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Quick answer from a licensed plumber: The confusion has a simple cause: a supply line has two different ends, and people quote the size of the wrong one. At the wall, the modern standard stop outlet is 3/8″ compression. At the toilet, the fill-valve nut is 7/8″ ballcock — a size that exists almost nowhere else in the house. A faucet connector is 3/8″ × 1/2″ instead, which is why the two are not interchangeable no matter how similar they look on the shelf.
At a Glance — who connects to what
| Connector | Wall end | Fixture end |
|---|---|---|
| Toilet supply line | 3/8″ compression | 7/8″ ballcock |
| Faucet supply line | 3/8″ compression | 1/2″ |
| Older/odd installs | 1/2″ stops exist — measure yours | — |
Those sizes are straight off the manufacturer’s own listings: Fluidmaster’s toilet connector is sold as “3/8-Inch Female Compression Thread X 7/8-Inch Female Ballcock Thread,” and their faucet connector as “3/8″ X 1/2″” — same wall end, different fixture end. That one difference is the whole store-aisle problem.
The two ends, untangled
The wall end: usually 3/8″, but measure, don’t assume
The angle stop (shut-off valve) under a modern toilet or sink almost always has a 3/8″ compression outlet — that has been the residential standard for decades. But older homes and some remodels carry 1/2″ outlets, either compression or threaded iron-pipe, and a 3/8″ nut will spin uselessly on one.
How to tell without guessing: take the old line with you, or measure the stop’s outlet thread across its outside — a 3/8″ compression outlet measures about 1/2″ across its threads (compression sizes name the tube, not the thread, which is half of why this topic confuses everyone). When in doubt, the old line’s wall nut is the truth. And if the stop itself is the seized, crusty original, replace it while you’re there — that decision is covered in the best under-sink shut-off valves.
The toilet end: 7/8″ ballcock, always
Every modern fill valve — Korky, Fluidmaster, the lot — takes the same 7/8″ ballcock nut, and that part really does hold. BrassCraft list thirteen toilet connectors with nine different wall ends between them — compression, flare, barb, push-connect, sweat — and every one of them finishes 7/8″ ballcock at the toilet. Sioux Chief’s closet supply tubes are 7/8″ too. So the toilet end is the one size you never have to worry about matching.
The nut’s material is a different question, and the page used to get this wrong. Most ballcock nuts are a big plastic coupling you tighten by hand, but a metal one is a stocked item rather than an oddity: BrassCraft catalog their Speedi Plumb PLUS connector as “3/8 in. Comp x 7/8 in. Ballcock With Metal Nut.” Look at the nut in your hand before you decide how hard to tighten it. “Ballcock” is simply the old name for the fill valve, fossilized into the fitting name.
Why the faucet line won’t work
A faucet connector’s fixture end is 1/2″, made to thread onto a faucet shank. Offer it to a toilet fill valve and it rattles around the 7/8″ threads; offer a toilet line to a faucet and it will not start. The packaging colors and braiding look identical — read the printed sizes, not the picture. (Faucet lines have their own buying decisions — see the best braided faucet supply lines.)
If you found this useful, I send an occasional note when a new guide goes up. No spam, no course funnel — written by a licensed Georgia plumber.
Choosing the line itself
- Braided stainless over vinyl, every time. The braided jacket contains the polymer core if it fails; bare vinyl lines age brittle and burst. The price difference is a couple of dollars against a flooded floor. Our picks are in the best toilet supply lines.
- Buy longer than the gap, never exactly the gap. A supply line should make a relaxed loop, not a straight taut run — tension works the joints at both ends. Common lengths run 6″ to 20″; when between sizes, size up.
- Never reuse a line that’s been in service, and never reuse one whose plastic nut has been wrenched. The gaskets take a set; the nuts crack invisibly.
- No tape, no dope on either end. Both connections seal on gaskets or the compression ferrule, not on threads. Sealant on a gasket joint is how people talk themselves into over-tightening.
- Hand-tight plus a quarter turn on a plastic ballcock nut — a wrench on that nut is how tanks end up dripping through cracked couplings. If yours is one of the metal-nut connectors instead, it will not split the way plastic does, but it still seals on a gasket and not on the threads, so snug is still the target and hard is still wrong. The metal wall nut takes a gentle wrench snug, no more.
The five-minute swap, in order
- Water off at the stop; flush to empty the tank; sponge or towel for the dribble.
- Old line off — fixture end first, then the wall nut. Expect a cup of water.
- Check the stop actually shut fully. If it weeps with the handle hard over, the stop is the real project — and if it’s the old multi-turn style, quarter-turn is the replacement to fit.
- New line: wall nut snug with a gentle wrench, ballcock nut hand-tight plus a quarter turn.
- Water on slowly, then the paper-towel test: wipe both nuts dry, wait ten minutes, wipe again. Dry paper is the pass.
FAQ
What size supply line does a toilet use?
The standard is 3/8″ compression at the wall stop and 7/8″ ballcock at the fill valve — Fluidmaster’s own listing reads “3/8-Inch Female Compression Thread X 7/8-Inch Female Ballcock Thread.” Older homes sometimes have 1/2″ wall stops, so match the wall end to your valve, and the toilet end takes care of itself.
The same outside-diameter logic governs the ferrule inside that wall-end nut, and it trips people up the same way — see compression ferrule sizing.
Are toilet and faucet supply lines interchangeable?
No. Both commonly have a 3/8″ compression wall end, but the fixture ends differ — 7/8″ ballcock for a toilet, 1/2″ for a faucet — so each fits only its own fixture. The lines look identical on the shelf; the printed sizes are the only reliable tell.
What is a ballcock connection?
The 7/8″ threaded inlet on the bottom of a toilet fill valve, closed by a large plastic nut you tighten by hand. “Ballcock” is the traditional name for the float-operated fill valve, and the fitting size outlived the name. It is universal across modern fill-valve brands. If the valve itself is what you are replacing, see our toilet fill valve picks.
Should I use Teflon tape on a toilet supply line?
No — neither end seals on threads. The wall end seals on its compression ferrule or washer and the ballcock end on its gasket, so tape adds nothing and encourages over-tightening. Hand-tight plus a quarter turn on the plastic nut; gently snug with a wrench on the metal wall nut.
How tight should the fill valve nut be?
Hand-tight plus about a quarter turn. It is a plastic coupling sealing on a rubber gasket — wrench-tightening it cracks the nut or distorts the gasket, and the crack usually shows up as a slow drip days later rather than immediately.
Sources
- Fluidmaster B1T16 toilet connector listing (read on the opened product page): “Toilet Connector, Braided Stainless Steel — 3/8-Inch Female Compression Thread X 7/8-Inch Female Ballcock Thread” · spec bullet “3/8-Inch compression by 7/8-Inch ballcock.”
- Fluidmaster B1F12 faucet connector listing (read on the opened retailer search result): “B1F12 3/8″ X 1/2″ X 12″ Braided Stainless Steel Faucet Connectors” — the faucet end is 1/2″, not 7/8″ ballcock.
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