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Toilet flapper — check a twenty-minute pencil-mark drop

Running toilet

After a quiet fill, pencil-mark the toilet tank waterline and check a twenty-minute drop to spot a slow toilet flapper leak — one beat.

twenty minute tank pencil mark drop — related household tip photo
Photo: Authors of the study: Madison P. Goforth, BS Stephanie A. Boone, PhD Justin Clar / Wikimedia Commons (CC BY 4.0)

You do not always need an overnight wait to catch a slow toilet flapper leak. After a quiet fill, pencil-mark the tank waterline and check a twenty-minute drop — a clear fall means the flapper is seeping.

Twenty-minute pencil-mark drop in the toilet tank

  1. Flush once, let the tank fill quietly, then turn the supply off or note the fill has stopped so the waterline is stable.
  2. Make a light pencil mark at the waterline on the porcelain or on a clean stick resting on the rim (do not leave graphite where it will wash into the bowl repeatedly).
  3. Wait twenty minutes without flushing. If the mark sits above the new waterline, the flapper (or a related seal) is dropping water into the bowl.
  4. If the line holds for twenty minutes, stop the leak hunt for now — an overnight mark is optional confirmation, not required for every check.

Honest stop: a twenty-minute hold does not rule out a fill-valve overflow whisper — listen at the overflow tube if the flapper seat looks fine.

If the mark dropped, follow the step-by-step flapper check and fix to clean the seat or replace the flapper.

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