How to fix a running toilet in 20 minutes
Three parts cause almost all of it. Start with the one that costs the least.
20 minEasy$12
Start with the dye test
Take the lid off the cistern and add a few drops of food colouring to the water. Wait ten minutes without flushing. If coloured water appears in the bowl, the flapper is leaking.
Check the chain
The chain should have a little slack when the flapper is closed. If it is too tight, the seal never closes completely. If it is too loose, it can catch under the flapper.
Replace the flapper
Turn off the water, flush once, unhook the old flapper and fit the matching replacement. Do not overtighten anything. Turn the water back on and repeat the dye test.
If the water still runs, the fill valve is probably the next part to inspect.