What are the differences between pipe threads?

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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

MIP vs NPT vs FIP

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 and NPS

NPT (national pipe tapered) and NPS (national pipe straight) are the tapered thread standard that has been used in the United States since . 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 These threads were originally made from steel and brass, but they can now be made with materials such as bronze, PVC, and cast iron.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 and FIP

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

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

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. 

   

   

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.   

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. 

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. 

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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. 

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.  

Elbows are couplers with a 90-degree turn.

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. 

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.  

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For those working with thread arrangements for piping, it&#;s important to know the differences between types of threads. Whether NPT vs. MNPT, FTP vs. NPT, or even NPT vs. BPT, understanding the differences between these commonly used threads will help avoid issues from utilizing incompatible pipe threads.

Understanding & Determining Common Thread Types

Identifying common piping thread types requires a caliper, thread gauge, and reference chart. The gauge helps determine the thread&#;s pitch, diameter, and angle. Featuring jagged teeth like a saw, it ensures a thread falls between an upper and lower limit, with a specific number of serrations marking the parameters that are also normally marked. For US standard threads, the pitch is determined by the number of threads per inch, while metric-based standards measure the distance in millimeters between each thread.

Steps to help determine the type of thread: 

• Identify whether the thread is parallel or tapered. This may be possible by visually inspecting the threads, with parallel threads having the same diameter throughout and tapered threads growing smaller toward the fitting&#;s end. A caliper can compare thread diameters if a visual inspection doesn&#;t reveal this.
• Establish the thread&#;s pitch. A pitch gauge is used to either compare threads against a lighted background when utilizing the American standard or to measure and calculate the number of threads within a given distance when using a metric standard. Results from this step determine the different thread forms by narrowing down the possibilities.
• Verify the size of the thread. For pipe threads, determining size is done by comparing the thread&#;s size profile; for pipes below 2 inches, the nominal size can be verified by measuring the outside diameter, subtracting ¼ inch, and rounding the measurement.
• Describe the thread. This step involves designating the industry standard under which the thread will be described to allow maintenance or installation personnel to understand the type and size of the thread with which they&#;re working, along with the pitch in certain instances. This is where the reference chart comes in, which enables workers to

Following the above four steps helps minimize downtime and expenses from ordering the wrong components.

Comparisons of Common Threads

When comparing NPT vs. BPT, FTP vs. NPT, or NPT vs. MNPT, it&#;s important to establish whether the thread is parallel or tapered. To do this involves measuring the first and last full threads, along with the fourth or fifth one. It&#;s tapered when the diameter increases between the first and last threads on a male thread or decreases for a female thread. However, it&#;s considered a parallel thread if the thread diameter stays the same.

Once determined to be tapered or parallel, its trade size can then be measured. Tapered threads are measured at the fourth or fifth full thread to ascertain trade size, whereas parallel threads can be measured at any full thread. Once properly measured, the measurement can be referenced via a table to match the thread and trade size.

Some common types of pipe threads used in stainless steel applications include:

• National Pipe Tapered Thread (NPT) is a US thread standard with a 60-degree thread form on a tapered thread, with a taper of ¾ inch per foot.
• Male national pipe thread (MNPT) is the male version of NPT pipe thread, sometimes abbreviated as MPT or NPT(M).
• Female pipe thread (FPT) is the female version of NPT pipe thread, sometimes abbreviated as NPT(F).
• British pipe thread (BPT) is utilized by the United Kingdom and many member countries of the British Commonwealth that feature an un-tapered thread form of 55 degrees.

NPT vs. MNPT

NPT is the most used thread for pipes and fittings for connecting hydraulic seals and mechanical joints. Two different kinds are used for sealing hydraulic circuits. Jointing threads tighten pressure by sealing on the threads, with a jointing compound improving this sealing effect; these feature taper external threads, along with either parallel or taper internal threads. In contrast, fastening threads don&#;t seal on the threads, using two threads in parallel to provide pressure upon a flat gasket or soft material against the external thread.

The general features of NPT are:

• Measurement of pitch is the number of threads per inch.
• Taper to the center axis is at an angle of 1.° for the pipe or fitting.
• Root shortness and peaks are more level.
• Thread angle of 60°.

MNPT is simply the male version of the NPT standard, with male or external tapered threads. When considering whether to utilize NPT vs. MTNP, it should be noted that the latter is interchangeable with the NPT standard.

FPT vs. NPT

When considering the differences between FPT and NPT standards, it should be noted that the FPT standard features internal tapered pipe threads. Essentially the female NPT standard can connect with their male NPT counterparts, joining pipe sections and fittings. They&#;re used in dry-seal connections, especially those involving fuel applications. FTP connections don&#;t require tape or other sealants like regular male NPT threads. Unlike NPT vs. MNPT, they aren&#;t interchangeable but rather interlocking.

These threads feature the same basic shape as the NPT standard but eliminate spiral leaking by adjusting root and crest heights, creating an interference fit. FPT vs. NPT largely differs between the former having internal threads, whereas the latter male standard is external. When making tapered connections with FPT, NPT fittings lock into their female counterparts, expanding until the force between them can withstand the pressure of whatever material flows through the pipes. Though FPT threads require no sealant on their own when connecting with male NPT threads sealant is required because of the narrow clearance between the male piece&#;s root and crest.

BPT vs. NPT

However, when comparing BPT vs. NPT threads, the differences are much more marked. Whereas American standard measurements count the number of threads per inch with NPT, BPT utilizes the metric system to measure the distance between threads. BPT also has a thread flank angle of 55° for parallel and tapered threads, whereas NPT&#;s thread form is 60°. They do have a couple of similarities, however. When considering BPT vs. NPT fittings, both can be used with pipes of standard size. Additionally, both are difficult to seal, requiring sufficient compression on the pipe&#;s threads to form a seal.

For more information about how our company can help with your application &#; or to learn more about comparisons of common pipe threads like NPT vs. MNPT, FTP vs. NPT, and BPT vs. NPT &#; we invite you to contact the experts at Arthur Harris today.