How to measure NPT Fittings and pipe sizes

What's the deal with measuring pipe threads? Do you order the wrong size and wonder why? This very easy rule will save you headaches for the rest of your pipe measuring life. 3/4" NPT pipe refers to the nominal INSIDE diameter of the pipe, not the outside diameter.

[Chris-dcpc]

just started a job doing compressor service calls. i asked my service manager how to measure pipe threads and he couldn't tell me. thanks man.

[hakukuze]

When I am at the Pearly gates, I was only gonna ask to see the one who invented email to beat up, but now I have to add the one who did the subtract 1/4 inch to the list too ;) Thanks much for the simple and quick lesson.

[terrys]

Informative. Thanks. I've always known that NPT measurement was different from other things, like fasteners, etc., but I didn't know exactly why until just now.

[gmax]

I've lost my mind on this several times. Thank you.

[TheDroppedAnchor]

Dang good video by a pro's pro.
I'm an old man who was taught steam pipe-stuff by WWII guys. Not being too educated I have to use memory tricks.
NPT I think of as NOMINAL pipe.
Putting the words "pipe" and "tube" next to each other reveals that the 2nd letter "i" means pipe is measured by its INSIDE diameter.
And the 2nd letter "u" in tubing is bc it's measured by the "OUTside" diameter.

Nominal is an important characteristic because even tho 100% of home owners see is Schedule 40 there are thicker schedules of pipe. Example; 1" Schedule 40 has a nominal inside diameter of 1". A 1" Schedule 80 has a nominal inside diameter of much less. (My Pipefitter's Manual isn't handy.).
Another example; when holding a length of 1" pipe the threaded end inside diameter will measure less than its non-threaded end inside diameter. Hence, nominal.
Dang good video-this guy is a pro's pro.

[notisgeorgakopoulos]

One could fill volumes of what "teachers" don't teach young men who want to be plumbers...Thanks for the informative tutorial!

[tonyaddington]

You explained in 4 minutes something that has confused me for many years! lol. Can't thank you enough.

[dianewach]

Thank you for so clearly explaining at the beginning of this video the pipe fitting measurements, and how and why typical measurements of diameter and circumference differ from NPT. VERY HELPFUL!

[barbaraburnopp]

Excellent straight forward no nonsense professional explanation.

[tedijune]

Master Tool; Thank you, easy to see-hear and understand! Bravo!

[catbeans]

Your the best, thank you for saving me multiple trips to home depo

[xzero]

BSP pipe also comes tapered, like NPT. And some conduits have a different taper and some straight an some the same taper as NPT.
Pipes 2" and larger have almost exactly the same size inside diameter as the nominal pipe size for SCH 40.
Taking a quarter inch off the OD of a 1.5" pipe will yield 1-3/4". The rule of thumb won't work there.
The rule of thumb here ONLY works for little pipes, then doesn't work for 1.5, does work almost for 2, 2.5, doesn't work again for 3 and gets worse and worse bigger than that.

Best to use the chart, look it up or find someone that knows. I frequently ask for second opinion when 'eyeballing' threaded pipes and I have been doing it for 35 years....usually the 1.25 and 1.5 sizes are mis-estimated--especially when they are up high and can't be inspected closely.

For 2" versus 2.5 ", the thread pitch gives it away....2" is the biggest with 'fine looking (11.5 tpi)' threads and 2.5 is coarse like 3, 3.5, 4, 5-inch and all the larger pipes up to giant sizes (8 tpi).

[SamChaneyProductions]

THANK YOU for explaining why the heck NPT sizes are all 1/4 inch off. Unfortunately, even the inside diameter isn't actually very close to the measurement in the name

[appalachianman]

This is awesome info. I wish I watched your video before I ordered mine. Lol
But at least I know I made the mistake and not the supplier.

[tonycorazalla]

Thank you, you did a very very good job at explaining that. Good job

[RHCPFAN-yksw]

Thank you for the explanation. As a machinist, I was throughly confused why my “5/8” pipe cap wouldn’t fit lol.. I’m about 1\4 off lol…

[berryae]

Short sweet and too the point, excellent video!

[thenexthobby]

Galvanized pipes all in my old house. I'll use calipers to measure the outside ... do the subtraction and hopefully that'll be correct. My goal is to be able to buy replacement parts BEFORE I destroy the old plumbing.

[TanukiDigital]

for smaller pipe fittings, do you you subtract 1/8" from the inside measurement rather than 1/4"?

[jamesmanzy]

Great video, just wondering why you always subtract a 1/4 inch on almost all the fittings