MIP, FIP and NPT: What the Fitting Labels Mean

MIP, FIP, NPT, sweat, compression, PEX barb. The labels on a fitting bin tell you exactly what you are holding, once you know that most of them are answering two questions: is the thread on the outside or the inside, and is it tapered.

As an Amazon Associate I earn from qualifying purchases.

The labels, decoded

• MIP / MPT — male iron pipe. The thread is on the outside, and it screws into something.
• FIP / FPT — female iron pipe. The thread is on the inside, and something screws into it.
• NPT — National Pipe Taper. This is the thread standard, and it is what MIP and FIP are cut to. The threads get slightly narrower along their length so they wedge and seal as they tighten.
• Sweat / C — no thread at all. A smooth copper socket for a soldered or pressed joint.
• Comp — compression. A nut and ferrule squeeze onto smooth tubing.
• Barb / F1807 / F1960 — PEX connections, held by a crimp ring, a clamp, or an expanded sleeve.

So a fitting sold as 1/2 MIP x 1/2 sweat has an external tapered thread on one end and a copper socket on the other. Once you read them that way, the bin makes sense.

One footnote worth knowing: MPT and FPT are shop shorthand, not standard designations. The standard’s own terms are external and internal NPT. Nobody will correct you at the counter, but it explains why the same fitting is labeled three different ways across three catalogs, and why the spec sheet says something else again.

The trap: NPT versus NPS

Not every pipe thread is tapered. NPS (straight) threads exist and they look almost identical in the hand. They do not seal on the threads — they rely on a washer, an O-ring, or a gasket face.

🔴 Forcing a tapered fitting into a straight one, or the reverse, is how you crack a fitting. It starts easily, tightens up sooner than expected, and then either splits the female part or leaves you with a joint that weeps forever. If a thread stops abruptly and will not pull up, stop and look at what you have.

The other everyday example is a hose thread — GHT is a completely different, coarser thread and shares nothing with NPT despite the similar diameter.

How to tell them apart in your hands: lay a straightedge along the crests of a male thread. On NPT you will see daylight opening up under it toward the end of the thread, because the taper is real and visible — roughly 1°47′ off the axis. On a straight thread the crests sit flat along the whole length. On a female thread, run a finger in: a taper feels tighter at the back. And the honest test is the dry one — hand-thread the two parts together with nothing on them. A matched tapered pair goes several turns and gets progressively firmer. A mismatch either spins freely and then stops dead, or binds in the first turn or two.

Which sealant, and where it goes

Sealant goes on male threads only, and only on tapered ones. There is nothing for it to do on a straight thread that seals against a washer, and putting it there just makes a mess.

• Oatey Great Blue pipe joint compound — a brush-in-cap dope that fills the thread and stays workable, which is what you want on larger or rougher threads.
• Oatey 31199 white PTFE thread seal tape — the standard tape for water lines.

⚠️ Wrap the tape in the direction the fitting turns (clockwise as you look at the end of the male thread), or tightening the joint unwinds it. Two to three wraps, starting a thread back from the end so nothing sheds into the line. Many plumbers use both tape and dope on tapered threads; that is fine. What is not fine is either one on a washer-sealed joint.

“Male threads only” is not a preference, incidentally — it is the code. Both the water-supply and the drainage chapters of the plumbing code carry the same sentence for threaded joints: threads shall conform to ASME B1.20.1, and pipe-joint compound or tape shall be applied on the male threads only.

Full comparison, including where plumber’s putty and silicone belong instead: Teflon tape vs pipe dope vs putty, plus pipe dope and PTFE tape for plumbing.

I send an occasional note to the trade when I write up a fitting question like this. Unsubscribe any time.

Nominal size is not a measurement

A 1/2-inch NPT thread does not measure half an inch across anything. Nominal pipe size refers to an approximate inside diameter of pipe from an era of much thicker walls, and it has been a naming convention rather than a dimension for a long time.

Measure the outside diameter of the male thread and work back from a chart. The common surprise: 1/2-inch NPT measures roughly 0.84 inch across the outside. Guessing from a tape measure is how you end up with the wrong bag of fittings.

Here is enough of the standard’s table to sort out the sizes that actually turn up under a sink:

Two things fall out of that table and both cost people time:

• 1/2 and 3/4 share the same 14 threads per inch, and 1/4 and 3/8 share 18. So a thread pitch gauge alone cannot tell you the size — you need the pitch and the outside diameter together. That is not a quirk of this table; it is why the reference works that way.
• 3/8 and 1/2 are the pair that gets mixed up on supply stops, and 0.675 against 0.840 is a difference you can see with calipers and cannot reliably judge by eye.

While you are sizing things: the code’s minimum supply pipe sizes are 3/8 inch for a lavatory and for a flush-tank toilet, and 1/2 inch for a kitchen sink, a dishwasher, a shower and a hose bibb. Useful when somebody has fed a fixture with whatever was on the truck.

