What Faucet Supply Line Thread Type Do You Actually Need — and How Do You Avoid a Cross-Threaded Leak?

Getting the faucet supply line thread type right is the difference between a five-minute connection and a slow drip you don’t notice until the cabinet floor swells. The confusing part is that a single flexible supply line usually has two different fittings — one end mates to your angle stop (the shutoff valve under the sink), and the other end mates to the faucet’s threaded inlet shank. Those two ends almost never share the same thread type, which is exactly why so many people buy the wrong hose, force it on, and end up with a cross-threaded, weeping joint.

This guide breaks down the common thread types, how to identify what you actually have, and how to match a supply line without guessing. It’s written for real under-sink situations — a new bathroom faucet, a kitchen faucet swap, a hard-to-reach angle stop — not textbook plumbing theory.

What are the common faucet supply line thread types, and which one is on my faucet?

There are really only a handful you’ll meet under a home sink: 3/8″ compression, 1/2″ and 3/4″ IPS (iron pipe straight/tapered), 15/16″-27 faucet-inlet thread, and captive ballnose faucet connectors. The faucet side is usually either a 1/2″-14 IPS female nut or a fixed connector that’s built onto the faucet’s copper/PEX inlet; the valve side is usually a 3/8″ compression nut. Look at both ends before you assume.

Here’s the practical way to think about it. Thread « type » is really three things bundled together: the size (3/8″, 1/2″, 3/4″), whether the thread is straight or tapered, and whether the seal is made by a compression ferrule, a rubber washer/cone, or thread sealant. A 3/8″ compression connection seals with a brass ferrule and does not need tape. A 1/2″ IPS threaded connection seals on the tapered threads and usually does want PTFE tape or pipe dope. Mixing those up — putting tape on a compression fitting, or skipping it on a tapered pipe thread — is the second most common cause of leaks after cross-threading.

• 3/8″ compression — the dominant valve-side connection in North American homes. The nut spins freely and squeezes a small brass ring (ferrule) onto the tube. No tape needed.
• 1/2″ IPS (aka 1/2″-14 NPS/NPT) — a threaded pipe connection, common on the faucet inlet shank and on some wall stub-outs. Needs sealant on tapered versions.
• 3/4″ garden-hose thread (GHT) — you’ll see this on washing-machine and utility supply lines, not standard sink faucets.
• Faucet-shank / captive connectors — many modern faucets ship with the supply line already attached to the faucet as a fixed hose; you only connect the valve end.

How do I tell the difference between compression and IPS threads under my sink?

The fastest tell: compression threads are on the outside of a smooth, unthreaded valve body and the nut is separate; IPS/pipe threads are cut directly into the fitting and taper slightly. If the nut slides off to reveal a bare tube stub and a loose brass ring, it’s compression. If the threads run right up the male fitting and a female nut screws onto them with sealant, it’s IPS/NPT.

A few more field checks that take ten seconds:

1. Count and feel the threads. Compression nut threads are finer and the nut turns very freely by hand for several turns. Tapered pipe threads get tighter as you go and you’ll usually see old white tape residue.
2. Look for a ferrule. Pull the old line off the valve. A small olive-shaped brass ring left on the copper stub = compression, 3/8″ almost always.
3. Measure across the threads. A 3/8″ compression valve outlet measures about 5/8″ (roughly 16 mm) across the male threads — the « 3/8 » refers to the tube it fits, not the thread diameter. This trips up a lot of first-timers.
4. Check the faucet inlet. Bathroom faucets frequently have 1/2″-14 male shanks or a 3/8″ female compression connector molded into a plastic/brass adapter.

If you’re already under there replacing a handle or servicing the faucet body, it’s worth confirming the shank material and thread while you have access — the same logic applies whether you’re doing a full swap or a faucet handle replacement on a bathroom sink. Knowing exactly what your inlet threads are now saves a second trip to the store.

Which supply line size do I buy for a standard bathroom or kitchen faucet under $30?

For nearly every standard residential job, buy a braided stainless flexible line with a 3/8″ compression nut on the valve end and either a 1/2″ IPS or 3/8″ compression nut on the faucet end — read the faucet’s inlet spec first. Most bathroom faucets use 3/8″ comp × 1/2″ IPS; many kitchen faucets ship with their own captive lines, so you may only need the valve-end fitting. A quality line runs $6–$15; don’t cheap out on the ferrule and nut metal.

Length matters as much as thread type. Measure from the valve outlet to the faucet inlet and add 3–4 inches of slack so the line makes a gentle loop instead of a tight, stressed bend. A too-short line strains the threads and is the hidden reason some « correctly sized » connections still weep after a year. Braided stainless is the default for good reason — it resists burst and kinking far better than the old vinyl or plain-PVC lines, and most carry a manufacturer burst-pressure rating well above household supply pressure (typically 60–80 psi).

What’s the difference between the thread types at a glance?

Why does my new supply line still leak even though the size seems right?

If the size matches but it still drips, the usual culprits are cross-threaded start, a missing or doubled washer, over-tightening, or the wrong seal method for that thread type — not the thread size itself. A correct thread type installed carelessly leaks just as fast as the wrong one.

Walk through these in order before you assume you bought the wrong part:

• Start every nut by hand. Spin it on with your fingers for at least two full turns before touching a wrench. If it won’t spin freely, stop — you’re cross-threading and about to ruin the threads.
• One washer, seated flat. IPS and cone connections seal on a rubber washer. Two washers, or a pinched one, leaks. Compression connections use no washer at all — just the ferrule.
• Snug, not gorilla-tight. Compression fittings need about a quarter to half turn past hand-tight. Crushing the ferrule deforms it and causes a permanent weep.
• Right sealant for tapered threads only. Use 2–3 wraps of PTFE tape (clockwise) on NPT tapered threads. Never tape a compression fitting or a washer-sealed connection — the tape prevents the metal-to-metal or washer seal from mating.

