Do the meter test
Turn off every faucet, appliance and irrigation zone. Find your water meter (often near the curb or where the water line enters the house) and note the reading or watch the small leak indicator dial. Wait an hour without using any water. If the reading changed, water is going somewhere.
To narrow it down, close the main shut-off valve into the house and repeat the test. If the meter still moves, the leak is likely in the line between the meter and the house. If it stops, the leak is inside.
Check the usual suspects
- Toilets: put a few drops of food coloring in the tank. Color in the bowl after 15 minutes means a leaking flapper.
- Water heater: look for moisture under the tank or dripping from the relief valve pipe.
- Under sinks: feel for damp cabinet floors and check the shut-off valves and supply lines.
- Outside: look for soggy spots in the yard, or sprinkler heads that keep weeping.
Signs it's inside a wall or floor
Brown stains, bubbling paint or soft drywall usually mean a leak above or behind the spot. Warped or cupping floors, loose tiles or a spot on the floor that feels warm can point to a leak under the floor. Mold or a musty smell in one room is another clue.