How to Determine Pipe Thread Type and Pipe Size

Posted by
John Woodard on August 16, 2022

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. 

Common pipe thread types:  

  • NPT or NPS (national pipe tapered or straight): Most common in North America 
  • MIP or FIP (male or female iron pipe): Same thread dimensions as NPT 
  • BSP(T) or BSP(S) (British standard pipe tapered or straight): Most common in Europe 
  • Compression: A unique threaded fitting that does not mate with other thread types
  • UNS (National Unified Special): Some compatible with compression fittings


NPT, MIP, and FIP work together but do not mate with any other types. An NPT will not fit a BSP.  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. Many times, the fitting itself will tell you what thread type you need. The letters may be inscribed or stamped on the top. 

Thread Type Definitions


NPT (national pipe tapered) and NPS (national pipe straight) are the tapered thread standard that has been used in the United States since 1886. The tapered threads offer a more reliable seal than straight threads and are used in plumbing, hydraulic, and gas transportation applications. These threads were originally made from steel and brass, but they can now be made with materials such as bronze, PVC, and cast iron. NPT fittings are distinguishable by the following characteristics:

  • Taper angle of 1°47’
  • Pitch measured in threads per inch
  • 60° angle between flat crests and troughs


MIP (male iron pipe) and FIP (female iron pipe) are both pipe fittings with NPT threads. MIP refers to fittings with male threads, and FIP refers to fittings with female threads. Despite their names, MIP and FIP are not always made of iron; they are commonly made from steel and brass as well.


BSP (British standard piped) is the British pipe fitting standard and is also used frequently in Australia and the commonwealth countries. Unlike NPT threads, BSP threads’ crests and troughs are rounded instead of flat. The angle between these crests and troughs with BSP threads is 55° instead of 60°. There are two types of BSP threads: BSPP and BSPT. BSPP threads are parallel (or straight), while BSPT threads form a tapered seal.


UNS (unified special thread) is the standard for screws, nuts, bolts, and other threads in the United States and Canada. They possess the same 60° angle between crests and troughs as NPT but are not interchangeable with them. Threads in this series include UNC (coarse), UNF (fine), UNEF (extra fine), and UN (fixed pitch).

NPT Thread Size Chart

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 Thread Size Chart 

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

What are the parts of a pipe thread?

Threads come on a variety of fittings, from PVC pipe fittings to quick-connect adapters. Here's how a thread is constructed:

  • The thread dimension refers to the major diameter or outside measurement.  
  • The pitch is the distance from the peak of one crest to the opposite of another. 
  • The thread angle is the difference between the threads or the valley between the peaks.   

parts of a pipe thread

    Determine thread type and pipe size in 5 steps

    1. What are you going to use the fitting for? 

    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.

    2. Do you need a male or female fitting? 

    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.   

    male vs female fitting

    3. Are the threads tapered or straight? 

    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. 

    tapered vs straight threads

    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. 

    4. What is the fitting thread size or pipe size?  

    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. 

    measuring OD of thread measuring ID of fitting

    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. 

    5. What pipe fitting is compatible? 

    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.  

    Tips for assembling pipe fittings 

    • Don’t cross-thread your threads. If you’re inserting a fitting and it feels like it’s going in incorrectly, it’s cross-threading, or going in at an angle.  
    • Use Teflon tape instead of pipe dope to seal the threads for simpler installation. Pipe dope can be messy, and if you use too much, it could flow into your filter housing or appliance. After applying sealant, make sure you test the seal to make sure it’s effective.  
    • Clean the ends of your fittings before use. You don’t want debris to get caught in the threads.  
    • Don't over tighten and crack or split the fitting. This is especially problematic when using plastic fittings. 

    Types of pipe thread fittings

    Couplings and unions join lengths of pipe or tubing. They can be male to female and reduce size or change thread type.  


    Nipples are straight fittings with threads on both ends. 

    nipple fitting

    Couplers are female fittings with threads on both couplers. 


    Tees are couplers that join three sections of pipe or tubing. They can be a combination of male to female or reduce size.  

    tee fitting

    Elbows are couplers with a 90-degree turn.

    elbow fitting

    Bushings are used to convert from one size to another. For example, your bushing could have a 3/8 male thread on one side and 1/4 female threads on the other.  


    Barbs are used for softer hose or tubing connections. 

    hose barb

    When to use a compression fitting

    Compression fittings are common, but their threads don’t match with any other thread type. This fitting received its name because the thread is compresses the pipe tube inserted into it to seal it. Compression fittings have straight threads and are made of brass or plastic and are often used on soft copper pipe and small-diameter tubing. The angle stop under your sink is one example of compression threads.

    A compression fitting includes an outer compression nut and an inner compression ring or ferrule (sometimes call the sleeve). A flare fitting is a type of compression fitting used with metal tubing, usually soft steel or soft copper or aluminum.  

    compression fitting

    Learn more about fittings in these articles:

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