Calculate the volume of your detention basin (in cubic feet) by multiplying its dimensions. For example, if your detention basin is 10 feet long, 10 feet wide and 8 feet deep, multiply 10, 10 and 8 to come up with a volume of 800 cubic feet.
Convert your detention bin's volume to gallons by multiplying the volume by 7.48. Using the previous example, this gives you a volume of 5,984 gallons.
Divide the volume (in gallons) by the detention time to find your flow rate. Based on the previous example, if the detention time is 30 minutes, divide 5,984 gallons by 30 minutes to come up with a flow rate of 199.46 gallons per minute.
Multiply the flow rate by the number of minutes in a day (1,440) to come up with the flow rate of the exchange bed per day. This step is optional.