pitch, and diameter as NPT, but with a modified thread form. The crests and roots are designed to make metal-to-metal contact when properly tightened, eliminating the spiral leak path. NPTF connections seal without sealant — hence "dryseal." Used in fuel systems, hydraulics, and applications where sealant contamination is unacceptable. NPTF is harder to manufacture (tighter tolerances on thread form) and less forgiving of imperfect tapping.
\nBSPT (British Standard Pipe Taper): Uses the same 1:16 taper but has a 55-degree thread angle instead of NPT's 60 degrees. BSPT is common in Europe, Asia, and countries that follow British standards. NPT and BSPT are NOT interchangeable — the different thread angles mean they will cross-thread and leak if mixed. Always verify the standard before mating pipe fittings.
Tap Drill Selection and Thread Engagement
Tap drill selection for pipe threads is more complex than for straight threads because the taper means the tap cuts progressively deeper as it advances. The drill must be large enough to prevent the tap from bottoming out immediately (which would break it) but small enough to produce full threads over the engagement length.
\nThe tap drill diameter for pipe threads is based on the minor diameter at the small end of the engagement zone. Published tap drill sizes for NPT threads are specific to each pipe size — they are not calculated the same way as straight thread tap drills. Use the published values from the ASME B1.20.1 standard or the pipe thread calculator.
\nThread engagement length for pipe threads is measured as the number of turns past hand-tight that produce a seal. For NPT with sealant: 3 to 5 turns past hand-tight is the standard recommendation for most sizes. For NPTF (dryseal): 2 to 3 turns past hand-tight. Over-tightening tapered threads is dangerous — the expanding wedge action can crack the female fitting, especially in cast iron, brass, and stainless steel.
Drill the pilot hole deep enough to produce full threads over the engagement length plus at least 2 additional threads for tap lead. For a 1/2" NPT fitting requiring 0.50" engagement, drill at least 0.65" deep and tap to at least 0.55" of full thread depth.
Advanced Tap Drill Calculator
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Gauging Pipe Threads
Pipe threads are inspected with L1 and L2 ring and plug gauges. The L1 gauge checks the hand-tight engagement — when the L1 gauge is flush with the end of the pipe (plus or minus one turn), the thread pitch diameter is within specification. The L2 gauge checks the wrench-tight engagement, verifying that the thread will develop a seal with the appropriate number of tightening turns.
\nFor shop floor work where L1/L2 gauges are not available, a practical check is to screw a known-good fitting onto the male thread by hand. It should start easily, engage 3 to 5 turns by hand, and then require a wrench for the final 2 to 3 turns. If the fitting goes on too far by hand, the thread is undersize. If it barely starts, the thread may be oversize or the taper is incorrect.
\nThread form is as important as size for NPT threads. The 60-degree included angle and the truncation of crests and roots must be correct for the sealant (NPT) or metal-to-metal contact (NPTF) to function. Single-point threading on a lathe gives you direct control over thread form. Tapping uses the tap's form — which means tap quality matters for pipe threads more than for most straight threads.
Gauge flush with pipe end ± 1 turn = PASS
Gauge goes in more than 1 turn past flush = thread undersize (too loose)
Gauge does not reach flush = thread oversize (too tight)
For critical pressure applications, use calibrated gauges from a certified source.
Pipe Thread Dimension Calculator
Calculate NPT, NPTF, BSPT, and BSPP pipe thread dimensions. Tap drill sizes, taper rates, effective thread length, and pitch diameters for 1/8 through 4 inch pipe.