When the water level in a toilet bowl drops on its own, the usual causes are a partial clog that slowly siphons water out, a blocked plumbing vent, or a crack in the bowl. A drop in the tank water level, on the other hand, usually means a leaking flapper.
Bowl or Tank?
First, figure out where the water is going. If the bowl level drops between flushes, the problem is in the bowl or the drain. If the tank level drops and the toilet refills on its own, the flapper or fill valve is the likely cause.
Common Causes of a Dropping Bowl
- Partial clog: paper or a small object can form a wick that slowly pulls water down the drain.
- Blocked vent: without enough air, draining fixtures nearby can suck water from the toilet bowl.
- Cracked bowl: a hairline crack can let water seep out, often leaving a damp spot near the base.
- Evaporation: in a toilet that is not used for weeks, water slowly evaporates.
Simple Checks
Mark the water line in the bowl with a piece of tape. Check it after an hour without flushing. If the level drops quickly, look for water around the base and listen for gurgling when other drains are used. Try plunging with a flange plunger to clear a partial clog.
Odors and Floor Damage
A bowl with too little water lets sewer gas into the bathroom, which can cause odors. A crack can also leak onto the floor and damage the subfloor. Gurgling and a dropping water level together may point to a drain or vent problem that affects more than one fixture.
If the toilet is in a guest room that is rarely used, flush it once a week to keep the bowl full.
If plunging does not help or you suspect a crack, having the toilet checked can find the cause. If nearby drains also gurgle, you can ask a plumber to look at the venting at (833) 603-1840.