What Adapter For

What size is a toilet supply line?

You need a 3/8" compression x 7/8" ballcock toilet supply line. The 3/8" end goes on the small valve on the wall; the 7/8" end threads onto the fill valve under the tank. Common lengths are 9, 12, 16 and 20".

What fits

3/8" compression x 7/8" ballcock toilet supply line

On the package: 3/8 in. Comp x 7/8 in. Ballcock

sold as a “toilet connector”

Available on Amazon

3/8" comp x 7/8" ballcock toilet supply line

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  • Length: measure from the valve outlet to the fill-valve stub under the tank. Toilet lines come in 9, 12, 16 and 20" (BrassCraft also 6").
  • Turn the water off at the shutoff valve first. When the line is on, turn the water back on slowly and check both ends for leaks.
  • Install: the plastic nut on the tank end is hand-tight only. On a Fluidmaster line, turn the metal nut on the valve 1/2 turn past hand-tight; don’t overtighten.
  • Replace a connector that is worn or over 5 years old.

The tank end isn’t a faucet size. The toilet’s fill valve (sold as a “ballcock”, its older name) has its own 7/8" thread, so a faucet supply line won’t fit the tank.

More: Supply line sizes

Keep going

Toilet flapper sizeFaucet supply line sizeShutoff valve for copper pipeGarden hose to 1/2" pipeGarden hose to 3/4" pipeConnect two garden hosesGarden hose to drip irrigation

For a different part on either end, open the adapter finder with this pair picked.

Where we get our information from

The sizes on this page come from published standards and the makers’ own documents. Checked against published data from Fluidmaster and BrassCraft.

See all 3 sources
  • BrassCraft, toilet connector 3/8" comp x 7/8" ballcock and submittal 400.339
  • Fluidmaster, faucet and toilet connector catalogs and Click Seal pages (FM91429, FM91233)
  • Fluidmaster, fill valve instructions (400CARSP5): replace connectors over 5 years old

General guidance from manufacturer and standards data. Parts vary by brand, so check the size printed on the package against what you measured. For gas lines or anything inside a wall, use a licensed plumber.