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Two readings, two hours apart
Read your meter, use no water at all for two hours, then read it again. Anything other than an identical reading is water leaving the property with nobody using it.
Leak rate
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Enter both readings and the time between them.
Running the test so the number means something
The calculation is arithmetic on two numbers and only as good as they are. Most leak tests that give a confusing result fail because water moved during the window and nobody knew. Find the meter first: indoors near the service entry in cold-winter regions, otherwise in a box at the property line.
Record every digit, including any painted on a fixed background rather than rolling — those permanent zeros mean the register counts in tens or hundreds, so a small flow takes a long time to show. Then note the sweep hand, the only pointer with the resolution to catch a small leak in two hours: 4 093 with the sweep at 6 is 4 093.6, and the .6 is where the answer lives. The guide to reading your water meter covers the register types.
Then use no water for two hours. That has to include the ice maker, the irrigation controller, a softener that backwashes on a timer, a pool auto-fill valve, and everybody else in the building. One flush invalidates the test.
Read again, enter the actual elapsed hours, and set the units correctly: one cubic foot is 7.48 gallons, and one CCF, the unit most US utilities bill in, is 748 gallons.
The low-flow indicator finds leaks the digits will not
Most meters carry a small separate dial on the face: a triangle, a star, a snowflake or a toothed gear. It is a low-flow indicator, geared to spin on any flow at all — far less than the sweep hand needs to move visibly.
Use it before committing to the two-hour test. Shut off every fixture, wipe the glass clean, and watch for a full minute. If it turns — slowly, intermittently, at all — water is moving on your side of the meter. That is a yes-or-no answer in sixty seconds.
It is also the more sensitive instrument over a short window. A seep of two or three gallons a day moves a cubic-foot register by less than half a cubic foot in two hours, which you may not resolve on the sweep hand and which disappears entirely if your last digit is a painted zero. The indicator will still be creeping — and two or three gallons a day is the signature of a flapper that is not seating.
Older meters may have no indicator, leaving the two-reading test as your only option. And a moving indicator confirms flow but says nothing about volume.
What the number means: severity bands
The rate of loss narrows the search considerably, because different failures leak at characteristically different rates.
| Loss per day | Most likely cause | Where to look |
|---|---|---|
| A few gallons | A flapper not seating, or a slow drip at a faucet | Toilets, then under sinks |
| Tens of gallons | A toilet running intermittently, or a fill valve overtopping | Listen at each toilet; check the overflow standpipe |
| Hundreds, nothing audible indoors | A break in the buried service line, or a stuck irrigation valve | The meter box, the run to the house, the manifold |
Toilets deserve the first look in nearly every case: the most common indoor leak, the most likely to be silent, the cheapest to correct. Put a few drops of dark food colouring in the tank, do not flush, and come back in half an hour; colour in the bowl means the flapper is passing water while the fill valve quietly replaces it.
The high band is different in kind. If the indicator spins with the main shutoff closed, the loss is underground. Supporting signs: grass greener or spongier than the rest, water standing in the meter box, a drop in pressure. It will not stop on its own.
Why a leak inside the house usually costs twice
Most utilities do not meter sewage. Metering a wastewater line is expensive and unreliable, so the near-universal practice is to bill sewer on the volume of water that entered the property, assuming most of it goes down a drain.
That assumption is what makes an indoor leak expensive. A gallon lost through a flapper is billed once as water and again as sewer, because as far as the utility's records go it passed the meter and therefore went to treatment. In many places the sewer component is comparable to the water component, so a leaking gallon can cost roughly double the figure on the water schedule. That is why the calculator asks for a combined rate: run the numbers on water alone and you understate the loss by about half.
An outdoor leak may be treated differently, since irrigation water never reaches the sewer. Utilities either ignore that, cap the sewer volume at your winter average, or let you install a separate irrigation meter excluded from the sewer charge. The distinction also explains why leak adjustments exist: the charge utilities credit back most readily is sewer on water that demonstrably never entered the sewer.
