What is NPT? National Pipe Thread
NPT is an abbreviation for National Pipe Thread. It is an American standard where the suffix determines whether it is a straight or conical thread. For example, NPT, National Pipe Tapered which is conical and seals in the threads. NPS, National Pipe Straight is straight and seals with washer or similar.
NPT is the category of threads called National Pipe Thread. A national standard pipe thread simply. This thread is available in different variants where it is clear whether the thread is conical or straight.
NPT, National Pipe Tapered
NPS, National Pipe Straight
NPTF, National Pipe Tapered Fuel
NPSF, National Pipe Straight Fuel
NPSM, National Pipe Straight Mechanical
Below we will go through National Pipe Tapered - Conical thread.
Mounting of NPT thread
When you thread in an adapter with a conical NPT thread, this will lock / pinch a little whenever it wants depending on the tolerances on the parts. Sometimes you can do 2-3 turns and sometimes 3-4 turns or more. This is due to the tolerances in the two parts and if you use sealing compound on the threads, which is because you can not screw as many turns.
Solid sealant such as PTFE tape or liquid form from Loctite is used as a seal for conical NPT threads.
NPT Size chart
NPT conical thread has a taper of 3/4" per foot 304.8mm (feet). Similar sizes are M10 and 3/8" -24 where NPT can be threaded down but this is not correct and will not seal properly. The largest and smallest measurement on an NPT thread depends on how long the thread is but usually looks like the table below.
1/16" -27 -
Largest thread size: 0,3125"/7,94mm
Smallest thread size: 7,4mm
1/8" -27 -
Largest thread size: 0,405"/10,29mm
Smallest thread size: 9,88mm
1/4" -18 -
Largest thread size: 0,540"/13,72mm
Smallest thread size: 13,11mm
3/8" -18 -
Largest thread size: 0,675"/17,15mm
Smallest thread size: 16,27mm
1/2" -14 -
Largest thread size: 0,840"/21,34mm
Smallest thread size: 20,6mm
3/4" -14 -
Largest thread size: 1,050"/26,67mm
Smallest thread size: 25,9mm
1" -11½ -
Largest thread size: 1,315"/33,40mm
Smallest thread size: 32,5mm
1¼" -11½ -
Largest thread size: 1,660"/42,16mm
Smallest thread size: 41,3mm
Select the correct NPT thread
If you do not know which NPT thread you have, you can with the table above get a really good indication between which values you are and thus find the right one.
Internal thread - Female
If you have an internal conical NPT thread, measure down into the hole against the threads and you will get the smallest value. Then you look at which minimum thread size fits in the table above.
External thread - Male
If you have an external NPT thread, measure the smallest diameter over the threads. Then you look at which largest thread measure fits in the table above.
NPT drill / bore / thread
There are tapered threaded pins to make NPT threads yourself. If you are going to make a conical thread, it is important to pre-drill with the right size as the thread may otherwise be too large or too small.
1/16" -27 - 6,2mm
1/8" -27 - 8,5mm
1/4" -18 - 11,0mm
3/8" -18 - 14,5mm
1/2" -14 - 17,8mm
3/4" -14 - 23,0mm
1" -11½ - 29mm
1¼" -11½ - 37,5mm