The same distinction bites on copper: type L and type M share an outside diameter and differ only in wall thickness — type L vs type M copper.

Getting it tight without cracking anything

Hand tight, then one to two turns with a wrench for smaller sizes. A tapered thread seals by wedging, so it will feel like it is still going when it is already sealed — and that is where people keep going and split a female fitting.

⚠️ Plastic female fittings are the ones that break. A brass male thread wedging into a plastic female will crack it, often days later rather than on the spot. Go gently, and never use a cheater bar on plastic.

Where you have a choice of orientation, put the plastic on the male side and the metal on the female side. A plastic male thread wedging into a brass female has nothing to split; the same pair the other way round is the classic delayed failure. It is not always possible, but when the fittings exist both ways it is free insurance.

Where you are joining dissimilar metals, thread compatibility is not the only issue — see dielectric union vs brass nipple.

Reading a threaded joint that weeps

A threaded joint tells you why it failed if you look at where the water is:

• Weeping from the last thread at the face, in a fine bead. Under-tightened, or not enough sealant in the thread root. Tape bridges the crests; the root is where the leak path is.
• A hairline crack running along the female fitting, usually visible only when wet. Over-tightened. This one commonly appears days later, which is why “it was fine when I left” is not reassuring on a plastic female.
• Shredded white flakes turning up at an aerator or a cartridge. Tape wrapped over the leading thread and shaved off into the line. Start the wrap a thread back from the end.
• Galled and seized on assembly, especially stainless on stainless. Stop; you will not un-gall it. Cut it out.
• Went in far too easily and will not pull up. A straight thread in a tapered female, or a badly worn die-cut thread. Check what you have before you reach for a bigger wrench.

FAQ

What is the difference between MIP and FIP?

MIP is a male iron pipe thread, cut on the outside of the fitting. FIP is the female version, cut on the inside. Both are normally NPT tapered threads, so a MIP end screws into an FIP end.

Is NPT the same as MIP?

Not quite. NPT is the thread standard — a tapered pipe thread. MIP describes which side of the fitting that thread is on. A MIP fitting is normally cut to NPT.

Does thread tape go on male or female threads?

Male threads only, and only on tapered threads that seal on the threads themselves. A joint that seals against a washer or O-ring needs no tape at all. The plumbing code says the same thing in both its water and drainage chapters.

How do I tell a tapered thread from a straight one?

Lay a straightedge along the crests of a male thread — NPT opens up a visible gap toward the end, a straight thread stays flat. Then dry-thread the pair by hand: a matched tapered pair goes several turns and gets progressively firmer, while a mismatch either spins free and stops dead or binds in the first turn.

Can a thread gauge tell me the pipe size?

Not on its own. 1/2 and 3/4 NPT both have 14 threads per inch, and 1/4 and 3/8 both have 18, so pitch alone is ambiguous. Measure the outside diameter as well — 3/8 is 0.675 inch and 1/2 is 0.840 inch, which calipers separate instantly and your eye does not.

Why did my plastic fitting crack a few days after I tightened it?

A brass male thread wedging into a plastic female keeps pushing after you stop. The stress relaxes into a crack over hours or days. Tighten plastic gently, never use a cheater bar on it, and where you have the choice put the plastic on the male side and the metal on the female side.

Sources

• Georgia State Minimum Standard Plumbing Code 2018 (IPC 2018 with Georgia amendments), Chapter 6 — §605.12.4 Threaded Joints: “Threads shall conform to ASME B1.20.1. Pipe-joint compound or tape shall be applied on the male threads only.” Table 604.5 minimum fixture supply sizes (lavatory 3/8, water closet flush tank 3/8, kitchen sink 1/2, dishwasher 1/2, shower 1/2, hose bibb 1/2). The same threaded-joint sentence appears in Chapter 7 at §705.8.1 and §705.10.3.
• Engineering ToolBox, ANSI/ASME B1.20.1 NPT thread table — threads per inch and nominal pipe outside diameters (1/4″ 18 TPI / 0.405″, 3/8″ 18 / 0.675″, 1/2″ 14 / 0.840″, 3/4″ 14 / 1.050″, 1″ 11-1/2 / 1.315″), the 1°47’24” taper angle, and the note that both TPI and OD are required for positive identification because more than one size shares the same TPI.

Related guides

• Teflon tape vs pipe dope vs putty
• PTFE tape for plumbing
• Best pipe dope
• Type L vs type M copper

For the plumbers reading this

Most of this site is written for homeowners — this page clearly isn’t. If you’re in the trade and want the occasional straight-shooting note when I publish something like this (fittings comparisons, tool tests, code changes), drop your email. No spam, no course funnel, written by a licensed GA journeyman.