How Much Water Does a Dripping Tap Waste?

The numbers, drip by drip

The usual working figure for a single tap drip is a quarter of a millilitre, which is the value the US Geological Survey uses in its drip calculator (15,140 drips to a US gallon). Real drops vary with the tap and the pressure, but the figure is close enough to show the scale. From there the arithmetic is simple.

• One drip every ten seconds: about 2 litres a day, roughly 800 litres a year.
• One drip every five seconds: about 4.3 litres a day, roughly 1,600 litres a year.
• One drip every two seconds: about 11 litres a day, roughly 3,900 litres a year.
• One drip a second: about 21.6 litres a day, roughly 7,900 litres a year. That is around a hundred 80-litre baths.
• A thin unbroken trickle: no longer a drip, and often several hundred litres a day. Measure it rather than guess.

To measure your own tap, put a measuring jug under it for ten minutes. Multiply the millilitres by 144 to get millilitres a day, then divide by 1,000 for litres. That beats counting drops.

What it costs you depends on your meter and which tap it is

If your water is not metered, the bill is based on the property rather than use, so a cold drip costs you nothing directly. On a meter you pay for every cubic metre (1,000 litres) that goes through it, and in most of London you pay a wastewater charge on it too, because what goes down the plughole is assumed to go to the sewer. Take the combined per-cubic-metre rate from your bill and multiply: a drip a second is about 7.9 cubic metres a year.

A hot tap costs more, because each litre that leaves the cylinder or combi has to be heated. Raising a litre of water from about 10°C to 60°C takes roughly 0.058 kWh. At one drip a second that adds up to around 450 kWh a year before boiler losses, which you can price with the unit rate on your gas or electricity bill. It is highest where the hot water comes from an immersion heater on a standard electricity tariff.

The water itself is not the only cost. A drip that has been going for months usually leaves a limescale or rust stain in the basin, and a constant drip into a bath with a slow waste can mark enamel permanently.

Why taps start dripping in the first place

Which fix you need depends on how the tap closes. Turn the handle from off to fully on and count the turns.

• Several full turns and it gets tighter as it closes: a compression tap with a rubber washer. A drip means a worn washer or, if new washers keep failing, a scored brass seat. See replacing tap washers.
• A quarter or half turn to a firm stop: ceramic disc headgear. A drip means grit or scale on the discs, or a chipped disc.
• A single lever: a ceramic cartridge, specific to the maker. See a leaking mixer tap.
• Stops dripping only when you force the handle shut: the washer is worn thin, split or hardened, so it no longer seals under normal hand pressure. Forcing it cuts the washer and the seat faster.

Is a dripping tap ever an emergency?

Usually not. A drip is waste, not a flood risk, as long as the basin or sink waste is clear. It becomes urgent in three situations: when the drip turns into a flow you cannot stop by closing the tap; when the waste is blocked and the basin is slowly filling; or when the water is escaping round the base or handle and running into a cabinet, floor or ceiling below.

In any of those cases, close the small isolation valve on the pipe under the basin with a flat screwdriver, turning the slot a quarter so it sits across the pipe. If there is no valve or it will not turn, use the main stopcock. Then get the tap repaired or replaced. If you need a leaking tap repair out of hours, the £60 call-out applies at any hour and the repair price is agreed with you on site before any work starts.

The Water Supply (Water Fittings) Regulations 1999 require water fittings not to cause waste or undue consumption, so a tap that drips for months is something to fix rather than live with, especially in a rented flat where the landlord is usually responsible for repairs to taps.

Check the drip is the only thing wasting water

If your metered bill looks too high for one dripping tap, something else is losing water. The two usual suspects are a toilet that trickles into the pan all day, which wastes far more than any drip (see a toilet that keeps running), and a leak on the supply pipe between the meter and the house.

A quick test: turn every tap off, make sure nothing is filling (toilet cisterns, washing machine, loft tank), and watch the meter for fifteen minutes. If the dial still moves, the loss is somewhere in the pipework, not at the tap. Our guide to a water meter running with no taps on walks through narrowing that down.