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you should get it!! The most accurate way to determine gallons is to take a 1g container and fill the pond till you can't fill anymore!! just make sure you keep track of how many gallons you use!!
Time how long it takes to fill a 5 gallon bucket and then fill the pond at the same flow with a hose and then calculate how many gallons based of the time it took to get full. (Much easier than doing 1 gallon at a time, especially if it is bigger)