A metal tube connects the wall water supply to the under surface of the toilet. The leak is visible at the plastic nut with wings that attaches this tube to the toilet proper. If the intake water is off, even if the tank is full, it will not leak. If the the water is turned on it will begin to drip immediately, long before there is much water in the tank. When I slightly turn the plastic nut to either the right or left, the dripping increases. What has gone bad here, and how much do I need to replace?
Easiest thing to do is get a new fill valve about 9 bucks and a new supply line about 6 bucks.
Get the korky Quietfill it’s easy to install.
You most likely have a 7/8x3/8 toilet supply line. Bring it to the store to match the end that goes to the shutoff. The 7/8 side on the fill valve is universal.
Depending on where you live you should be able to swap the hard tube for a flex supply. It’s only an issue if you are selling the house or getting an inspection and your local code require hard tube.
Sounds like a bad (or missing) cone washer.
A 50¢ fix.
Possibly just the hose that goes from the shut-off valve against the wall to the bottom of the toilet tank. Frequently this is covered in braided metal. They are cheap, buy a new one anyway.
I JUST went through this. Mine was leaking at the same place. I bought a new jumper line. Still leaked. I removed the fill valve assembly which was a plastic replacement I installed perhaps 15 years ago. The threaded portion of the pipe was plastic, and had several cracks in it. The leak path was through the cracks, and up the threaded nut, and out.
Replace the fill valve assembly.
Went out and bought an new supply line and fill assembly. After deconstructing I think the leak was caused by “stuff” that accumulated at the base of the fill valve. Replaced it all. Looked fine. Went with the Korky, as recommended above, and it really was easy to instal. Next morning, water again. This time it was from the damn compression nut. If I even look at the toilet funny those compression nuts start to leak. Quarter turn solved it and now everything is (seems) fine.