From Stan & Debbie Hauter,
Your Guide to Saltwater Aquariums.
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Step 5 - Determine Gallons Per Minute

Take the time in seconds it took to fill the one gallon container and divide that into 60 seconds to get the number of gallons it pumped per minute.
• Our powerhead example: 60 seconds divided into 15 seconds = 4 gallons per minute.

Step 6 - Determine Gallons Per Hour (GPH)

Now multiply the number of gallons per minute by 60 minutes and you get the number of gallons per hour pumped.
• Our powerhead example: 60 minutes multiplied by 4 gallons = 240 GPH (gallons per hour).

Step 7 - Determine How Many Times Per Hour the Tank Water Volume is Turned Over

Divide the number of gallons per hour by the actual gallons of tank water volume you got from Step 2.
• Our powerhead example: 240 GPH (gallons per hour) divided into 38 gallons = 6.32 times per hour our tank water passes through the powerhead.

If you are purchasing a new piece of equipment and want to estimate what the flow rate will be before you buy it, take the manufacturers flow rate and divide that into the actual tank water volume you got from Step 2. This will give you how many times per hour the water pump will turn over the tank water, without any resistance factors of course.

• Example: You are looking at a water pump that is rated at 400 GPH. You have a total of 55 gallons of actual tank water. This water pump will turn over the tank water 7.27 times per hour. Estimate in any factors for resistance and decide if this pump is going to be sufficient for your needs.

That's it plain and simple. These 7 steps can be applied to any piece of equipment to determine water flow rates.

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