The most important information you should know about your pond is the number of gallons of water it holds. Your pump size and water treatments are based on this number. You can use the following calculations to determine how much water is in your pond. There are 7.48 gallons of water per cubic foot. To figure gallons in your pond: (All measurements are in feet)
Length x Width x Depth x 7.48 = Gallons in your Pond
3.14 x r2 x Depth x 7.48 = Gallons in your Pond. r2 = Radius x Radius (Half pond diameter times itself)
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)
You could also measure the amount of water that goes into your pond to fill it if you have just made your pond or have emptied it. However most if us are not just making their pond and even if you are measuring water as it goes into your pond it is time consuming. Therefore the above calculations are very useful.