Congratulations! If you're getting ready to read this article, we're assuming that you're about to begin your very own project. And, whether this is your first project or your fiftieth, it's important to make sure that you're getting the right supplies.

And, one of the most important considerations that you have to make is the type of thread that you're going to use. There are hundreds of kinds of threads, but each kind is best for specific projects.

The most common thread types are useful for everyday projects.

Whether you're going to use these for personal, industrial, or commercial reasons, keep reading. We're going to cover what you need to know about the most common thread types.

The Most Common Thread Type

There are six common types of threads:

  1. UN/UNF
  3. BSPP
  4. BSPT
  5. Metric parallel
  6. Metric tapered

Each of these is commonly used in construction projects, whether personal or commercial.

The most common thread type from this list is the Unified coarse/fine threads that are measured in inches. The second most common type of thread is the metric coarse/fine threads. These are measured in millimeters.

Each kind of thread has its own purpose based on its thickness and material.

When you're planning for your next project, you need to know what you're going to need your threads for. So, you have to be able to identify the kinds of threads that you're working with. This will help you decide what kind of equipment you need.

The Difference Between Coarse Threads and Fine Threads

If you're choosing between coarse and fine threads, you're likely going to be reaching for coarse threads.

However, this changes if you're using sheet metal. In that case, you should opt for fine threads.

Coarse threads have fewer threads per inch than fine threads. And, coarse threads are more common than fine threads. This is because coarse threads have more uses.

Fine threads are more likely to cross-thread or jam. Coarse threads go in at an angle so that these kinds of complications don't happen. In the end, this makes coarse threads easier to install.

On the other hand, screws that have fine threads are stronger than screws with coarse threads. Fine threads are thin, so they take up less of the available surface area on the screw.

However, coarse threads are better against potential stripping. Coarse threads are more tolerant to any damage or corrosion that may happen. So, there's more room for error.

Fine threads allow professionals to achieve a fine level of adjustment. Each rotation of a fine thread is smaller than the same rotation of a coarse thread. So, those using fine thread screws have more control as they're making the placement.

Characteristics of Threads

There are a few characteristics that you should be able to identify with each screw:

  • Gender
  • Handedness
  • Design
  • Pitch
  • Diameter
  • Angle
  • Crest

Let's run through each characteristic and explain how you can identify them.


Threads work in pairs. One thread is internal, while the other is external.

The internal thread is female, and the external thread is male.

For example, a screw has a male thread, while the hole that it goes into has a female thread.


The helix of the thread can go either way. The direction that the helix turns determines whether the screw is left-handed or right-handed.

Most threads are oriented in such a way that the centre of the helix moves away from the viewer when it's turned in a clockwise direction. At the same time, it moves towards the viewer when it's spinning counter-clockwise.

This kind of orientation is right-handed. Since most people are right-handed, right-handedness in screws is the default. In turn, right-handedness is also the default for parts and fasteners.


The design of every thread is either tapered or parallel. Tapered designs pinch in at the end of the screw, while parallel designs stay consistent as you move along the screw.

It's important to match each design with its opposite. Otherwise, there may be structural issues.


The pitch is the distance from the crest of one thread to the crest of the next thread. Usually, experts measure pitch in millimetres.

However, there is a measurement that's called TPI. This stands for "threads per inch." As the name implies, TPI is in inches.


There are three kinds of diameters when it comes to screws: major diameter, minor diameter, and pitch diameter. The major diameter of a screw is determined by the thread tips, while the minor diameter of a screw is determined by the groove of the thread.

The pitch diameter refers to the distance between two tips of opposite flanks. In other words, the pitch diameter is the length of the centerline of the profile of the screw.


There is one kind of main angle: the flank angle. Screws with tapered designs also have a taper angle.

The flank angle refers to the angle that is between the flank of a screw thread and the perpendicular line to the axis of the screw. The taper angle in tapered screws is the angle between the taper and the centre axis of the pipe.

Crest vs. Root

The crest is the outermost part of the thread of a screw, while the root is the innermost part of the thread of a screw.

You may be able to better identify the crests and roots by thinking of the thread of the screw as a long ridge. The top of the ridge is the crest, while the bottom of the ridge is the root.

Notating Threads

There are four steps when it comes to recognizing and identifying different kinds of threads. To follow these steps, you need two tools: the pitch gauge and the calliper.

The pitch gauge is a tool that can measure the distance between two threads on a screw. In NPT, UN/UNF, and BSPP screws, these measurements occur in inches. For metrics screws, we measure in millimetres.

The calliper measures the diameter of the thread. The caliper measures outside of the thread for male threads and inside of the thread for female threads.

Digital callipers are able to take these measurements quickly and easily. So, it simplifies the process.

1. Parallel Thread vs. Tapered Thread

In order to tell the difference between a parallel thread and a tapered thread, you need to look at the diameter of the thread.

If the diameter of the thread gets thinner as you go, it's a tapered thread. If the thread diameter stays the same, it's a parallel thread.

Parallel threads include UN/UNF, BSPP, and metric parallel threads. Tapered threads include NPT/NPTF, BSPT, and metric tapered threads.

2. Find the Pitch

You can use the thread pitch gauge to determine how big the thread of a screw is. The gauge can help you measure and calculate the number of threads within a certain distance.

These are the common pitch sizes for common thread types:

  • UN/UNF (SAE) - 12, 14, 16, 18, 20, 24
  • NPT/NPTF (American Pipe) - 11.5, 14, 18, 27
  • BSPP (British Pipe) - 11, 14, 19, 28
  • BSPT (British Pipe) - 11, 14, 19, 28
  • Metric Parallel - 1.0, 1.5, 2.0
  • Metric Tapered - 1.0, 1.5, 2.0

As you're determining the pitch size of your screws, you should use these measurements to guide you as you use the pitch gauge.

3. Determine Thread Size

Now, it's time to determine the thread size.

Before you begin, you should determine whether or not your thread is a pipe thread. If you are looking at a pipe thread, you need to compare its size with a nominal size profile. If you are looking at a non-pipe thread, you need to use the calliper tool to measure the outside diameter of the thread.

4. Pick the Thread

During this final step, it's time to put all of the information together. You can determine the thread type by figuring out whether the thread is tapered vs parallel, measuring the pitch, and determining the thread size.

The most common thread type, UN/UNF, has the following specifications:

  • Tapered vs. parallel - Parallel
  • Pitch - 12, 14, 16, 18, 20, 24
  • Thread size - Measured with calliper
  • Thread designation - size-pitch, type 3/4-16 (UN/UNF)

If you can determine the kind of thread that you're looking at, you can make sure that you're using the right kind of threads for your projects. If you choose the wrong part, you could cause property damage. So, when in doubt, you should measure everything again.

Getting the Types of Threads You Need

As any professional knows, it's important to understand and be able to identify every single type of thread. Given that the most common thread type is the UN/UNF thread type, you may become more familiar with this kind of thread type.

Although, there are six common types that you should be able to identify and differentiate between. At the same time, you should be able to determine the uses for these different thread types.

Once you've identified the kind of equipment that you need, you should check out our supply. From measurement tools to different kinds of threads, we have everything you need.

If you're not sure what kind of equipment that you need, feel free to contact us. Whether you're choosing from one of the most common thread types or another kind of thread, we can help you find the right tools for the job.