Short answer
A high water bill usually comes from one of four things: a running toilet, a leak in the yard or under the house, an irrigation problem, or seasonal use. Turn off every fixture and the irrigation controller, then watch the meter's leak indicator. If it moves, you have a leak, and a few simple shutoff tests tell you where.
First, compare the right months
Water use swings with the seasons, especially where lawns, irrigation and pools use far more in summer. Compare this month's bill to the same month last year, not to last month. A 30% or larger jump over last year with no change in habits is worth investigating.
Do the meter test
Your meter is usually in a box near the street or sidewalk. Lift the lid carefully, since scorpions and spiders like meter boxes.
- Turn off every faucet, the dishwasher, washer, ice maker and irrigation controller.
- Find the small leak indicator on the meter face (often a red triangle or star).
- Watch it for two minutes. If it turns, water is going somewhere.
Narrow it down
Now split the house from the yard.
- Close the house shutoff valve. If the meter stops, the leak is inside the house. If it keeps turning, it's between the meter and the house, or in the irrigation mainline.
- Inside, put a few drops of food coloring in each toilet tank. Color in the bowl after 15 minutes means a leaking flapper.
- Close the cold inlet on the water heater. If the meter slows or stops, suspect a hot-side slab leak.
The usual suspects
Toilet flappers and fill valves are the most common culprit, and they wear out faster in hard water. Irrigation mainlines leak quietly underground. Slab-foundation homes get leaks on copper hot lines under the concrete, and basement homes can have a leaking service line or a pipe that split in a freeze.
When to call a plumber
- The meter moves with everything off and it isn't a toilet.
- You find a warm spot on the floor or hear running water.
- The yard has a soggy or unusually green patch between the meter and the house.
What the plumber does: see slab leak detection.
Common questions
Can a running toilet really raise my bill that much?
Yes. A constantly running toilet can waste thousands of gallons a month, which is often more than a slow slab leak.
Will my water utility adjust the bill for a leak?
Many water providers offer a one-time leak adjustment once it's repaired. Ask your provider and keep the repair invoice.
Editorial note: this guide is general information for homeowners, not a diagnosis. Conditions vary by home, so a licensed plumber should confirm the cause before any repair.