Measuring pipe thread size to find the right thread type can be confusing. Thread dimensions are considered a nominal size and don't match standard units of measurement. Using the proper pipe thread type is crucial for maintaining and installing equipment, so helping you determine what pipe thread you need is important to us. We have created a thread guide with five easy steps and simple thread size charts to help you avoid confusion and find the proper fitting.
ㆍNPT or NPS (national pipe tapered or straight): Most common in North America
ㆍMIP or FIP (male or female iron pipe): Same thread dimensions as NPT
ㆍBSP(T) or BSP(S) (British standard pipe tapered or straight): Most common in Europe
ㆍCompression: A unique threaded fitting that does not mate with other thread types
ㆍUNS (National Unified Special): Some compatible with compression fittings
You can force threads together, but if they don't completely match, then your fittings won't seal. Most pipe thread types are not interchangeable. NPT, MIP, and FIP work together but do not mate with any other types. An NPT will not fit a BSP. Many times, the fitting itself will tell you what thread type you need. The letters may be inscribed or stamped on the top.
| NPT Nominal Size | Actual OD | Hand Tight Turns | Wrench Turns |
| 1/8 | 0.405" | 4.5 | 2.5 |
| 1/4 | 0.54" | 4 | 3 |
| 3/8 | 0.675" | 4.5 | 3 |
| 1/2 | 0.84" | 4.5 | 3 |
| 3/4 | 1.05" | 4.5 | 3 |
| 1 | 1.32" | 4.5 | 3.25 |
| 1 1/4 | 1.66" | 4.5 | 3.25 |
| BSP Nominal Size | Major Diameter (OD) | Minor Diameter (ID) |
| 1/8 | 0.38" | 0.34 |
| 1/4 | 0.52" | 0.45 |
| 3/8 | 0.65" | 0.59 |
| 1/2 | 0.83" | 0.73 |
| 3/4 | 1.04" | 0.95 |
| 1 | 1.30" | 1.19 |
| 1 1/4 | 1.65" | 1.53 |
Threads come on a variety of fittings, from PVC pipe fittings to quick-connect adapters. Here's how a thread is constructed:
ㆍThe thread dimension refers to the major diameter or outside measurement.
ㆍThe pitch is the distance from the peak of one crest to the opposite of another.
ㆍThe thread angle is the difference between the threads or the valley between the peaks.
Let's say you live in the U.S. and purchase an NPT to use on your coffee maker. You didn't realize, however, that your coffee maker was made in Italy and needs BSP threaded fittings. If you try to wrench the NPT thread into the BSP, you'll wreck the threads on your machine. Many times, such damage can't be repaired.
If your equipment does not display the thread type, then you can't tell whether you have an NPT or BSP just by looking at the threads. Chances are, if you live in the United States, the you need NPT. If you live in Europe, then you likely need BSP. You can always measure the threads or test different types of fittings to find which type you need.
Threads have a gender—either male or female. The threads are on the outside of a male fitting and on the inside of a female fitting. The outer diameter is smaller on a male thread than a female thread because the male end compresses into the female end.
NPS stands for national pipe straight, and NPT stands for national pipe tapered. Tapered threads become narrower as they extend outward, while straight threads retain the same diameter.
Straight fittings have no taper to the body and are sealed to another fitting with an O-ring or gasket. Both the male and the female have a tapered flare at the end of the fitting that allows them to screw together and create a tight seal. The threads interlock as the male and female ends screw into one another.
Both straight and tapered fittings required a seal to prevent leaks. The male end of a straight fitting must be sealed with an O-ring, gasket, or washer. A tapered fitting should be sealed with Teflon tape, pipe dope, or some other pipe sealant.
You must have the right mate to make sure your connection is sound. If you try to thread a tapered fitting into a straight fitting, you'll only get a few threads deep. Although the fittings may seem like they fit tightly together, the seal won't be strong enough to prevent leaks.
Measuring pipe thread size is the most confusing part for many homeowners since thread dimensions aren't based on standard units of measurement. Thread dimensions are based on the ID of the pipe. Use a caliper, measuring tape, or ruler to measure the thread diameter of a male thread or female thread. Measure the inner diameter (ID) of the female thread and the outer diameter (OD) of the male thread. This number will help determine the thread dimension.
After measuring the threads on a fitting, you must match the number to a thread chart to determine the nominal size. Thread size and pipe size are measured according to the same scale.
Choosing what pipe fitting you need is like selecting the proper Lego when constructing a Lego set. Your project will determine the arrangement of your fittings. If you're going around the back of a cabinet to get to the ice maker or trying to hide tubing, you'll want to avoid creating leak points. Where you need the water, fluid, or air to go—around a corner or up a pipe—determines what shape or style of pipe fitting to use.
Note: Having many fittings, turns, or elbows could cause a loss of pressure and a slower flow rate.
Material matters. Putting together dissimilar metals could cause corrosion. The material of the fitting and the O-ring or gasket must be chemically compatible with the substance that will flow through it.
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