The most important information you should know about your pond is the number of gallons of water it holds. Correct filter sizing and water treatments are based on this number. A natural pond is a bit harder to calculate than a rectangular or circular pond but many of us want a pond that looks natural so its a necessary calculation. You can empty your pond and measure how much water it takes to refill it but most of us would rather not have to take this route when a calculation is available.
There are 7.48 gallons of water per cubic foot.
To figure gallons in your pond: (All measurements are in feet)
Natural Ponds: Length x Width x .8 x Depth x 7.5 = Gallons
(.8 compensates for lack of corners. Use .85 for ponds 400 plus square feet)