Most faucet leaks are plainly visible as drips coming out of the faucet aerator. Occasionally, faucets may leak in other places such as the on/off handle or in the pipes below the basin. If you are unsure if your faucet is leaking you might try using your water meter to detect the leak.

Most faucet leaks are easy to identify and should be
fixed immediately to avoid water waste.

Leaks should be fixed immediately since they can quickly develop into a serious break. Temporary repairs are only temporary, and wrapping the pipe usually fails. If you find a fixture that is leaking, turn off the shutoff valve for that device. Then locate the leak. Try to tighten any fittings that leak. A quarter or half turn with a wrench might do the trick. Be careful not to over-tighten fittings. Another possible cause is that the pipes are undersized and the water velocity is too high.

Estimated faucet leakage rates

Drips
60 drops per minute = 192 gallons per month
90 drops per minute = 310 gallons per month
120 drops per minute = 429 gallons per month

Stream
3" stream = 1,095 gallons per month
6" stream = 2,190 gallons per month
9" stream = 3,290 gallons per month

For more detailed leakage analysis check out the WaterWiser drip calculator.

For information on how to repair faucet leaks see the faucet repair and faucet maintenance pages.

Source:
CUWCC Practical Plumbing Handbook