Brass quality matters here too. A thin, poorly cast nut or an out-of-round ferrule won’t seal no matter how careful you are. This is the same reason the fitting the faucet threads into — the brass faucet socket — is worth inspecting when you replace a tap; worn or corroded threads on the faucet side will leak against a perfectly good new supply line. If you’ve chased a drip on a single-handle unit and the threads look chewed, our Moen centerset faucet repair guide walks through when the inlet or cartridge, not the supply line, is the real problem.

Can I connect a 3/8″ compression line to a 1/2″ faucet inlet without an adapter?

No — not directly. A 3/8″ compression nut will not thread onto a 1/2″ IPS shank; you need either a supply line that already has both correct ends, or a brass adapter that converts one thread type to the other. The cleanest fix is buying a line spec’d as « 3/8″ comp × 1/2″ IPS, » which is exactly what most bathroom faucets want, so you never touch an adapter at all.

Adapters are fine when you genuinely have a mismatch — for example, an old 1/2″ wall stub-out feeding a modern faucet — but every extra fitting is another potential leak point. Fewer connections is always better. If you find yourself stacking two or three adapters to make something fit, that’s a signal to buy the correct-length, correct-thread line instead.

Do supply line thread types differ between the US and Europe/metric faucets?

Yes. North American residential plumbing standardizes around inch-based 3/8″ compression and 1/2″ IPS, while European and many imported faucets use metric threads — commonly G1/2″ and G3/8″ (BSP parallel). G1/2″ and NPT 1/2″ look almost identical but the thread pitch is different, so they’ll start and then bind or leak. If you’re installing an imported or metric-spec faucet, confirm whether it’s BSP or NPT before buying the line, and use the flexible connector supplied with the faucet whenever possible — it’s matched to the inlet by the manufacturer.

How do I know the supply line meets safety standards and won’t burst?

Look for a line that’s certified to recognized plumbing standards and carries a real burst-pressure rating and warranty — braided stainless lines from reputable makers are typically tested well above normal 60–80 psi household pressure. Certification marks (such as those tied to NSF/ANSI and low-lead compliance) tell you the brass and the hose have actually been tested, not just labeled.

This is also where the wider fitting standards on your faucet come into play. Checking the model’s listing and its bathroom faucet UPC and certification before you buy tells you the inlet thread spec and confirms the fixture is built to a code-recognized standard — which in turn tells you what supply line to pair with it. A certified faucet plus a certified, correctly-threaded line is what keeps a connection dry for a decade instead of a season.

• Warranty: better supply lines carry multi-year or limited-lifetime warranties against burst — a signal the manufacturer stands behind the ferrule and hose.
• Low-lead brass: nuts and ferrules on the potable side should be low-lead compliant.
• Burst rating: look for a printed psi rating on the packaging; anything rated several times household pressure gives you real margin.

Quick buyer’s checklist before you order

1. Identify the valve outlet thread — almost always 3/8″ compression in a US home.
2. Identify the faucet inlet thread — 1/2″ IPS, 3/8″ compression, or a captive connector.
3. Measure the distance and add 3–4 inches of slack.
4. Buy braided stainless with a burst rating and warranty.
5. Use PTFE tape on tapered pipe threads only; never on compression or washer seals.
6. Start every nut by hand and stop if it binds.

FAQ

What thread type is a standard faucet supply line?

Most standard residential faucet supply lines have a 3/8″ compression fitting on the shutoff-valve end and either a 1/2″ IPS threaded nut or a 3/8″ compression fitting on the faucet end. The two ends are usually different thread types, so always check both before buying.

Is faucet supply line 3/8 or 1/2 inch?

Both, depending on the end. The valve side is typically 3/8″ compression, while the faucet inlet is often 1/2″ IPS. The « 3/8″ refers to the tube size, not the thread diameter — a 3/8″ compression thread actually measures about 5/8 » across.

Do I need Teflon tape on a faucet supply line?

Only on tapered pipe (NPT) threads. Compression fittings seal with a brass ferrule and IPS/cone connections seal with a rubber washer — adding tape to either can actually prevent a proper seal. Use 2–3 clockwise wraps on tapered threads only.

Are all faucet supply lines the same thread?

No. Common types include 3/8″ compression, 1/2″ IPS, 3/4″ garden-hose thread, and metric BSP (G1/2″, G3/8″) on imported faucets. US and European faucets in particular use different thread standards that look similar but won’t seal together, so match the spec to your specific faucet and valve.

How do I stop my supply line connection from leaking?

Start the nut by hand for at least two full turns to avoid cross-threading, use exactly one flat-seated washer where required, tighten compression fittings only a quarter to half turn past hand-tight, and use PTFE tape solely on tapered threads. If it still leaks, inspect the faucet-side threads and ferrule for damage.

Can I reuse an old compression ferrule?

Usually yes, if the ferrule is intact and not deformed — reconnecting to the same tube often reseals fine. But if it’s cracked, out-of-round, or was over-tightened, replace the ferrule and nut. A damaged ferrule is a common reason a « correct » connection weeps.

Author note: This guide was written by the arcorarobinet fixtures team, drawing on hands-on faucet installation and under-sink troubleshooting across kitchen and bathroom setups. arcorarobinet designs and supplies faucets and bathroom fixtures built to code-recognized plumbing standards, and every connection recommendation here reflects the same fittings and tolerances we test our own products against. When in doubt about your specific valve or inlet, match the printed spec on your faucet or ask a licensed plumber.