Which side of the meter, and who pays
Ownership splits at the meter, and the split decides who does the work and who pays. The utility side is the main, the tap into it, and the service line up to and including the meter. The customer side is everything from the meter to the building, including the buried section crossing your yard — yours even though you did not lay it, cannot see it, and may not know what it is made of.
The exact boundary is set by your utility's tariff and occasionally sits at a curb stop instead, so confirm rather than assume. The consequence is uncomfortable: the most expensive failure in a residential water system, a break in the buried service line, usually falls on the homeowner.
Isolating the side is straightforward once you find the main shutoff. Close it and watch the indicator. If it stops, the leak is inside. If it keeps turning with the house isolated, the loss is on the buried run — still yours, but a different job. If it turns with the meter's own valve closed, the problem is at or before the meter, and that is a call to the utility rather than to a plumber.
What to do next, in order
- Dye-test every toilet. Ten minutes, no cost, and it resolves most small leaks.
- Walk the fixtures. Faucets, supply stops under sinks, washing machine connections, the water heater's relief discharge pipe — a relief valve weeping into a drain is invisible and moves real volume.
- Isolate at the main shutoff and re-watch the indicator to establish which side the loss is on.
- Look outside if the indoor search comes up empty: irrigation valve boxes, hose bibs, the meter box, any lawn that is greener or settling.
- Re-run the test after each repair. Homes commonly have more than one leak, and fixing the loud one can leave the expensive one in place.
Then ask about a leak adjustment. The request is worth making when the excess is large relative to your normal bill, the cause was a genuine failure rather than a tap left running, and you have fixed it and can prove it. Utilities that offer adjustments require the repair first and typically credit part of the excess rather than all of it.
Once the leak is closed, put a clean month into the water bill calculator and keep that figure to compare against. A leak is far cheaper to find in its first month than its sixth.
Frequently asked questions
How long should I shut the water off for a meter leak test?
Two hours is the usual recommendation and a reasonable compromise between sensitivity and inconvenience. Longer is better: over eight hours overnight, a leak of a few gallons a day moves the register enough to read without squinting at the sweep hand, and the arithmetic averages out small errors in either reading. Shorter windows work only for large losses. Whatever period you use, enter the actual elapsed hours rather than the intended ones, because the calculator scales the difference to a daily figure — an hour of error at the two-hour mark shifts the answer by half.
The meter is not moving but my bill doubled. What else could it be?
Several things, none of them a leak. A billing period can be several days longer than the previous one, which raises the total without raising daily use. An estimated read followed by an actual read produces one low bill and one that catches up. Seasonal irrigation, a new pool, house guests, or a water softener regenerating more often after a settings change all move real consumption. Rates and fixed charges are also revised, often at the start of a fiscal year. Check the meter reads and the number of days printed on both bills before assuming the plumbing is at fault.
Does a leaking toilet get charged for sewer as well as water?
Almost always. Utilities rarely meter wastewater; they bill sewer on the volume recorded by the water meter, assuming that what enters the property leaves through a drain. Water lost through a toilet flapper genuinely does reach the sewer, so both charges apply and there is nothing anomalous about it. That is why the calculator asks for a combined water and sewer rate — using the water rate alone can understate the cost of an indoor leak by roughly half. Leak adjustment policies, where they exist, more often forgive sewer charges on water that never reached the sewer at all.
Who pays to fix a leak in the pipe between the meter and the house?
The homeowner, in most jurisdictions. The utility owns the main and the service line up to and including the meter; everything from the meter to the building is the property owner's responsibility, including the buried section under the yard. Confirm the boundary in your utility's tariff, since a few set it at a curb stop instead. If the loss continues with your main shutoff closed, the leak is on that buried run and you need a plumber rather than the utility. Some insurers sell service line coverage as an endorsement, and some utilities offer an optional repair programme billed monthly.