Tank Fit

Behind the toilet

Supply line and shutoff: the twenty minutes that prevents a flood

A toilet supply line is the highest-consequence, lowest-cost part in the room. If yours is a rigid chrome tube, a plastic ribbed line, or anything you cannot date, replace it. If the shutoff will not close by hand, replace that too.

What you need to know before buying

  • Toilet end: 7/8 in ballcock thread, with a large plastic coupling nut you hand-tighten. Standard on essentially all modern fill valves.
  • Stop end: usually 3/8 in compression. Look at the fitting on your angle stop; 3/8 in is the residential norm, 1/2 in exists.
  • Length: measure from the stop to the tank shank and add a few inches for a gentle curve. A line pulled taut is a line under stress. 12, 16 and 20 in are the common stock lengths.
  • Material: braided stainless with a reinforced core. Skip the rigid chrome tube (needs bending, unforgiving) and the bare plastic ribbed line.
waterlinefill valveflush valve / seatflappersupply line + shutoff
Measure stop-to-shank and buy a line a few inches longer. The toilet end is 7/8 in ballcock, hand-tight only.
Braided stainless supply, 3/8 in comp × 7/8 in ballcockToilet connector, 12–20 intypically $8–$14
Fits
Any standard toilet fill valve shank with a standard 3/8 in compression angle stop. Buy longer than you measured; a lazy curve outlasts a taut line.
Does not fit
Do not reuse an old line — the rubber cone washer inside has taken a set. Do not use thread tape or dope on the ballcock nut; it seals with a gasket and tape makes it worse.
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Quarter-turn angle stop, 1/2 in nominal × 3/8 in compBall-type shutoff valvetypically $10–$18
Fits
Replacing an old multi-turn stop that no longer closes. Quarter-turn ball stops close reliably years later, which is the whole point of a shutoff. Compression versions install with two wrenches and no torch.
Does not fit
Guessing the inlet. Check whether your stub-out is copper, CPVC, or PEX, and whether the existing stop is compression, sweat, or threaded — they are not interchangeable. Sweating a new stop onto copper is a torch job; if that is what you have and you don't solder, this is a plumber's twenty minutes, not yours.
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Replacing just the supply line — 15 minutes

  1. Close the stop. Flush and hold to drain the tank; sponge the rest.
  2. Put a towel and a small pan under the connection. There is always more water.
  3. Unscrew the plastic nut at the tank by hand or with light wrench pressure, then the compression nut at the stop.
  4. Hand-tighten the new line at the tank — firm, then a light quarter turn at most. Over-tightening cracks the plastic shank, which is the single most common self-inflicted injury here.
  5. Snug the compression end with a wrench, holding the stop body with a second wrench so you don't twist the stub-out.
  6. Open the stop slowly and watch both ends for a full minute, then check with dry paper towel after an hour.

Replacing a compression angle stop — 30 minutes

  1. Shut off the water to the house or the branch. Not the stop itself — you are removing it.
  2. Open a lower fixture to drain pressure and standing water.
  3. Back the old compression nut off and pull the stop. The old brass compression ring usually stays on the pipe; a puller or careful cutting removes it.
  4. Slide the new nut and ring on, seat the new stop square to the pipe, and tighten while holding the pipe steady. Square matters more than force.
  5. Restore water slowly, then check the joint for ten minutes and again the next day.

Know when to stop

If the stub-out pipe turns when you apply force, if the stop is soldered to copper, if the pipe is galvanized steel, or if there is no accessible shutoff upstream — stop and call a licensed plumber. A supply line is a homeowner job. A stub-out that spins inside a finished wall is a hole in the drywall and a soldering iron.

Life expectancy and a habit worth having

  • Braided stainless lines: replace on sight if unknown age, and as a matter of course at 8–10 years or any time the toilet is off the floor.
  • Exercise every shutoff in the house once a year — open and close it fully. Stops fail from never moving.
  • Any line that is kinked, corroded at the crimp, bulging, or damp at the ferrule gets replaced immediately, not scheduled.

Insurance claim data on water damage from failed supply connections is why washing machine hoses get so much attention; toilet supplies are the same failure mode in a room people leave for a week. It is a $10 part.

Next, in the tank