If you own an air compressor and you need to replace air hose fittings, or even if you’re thinking about installing a permanent system for a shop, then air compressor hose fitting types and sizes will be one of your major concerns.
While most consumer air compressors, tools, and hose systems come with standard fitting sizes, it's important to understand what these sizes are, along with the common measurements that you'll be exposed to.
Air hose couplings and other connectors that are used for compressors, follow a system that is known as the NPT fitting standard. NPT stands for National Pipe Thread Tapered and it's a US standard for tapered threads which are used for fittings and pipes.
The NPT fitting standard means that if you're selecting equipment which is measured with standard air fitting types, then it will be compatible with your other fittings. This applies to the pre-installed couplings that come with your air compressor.
Let's first take a look at the size chart and some of the most common air fitting hose sizes, then we can look at some of the air compressor fitting types and accessories that you might need when upgrading your system or replacing worn parts.
Air Hose Fittings: NPT Size Chart for Air Compressors
Also known as American standard pipe, the NPT doesn't just refer to the size of a connector, but also the size of the thread on any connector.
The thread size is especially important, because non-standard fittings may not provide a full seal, and will allow air to escape from your hose lines and connectors. As you might guess, this will greatly reduce the efficiency, airflow, and pressure that an air compressor is able to deliver to any pneumatic tool which is connected to your system.
While it is fairly easy to understand the fitting types for industrial and automotive connectors, it is still helpful to be aware of the NPT fitting chart and how it relates to pipes and connectors of all types.
NPT fitting chart sizes are provided in inches and include threads per inch and the outside pipe diameter. Manufacturers that advertise NPT fittings are required to conform to these figures with a high level of accuracy.
The table below displays the most common NPT thread sizes and their respective threads per inch count, as well as the actual measurement of the outside diameter (OD). You can find a more elaborate explanation and an expanded version of the table containing additional NPT standard thread sizes in this article.
NPT Fitting Chart
Pipe Size (inches)
Threads per Inch
Outside Pipe Diameter
How to Determine the Size of an Air Compressor Fitting Port on a Tool or Connector You Already Own
Chances are you already have tools and connectors with working parts, but you may have lost the orginal documentation and/or packaging that provides the exact air hose fitting sizes. This can obviously be a problem when it comes to fitting a new hose or replacing a connector.
Most common tools use a 1/8 or 1/4-inch connector, particularly when it comes to contractor or consumer gear. However, depending on the age of a tool, or where it was manufactured, the size may not conform to these standards.
The best way to determine a port opening size is to measure the diameter of the opening. This is done by measuring right across the center of the port. Once you have an accurate measurement, you'll know the pipe size and be able to find the correct connector or replacement part.
How to Measure Pipe Threads
How to Get a Strong Seal When Connecting a New Fitting
Whether you're replacing a fitting between a compressor and an air tank or a fitting on a tool handle, you'll need to ensure that you not only use the right size thread, but also create a strong seal when attaching the hardware.
The best way to get an airtight seal, is to use Teflon tape on the thread.
Even when parts are manufactured to exacting standards that adhere to the NPT chart, there's still potential for air to escape from a fitting. Teflon tape will help create a tighter seal, which should retain its integrity through the lifetime of the fitting.
How To Apply Teflon Tape
Using Teflon tape is a very effective way to get a strong seal on a new fitting, but it is important to take care when wrapping the connectors. The tape should be wrapped around the male connector several times in the same direction the fitting will be turned to secure it in place (see video for a visual example). The teflon should not overlap the end of the connector.
Avoiding excess tape that extends beyond the end of the thread is critical. High air pressure could cause the excess tape to break away, which may create a blockage within the hose, other connectors, or inside the air tool itself.
Even if not dislodged, the excess tape could cause a blockage or partial blockage that reduces the efficiency and airflow of your compressor.
Watch The Video
There's quite a few Teflon tape products available today. This one by Dixon Valve is an excellent choice. It's affordable, rated for a temperature range of -212° to 500°F, and is a bestselling airtight sealant tape for air compressor fittings.
