The American Water Works Association calculates that the average per capita domestic water consumption is 69.3 US gallons per day. For a family of four, that equates to approximately 8,600 gallons per month. This does not include usage like watering the lawn, washing your car or filling a swimming pool, which can add significantly to your monthly water usage. Water wastage can increase your recorded usage by double, triple or more. For example, a â€œsilentâ€ running toilet leak can waste up to 7000 gallons in one month.