Place a measuring cup under your leaky faucet. Measure how much water drops from the leaky faucet each day into the cup. Move the cup each time you use the faucet and place it back when you finish using the faucet. Empty the water out each day at the same time and record the amount of water. For example, assume over three days the leaky faucet lost 5 cups, 4 cups and 8 cups of water respectively.
Add together your water loss from each day, then divide the result by the number of days you measured from your leaky faucet. In the example, 5 plus 4 plus 8 equals 17 cup, then 17 divided by three equals an average of 5.67 cups.
Divide your average amount of water loss by the average amount of minutes the leaky faucet could lose water. In the example, assume you tracked the leaky faucet for three days,so, 3 days equals 4,320 minutes. Then 5.67 cups divided by 4,320 minutes equals 0.0013125 cups per minute. So each minute you are wasting 0.0013125 cups of water on average due to the leaky faucet.