Water Meter Overnight Leak Test Calculator

How to take the two readings

Make sure nothing is using water: taps off, no washing machine or dishwasher running or set on a timer, no cistern refilling. Write down the meter reading and the time. Leave the building alone for a gap, then read it again.

Thames Water describes the same test on its page about identifying leaks, fetched on 12 November 2026. It has you turn off the inside stop valve, take a reading, wait 30 minutes and read again, and it says that if the readings differ the leak is between the meter and the stop valve. The calculator asks which way you ran the test, because the two setups tell you different things.

For a clearer result use the overnight gap. The household is asleep, nothing is on a timer you have forgotten, and a small flow has hours to show. Use every digit on the register, including those after the decimal point on a cubic metre meter. A cubic metre is a thousand litres, so the digits after the point are the ones that move.

What the result means

If the meter did not move over a gap of at least 30 minutes, nothing was registered. That is a good sign for the supply and the pipework tested, but a very slow leak can sit below what the meter displays.

If it moved, the calculator shows litres each hour and what that would be over a day if it continued. With the stop tap open and everything off, movement points to a leak or to something using water that you had not counted. With the inside stop tap closed, movement points to a loss on the pipe between the meter and that tap. Repeating the test both ways is the quickest way to tell inside from outside.

The figure is a rate from two readings, so a long gap smooths out a short burst and a short gap exaggerates it. That is why the calculator asks for at least 30 minutes.

Common false alarms

Before you decide there is a leak in a pipe, rule out the usual causes of movement that are nothing to do with a pipe.

• A toilet cistern refilling after a slow drip past the flush valve. It is a leak, but a repair to the toilet, not a hidden pipe.
• A water softener or filter cleaning itself overnight.
• An ice maker, a water dispenser or an appliance on a delay timer.
• An outside tap, a garden watering system or a water butt filling.
• A heating system topping up through a filling loop that is not fully closed.
• A smart meter reporting in different units, or readings taken from the wrong digits.

What to do with a positive result

Write down both readings and times, and repeat once to check. Keep the notes: a water company, a landlord or an insurer will want to see them. The evidence log keeps them dated.

A positive meter test shows that water is moving. It does not show where. Finding the point of loss is the job of the visit, which uses the methods it needs and is one price: one fixed price of £480, every method included, agreed before we attend. No VAT is charged. See the pricing page and what is included.