FIP vs NPT: A Simple Guide to Threads and Pipe Fittings

FIP and NPT are common threads and fittings in plumbing. At first glance, they look very similar, which is why many people assume they’re the same. They’re also used in residential and industrial plumbing, have threaded pipe connections, and often fit together. 

Despite these similarities, they’re not the same. They serve different roles, and understanding their differences is key to connecting fittings correctly.

In this guide, we will weigh up FIP vs NPT, including breaking down how they work, when to use each, and how to connect them correctly.

What is FIP thread?

FIP stands for Female Iron Pipe. It’s a pipe fitting with threads located on the inside. “Female” in plumbing means the fitting is designed to receive another pipe or fitting. Even though the name says “iron,” FIP fittings today are made from various materials like brass, PVC (plastic), stainless steel, or galvanized steel.

FIP threads are distinguishable by a few characteristics. They are tapered, meaning the threads get slightly tighter as you screw the pieces together. The inside threads are cut in a spiral shape that matches the threads on a corresponding male fitting. You usually still need thread sealant, such as a plumber’s tape, to ensure no small leaks between the threads.

FIP pipe fittings are used in residential and industrial settings. In homes, they are used for water heater hookups, faucet connections, shower arms, and toilet supply lines. In industrial environments, they are used in larger plumbing systems to move water, gas, steam, or other fluids.

What is NPT thread?

NPT stands for National Pipe Thread. It’s a standardized thread used primarily in the US for pipe fittings. It's designed to create a tight, leak-proof connection between pipes and other fittings. 

NPTs are cut in a tapered design, which means they get tighter as the male and female fittings are screwed together. To prevent leaks, small gaps between the threads are sealed with Teflon tape or pipe dope.

They are typically used for high-pressure systems, including oil pipelines, hydraulic systems, and other machinery requiring reliable fluid or gas containment in industrial settings. In homes, you’ll find NPT fittings on gas lines, water pipes, and heating systems.

How FIP and NPT threads work together

FIP and NPT threads work together through their design compatibility. FIP thread is internal, meaning it’s a cut on the inside of the fitting, whereas an NPT thread can be either male (external) or female (internal). The compatibility comes from the fact that a male NPT fitting, which has external threads, can be screwed into an FIP fitting with internal threads. As the threads engage and are tightened, the tapering creates pressure that helps to form the seal.

The threads on an FIP fitting and a male NPT fitting should match the size, material, and thread design. Hence, always check that they are correctly paired and use thread sealant like a plumber’s tape to create a strong, leak-proof connection.

Do FIP and NPT always match?

No, FIP and NPT do not always match. Sometimes, even though FIP and NPT threads appear similar, they may not align perfectly due to thread pitch, size, or design differences. Hence, it’s crucial to double-check the specifications and confirm that the fittings are compatible, as incorrect pairing can lead to leaks or weakened connections.

Key difference between FIP and NPT

FIP and NPT have key differences that affect their use and compatibility. Understanding these differences can help you choose the proper fitting for your project. Let’s dive into the details with a side-by-side comparison of their features.

How to connect FIP to NPT threads correctly

Connecting FIP to NPT is straightforward if you follow these three steps:

Step 1: Match threads

Check that the sizes of the FIP to NPT fittings match. If the fittings are incompatible in size or thread type, they may not seal correctly, leading to leaks or weak connections. Double-check the size and ensure both fittings are made to the NPT standard.

Step 2: Use sealant (Teflon tape or pipe dope)

Apply Teflon tape or pipe dope to the male NPT threads to ensure a tight seal and prevent leaks. Wrap the tape around the threads clockwise, starting from the base and ensuring the tape covers all the threads. If you're using pipe dope, apply a thin layer directly to the male threads. 

Step 3: Hand-tighten first, then use a wrench

Once the sealant is applied, hand-tighten the male NPT into the female FIP fitting. Then, use a wrench to tighten the fitting further. Ensure the connection is snug and secure, but avoid over-tightening, as this can damage the threads and compromise the seal.

Step 4: Check for leaks

After the connection is tightened, turn on the system and observe the connection for any signs of leakage. If you detect any leaks, tighten the fitting a bit more. If that doesn’t work, loosen it, reapply sealant to the threads, and retighten. 

Always inspect your work carefully to ensure the fitting is sealed correctly before fully operating the system.

Common mistakes to avoid

Avoid these mistakes that cause leaks, damaged fittings, or system failure.

• Cross-threading: When the male and female threads are not aligned properly during installation, they catch on each other, causing damage to the threads themselves. Cross-threading can result in a weak or improper connection, leaks, or stuck and unusable fittings. To avoid it, ensure the threads are lined up correctly, then hand-tighten the fitting. If you feel resistance or see uneven alignment, stop and realign the threads before continuing.
• Mismatched fittings or thread types: Not all FIP and NPT fittings are created equal, and the thread pitch, size, or taper differ between manufacturers or standards. Always double-check the size and thread type of the fittings to ensure compatibility.
• Skipping sealant: Skipping the use of Teflon tape or pipe dope can result in a poor seal, causing leaks over time. Even though the tapered threads create some friction and resistance, they don’t provide an airtight or watertight seal by themselves. Hence, always apply the proper sealant to the male threads before tightening the connection.

When to use FIP vs NPT fittings

FIP fittings are handy when the plumbing system's layout or design requires a stationary, female-threaded fitting mounted to a wall, appliance, or fixture. They’re common in residential plumbing projects such as hooking up faucets, toilet supply lines, shower arms, and water heaters. They’re also frequently found in garden systems and irrigation setups. 

Since FIP threads are internal, they allow for a clean and secure connection in tight spaces or behind walls where space is limited.

On the other hand, NPT fittings are best suited for high-pressure or safety-sensitive environments. They have tapered threads that tighten as they are screwed together, helping to create a mechanical seal that prevents leaks. 

As such, they are widely used in gas lines, including those for stoves, dryers, and water heaters. They are also preferred in HVAC systems, especially for refrigerant lines, compressors, and pressurized components. Outdoor plumbing systems like sprinklers or irrigation setups also use NPT fittings. 

Choosing the right pipe thread matters

FIP and NPT threads are widely used in plumbing and industrial applications, but they have significant differences that impact their function and compatibility. On one hand, FIP (Female Iron Pipe) threads are always internal and designed to connect with male-threaded fittings. 

Conversely, NPT threads refer to a thread standard with a tapered design. They come in male and female versions and can connect with an FIP.

Although FIP pipe thread is often NPT-compatible, compatibility does not always guarantee a leak-proof connection. Variations in thread pitch or fitting quality can still cause issues, so it's essential to check the specifications before assembling any parts. Additionally, always apply Teflon tape to the threads before tightening.