Do Couplings Come with Teflon Tape?
You might be able to find air fitting types that come packaged with Teflon tape, but typically, instead of using a tape, when manufacturers apply teflon sealing they use a paste or putty which creates a perfect seal between connectors.
While you most likely won't find this when looking for loose air compressor fitting types, you may find a sealing substance to be in major fittings on portable air compressors from leading brands.
Keep in mind, when you're replacing equipment and come across any sealant paste you'll need to clean off the excess before reapplying fresh Teflon. Just like tape, loose paste in the system can cause blockages in fittings, hoses, and air tools.
If you prefer to use a liquid pipe thread sealant on your own couplings instead of Teflon tape there are many excellent brands available.
Liquid Pipe Thread Sealant
Rectorseal contains teflon and synthetic fibers to create a non-hardening stronger seal. No need to wait for it to dry, it can be pressurized immediately after applying.
Common Coupling Products that Conform to the NPT Size Chart
When it comes to contractor and DIY air compressor accessories, the most common couplings are the elbow and straight fittings. Some of these use quick connect couplings (ie, a hose or tool that doesn't have a thread), but the main connection points will always use a threaded fitting.
When purchasing new fittings, use the NPT size chart and measure your air ports if you're unsure about the size, or simply refer to the manufacturer documentation which will state what kind of air hose fitting sizes you'll require.
One of the best ways to purchase the couplings you'll need, is to invest in a package of spare parts. EPAuto manufactures a useful set of couplings and quick connector parts. They're available in a 7-Piece EPAuto Industrial-Type D 1/4-inch Coupler and Plug Kit which conforms to the 1/4-inch NPT size chart measurements.
EPAuto Coupler Set
The EPAuto 7-Piece Industrial-Type D 1/4" Coupler and Plug Kit is a great choice if you own an air compressor, air blow gun, pressure washer, air nailer or any other air tools.
Another option is to purchase a coupling set manufacturered by your equipment. Since manufacturers produce coupling sets for their own equipment you won't need to worry too much about the NPT sizes. But be sure to cross-check the product with your own model.
One example is the Campbell Hausfeld 17-Piece Compressor Inflation Kit. They will work on their own compressors and tools as well as other equipment that uses the NPT 1/4-inch connections.
Campbell Hausfeld Inflation Kit
The Campbell Hausfeld Inflation Kit is a must have if you own a compressor. The components are made of brass and are designed for repetitive use.
When to Replace an Air Hose?
If you plan on upgrading your system so you can utilize different tools, or if you are looking to get even more out of the system you currently own, then it might be time to replace the air hose on your compressor. And of course, if you find that your current hose has a defect or is worn out, then it's definitely time for a replacement. Hoses naturally wear over time, and they can develop a crack or hole.
When replacing your air hose, you'll need to determine the following:
Length of the Hose - When choosing the length of the hose, consider how you'll be using your air tools and how they need to perform. The longer the air compressor hose, the more air and pressure will be lost along the way. The object is to find the right hose length while finding a balance between maneuverability and achieving minimum pressure loss.
Hose Diameter - When choosing hose diameter, you'll find that hose's are measured by their internal diameter (I.D.). In other words, the larger the I.D. of the hose, the more air it can carry. Common internal diameter sizes include: 6mm; 8mm; and 10mm hoses. One of the main advantages of these sizes is that the air compressor can run at its full capacity.
Hose Design - Finally, you'll want to be sure that the hose material is durable and flexible enough to do what you need to do. PVC has a tendency to coil, rubber is abrasion resistant, and polyurethane is much less flexible but makes good recoil hoses.
Find the Right Equipment
Now that you're familiar with the NPT size chart, why it matters, and how it relates to the accessories and fittings, you'll find it much easier to choose the right products for repairs as well as upgrading projects.
Take your time selecting a fitting with the right measurments, consult your manufacturer documentation, and always create a good seal using either Teflon tape or a liquid sealant when making permanent connections. Attention to detail will mean a more efficient tool that performs just as the manufacturer intended.