Symptoms
- Water sits noticeably lower in the bowl than normal
- No clog symptoms, drains fine
- Bowl looks the same after several flushes
- May be worse in a rarely used bathroom
Likely causes
- CommonPartial vent blockageA partly clogged roof vent can pull extra water out of the trap through siphoning.
- CommonEvaporation in a rarely used toiletWater slowly evaporates from the bowl over weeks of no use, especially in dry climates.
- PossibleSmall crack in the bowl or trapA hairline crack can let water slowly seep out, lowering the level.
- PossibleLow tank water level feeding the bowl lessIf the tank itself is set low, each flush leaves less water behind in the bowl.
What to do
- Flush the toilet and watch how much water refills the bowl versus the tank; a low tank level affects the bowl too.
- Check for a hairline crack around the base and inside the bowl by drying it and looking for wet lines forming.
- Add water manually to the bowl if it's just from evaporation in an unused bathroom, and flush regularly going forward.
- Ask about recent work on the roof vent stack if this started suddenly, since a blocked vent needs a professional check.
- Pour a gallon of water into the bowl to see if the level stabilizes normally afterward.
Tools and parts
- Bucket
- Flashlight
When to call a pro
Call a plumber if the level keeps dropping after refilling it, since that suggests a crack or vent problem needing inspection.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.