A leaking toilet, or a toilet that continues to run after being flushed (see additional information below)
Watering the lawn, new grass, or trees
Kids being home for summer vacations or school holidays
Water-cooled air conditioners
A broken water pipe or obvious leak; check the pipes in the basement or crawlspace
Water softener problems – cycles continuously
Letting the water run to avoid freezing water pipes during cold weather
If none of these seem to be the problem, try the following:
Read your water meter at night right before going to bed and then first thing in the morning before anyone uses any water. The numbers on the meter should not move at all. If the meter numbers moved, then water is going through the meter. Something is leaking and you may want to have a plumber come to your property to take a look.
The most common problem is running water from your toilet. See the toilet assessment below for help in determining if this is the cause of your high water bill.
Do-It-Yourself Toilet Assessment
First check for the most common leak: a deteriorated or defected flush valve (flapper) ball at the bottom of the toilet tank. If it does not make a tight seal water will leak into the toilet bowl.
To check for this:
Take the lid off of the tank behind the bowl, flush the toilet, then wait for it to fully refill.
Put a few drops of dye or a colored dye tablet (available at some hardware stores) in the tank.
Wait at least 20 minutes; longer if you suspect it is a small leak.
If there is any color in the toilet bow, there is a leak.
The second most common type of leak has to do with an improperly adjusted or broken fill (ballcock) valve. To check for this take the lid off of the toilet tank, flush, and see if water is draining into the overflow tubes when the tank is full.