How to Find a Hidden Water Leak With the Meter Test, Dye Test and Bill Check

The leaks that cause the worst damage are usually the ones nobody can see. A supply line weeping in a wall, a toilet flapper running into the bowl, a slab leak under a floor: they can run for months and show up first as a bill or a smell. Three tests, all of them from EPA’s WaterSense program, will tell you whether water is going somewhere it should not.

How much water leaks cost

EPA’s WaterSense pages put a number on it. The average family can waste 180 gallons per week, or about 9,400 gallons a year, from household leaks, and nine percent of homes have leaks that waste 50 gallons or more per day. The pages differ slightly on the annual average, with one saying more than 9,300 gallons, so treat the figure as approximate. The examples EPA gives for individual leaks show how small ones add up.

The two-hour meter test

EPA’s guidance is to check your water meter before and after a two-hour period when no water is being used, and if the meter changes at all, you probably have a leak. It is the single best test because it covers the whole system.

1. Stop all water use. Turn off faucets, appliances, the ice maker and irrigation. Do not flush.
2. Read the meter. Note the digits, including any small dial or triangle.
3. Wait two hours, using no water.
4. Read it again. A change means water is moving.
5. Narrow it down. Close the main shutoff and read again after a short wait. If the meter stops, the leak is on the house side. If it keeps moving, the leak is between the meter and the valve, or in the meter. See the shutoff guide.

The toilet dye test

Toilets are the most common source of a silent leak, usually a worn flapper that lets water seep from the tank into the bowl. EPA’s test: put a few drops of food coloring in the tank. If any color shows up in the bowl after 10 minutes, you have a leak. Flush right away afterward to avoid staining the tank. EPA also says a flapper should be checked periodically and replaced at least every five years.

The winter bill check

Irrigation and pools muddy summer bills, so EPA suggests looking at a colder month, such as January or February. If a family of four exceeds 12,000 gallons per month, there could be serious leaks. That is a screening threshold, not a diagnosis. A bill below it does not rule out a leak, and a bill above it may reflect a houseful of guests or a hobby. If the number is high, run the meter test next.

Signs of a leak you cannot see

• A meter that moves with everything off.
• The sound of running or hissing water in a quiet house.
• Warm spots on the floor, which can indicate a hot water line leaking under a slab.
• Damp or discolored flooring, carpet that stays wet or baseboards that separate from the wall.
• A musty smell with no visible mold, or paint that blisters on a wall.
• Cracks or damp at the base of a wall or at the slab edge.

These are signs, not proof. A moisture meter and, for leaks under slabs or inside walls, a plumber’s leak detection equipment are how they are confirmed.

Leaks you can fix and leaks to hand to a plumber

• A running toilet. If the dye test shows color in the bowl, the flapper is usually the cause. EPA says a flapper should be checked periodically and replaced at least every five years. It is an inexpensive part and a common home repair.
• A dripping faucet or showerhead. EPA says a faucet dripping once a second can waste more than 3,000 gallons a year. Replacing a washer, a cartridge or a showerhead is within reach for many homeowners.
• An irrigation leak. EPA says a 1/32 inch leak can waste about 6,300 gallons a month. Walk the system with it running and look for wet spots and geysers.

Hand it to a plumber when the leak is inside a wall, under a slab or in a ceiling; when the meter moves and you cannot find a fixture to explain it; when it involves a hot water line; when it is near electrical equipment; or when a valve will not close. If you do not know where it is, a plumber with leak detection equipment can narrow it to a small area.

Keep a log. Write down each test, the meter readings and the date, and what you fixed. If a leak persists after a repair, the log shows what you ruled out.

Where hidden leaks usually are

IBHS’s guidance on preventing interior water damage from plumbing and appliances covers the sources: water heaters, washing machine hoses, dishwashers, ice maker lines and toilets. In a house, those translate to a handful of places worth checking by touch and by eye every few months.

• Under every sink, for dampness on the cabinet floor and drips on the supply stops.
• Behind the washing machine, for hose bulges, rust at the couplings and water on the floor.
• Around the base of toilets, for soft flooring and wobble.
• Around the water heater, for rust, a wet pan and corrosion on fittings.
• The air handler closet and its drain pan.
• The attic, for wet insulation and stained decking near plumbing.

An evening routine that finds most leaks

1. Walk the house first, with the sound off. Look at ceilings, wall bases and cabinet floors, and listen for hissing.
2. Run the toilet dye test in every toilet. It takes ten minutes and runs in parallel with the next steps.
3. Do the meter test. Stop all water use, read the meter and read it again after two hours.
4. If the meter moved, close the main and check again, to place the leak on the house side or the street side.
5. If it stopped, shut valves one at a time to the water heater, the washer and the irrigation. When the meter stops moving after a valve is closed, you have found the branch.
6. Write down the readings and the time. A plumber and an insurer will both ask.

What a plumber adds

Leaks inside walls and under slabs are found with equipment: acoustic sensors that hear the leak through the structure, thermal imaging for hot water lines and pressure tests that isolate sections of pipe. They narrow the leak to a small area, so repairs open a small part of the floor or wall. If your own tests say there is a leak and you cannot find it, that is the point to call.

When the tests say there is a leak

If the meter moves, close the main and call a plumber, and photograph anything wet. If the leak is in a wall or ceiling, do not open it yourself unless you know what is behind it, and keep people out from under a sagging ceiling. If water has already damaged materials, the clock described in the mold timeline is running. The water heater and appliance leak page covers the cleanup and drying that follow, and the assessment tool organizes the details for a call.

Read about water heater and appliance leaks →

This article is general information from the Dallas Flood Guard editorial team. It is not medical, legal, engineering or insurance advice. Sources are linked above. See the editorial policy for how posts are researched and corrected.