If your water use is metered, review your water bill. Divide your water usage by the number of days in the billing period and also by the number of residents of your household. If your water is measured in cubic feet, convert to gallons by multiplying by 7.48.
If your water is not metered try this calculator:
If you live in a region that that has seasons where usage changes at different times of the year, compare the water bill from winter to summer and see how your usage increases.