What NPT is, and why it's tapered
NPT stands for National Pipe Taper (formally, American National Standard Taper Pipe Thread), the standard thread form for most pipe fittings, valves, and pipe nipples in North America. Unlike a bolt thread, which is straight (the same diameter along its full length), an NPT thread tapers at a fixed rate of 1:16, three quarters of an inch of diameter change for every foot of thread length. That taper is the entire point: as a male and female NPT thread turn together, the flanks wedge against each other with increasing interference the further they thread on, and that interference, not crest-to-root contact like a straight thread, is what creates the seal. It's also why NPT fittings are normally assembled with thread tape or pipe dope: the taper seals mechanically, but a small amount of sealant fills the residual spiral leak path between the flanks.
What each column means
Pipe OD is the actual outside diameter of the pipe or fitting, in inches. Threads/in (TPI) is how many thread crests occur per inch of length; Pitch is simply 1 divided by TPI, the axial distance from one thread crest to the next. The remaining four columns describe the thread at two specific reference points along the taper. E0 (pitch diameter at end of pipe) is the theoretical pitch diameter exactly at the end of the pipe, before any thread has engaged. E1 (pitch diameter, hand-tight) is the pitch diameter at the point a fitting reaches hand-tight engagement, one turn or so before final wrench-tight makeup; L1 (hand-tight engagement) is the axial length of thread engaged at that hand-tight point. L2 (effective thread) is the full usable thread length, the effective engagement a properly made-up joint achieves. Read together, E0/E1/L1/L2 describe exactly how far a fitting threads on before it seals and how much taper interference it develops getting there, which is what a machinist or engineer actually needs when cutting or specifying NPT threads by hand.
Nominal size versus actual OD
For the smaller, more common sizes (1/16 through 12), the nominal size is a legacy label inherited from old wrought-iron pipe sizing and does not equal the pipe's actual outside diameter. 1/2 NPT pipe, for example, is 0.840" OD, not 0.500". The gap between nominal size and actual OD narrows as pipe gets larger and disappears entirely at 14" and up: those sizes are labeled "OD" in this chart (14 OD, 16 OD, and so on) specifically because, at that size, nominal size and actual outside diameter are the same number. Below 14", always use the Pipe OD column, not the nominal label, for anything that requires an actual dimension.
NPT, NPTF, and BSPT, briefly
NPTF (Dryseal, per SAE J476/ANSI B1.20.3) shares NPT's 1:16 taper and 60-degree thread angle, but holds tighter tolerances at the thread crest and root so the threads themselves seal without tape or compound. NPT and NPTF fittings thread together, but a standard NPT fitting does not hold NPTF's dryseal tolerance, so the joint still needs sealant. BSPT (British Standard Pipe Taper) also tapers at 1:16 but uses a 55-degree thread angle and different pitch diameters at each size; it is not interchangeable with NPT despite looking similar at a glance; forcing an NPT fitting into a BSPT port (or the reverse) will cross-thread or leak.
Where these numbers come from
Every dimension in this table other than pipe OD and TPI is calculated from those two values by the formulas in the derivation section above, published in ANSI/ASME B1.20.1. That means every value here is independently checkable: plug in a pipe's OD and TPI and the formula reproduces its E0, L2, and E1 to five decimal places, exactly as this chart does. For related shop reference, see the drill bit size chart and the tap drill chart, both drawn from the same verified dataset family.