Tap Water Calculator
About 140 litres a person a day, and showers and toilets are most of it.
The formula
litres = drips per minute * 1440 * mL per drip / 1000
A drip a second is 3,000 gallons a year
That is the EPA's figure and it surprises people, because a drip is about a third of a millilitre and looks like nothing. Multiplied by 86,400 seconds in a day it is thirty litres, and over a year it is eleven thousand. The arithmetic of small things repeated often is the whole lesson.
Hot water costs twice
A dripping hot tap wastes the water and the energy used to heat it, and the energy is usually the larger cost. Heating thirty litres a day from 10 to 60 degrees takes about 1.7 kWh — more than a fridge uses in a day, running out of a tap nobody turned on.
Where household water actually goes
Showers and toilets dominate, and a bath is roughly nine minutes of showering. The drip matters less than the shower length in absolute terms — but the drip is free to fix and the shower is a habit, which is why plumbers get better results than posters.
How much to trust a footprint
Life-cycle figures for the same product routinely differ by a factor of two between studies, because deciding what to count — the farm, the packaging, the shop, the drive home, the disposal — is a methodological choice rather than a measurement. The numbers here are central estimates from published meta-analyses. They are reliable for comparing one option against another, which is what people actually use them for, and not reliable to the second significant figure.
The other thing worth keeping in view is scale. Individual choices matter and they are not where the emissions are: the difference between the best and worst quartile of beef producers is larger than most people's entire dietary footprint, and that is a question about agriculture policy rather than about shopping.