Estimate the cost of a hidden plumbing leak from usage and water rate.
A hidden leak is expensive because it never stops. Two litres an hour sounds trivial — it is a fast drip — but it is forty-eight litres a day and 17,500 a year, and you pay twice for it, once on the water charge and again on the wastewater charge that most utilities levy in proportion to metered supply. If the leak is on the hot side the loss compounds: every litre also carries the energy used to heat it, roughly 0.052 kWh for a 45 degree rise, which for a continuously dripping hot tap can exceed the water charge itself. This is why the standard advice is to read the meter with every fixture closed — an advancing dial with nothing running is the only reliable way to catch a leak inside a wall or under a slab before the damage bill dwarfs the water bill.
Cost of a continuous leak
Litres lost = leak rate x 24 x days; cubic metres = litres / 1000; water cost = cubic metres x (water rate + wastewater rate); heat cost = hot litres x 4.186 x 45 / 3600 x energy rate
Cost estimate only. Concealed leaks cause structural damage, mould and electrical hazards that usually cost far more than the water, and insurers may reduce a claim for damage left unaddressed. Have suspected hidden leaks investigated by a licensed plumber promptly.
Close every fixture, read the water meter, wait an hour and read it again. The difference in litres is the rate directly. For a dripping tap, catch the water in a measuring jug for ten minutes and multiply by six.
Because the flow is large and completely silent once the overflow path is established — commonly 50 to 200 litres an hour. A single failed flush valve can add hundreds to a quarterly bill.
Many offer a one-off leak adjustment if you can show the repair was made promptly, and some waive the wastewater portion since the water never entered the sewer. Ask, and keep the plumber's invoice.