It is very important when starting a pond project to know exactly, or as close as possible, to the actual size of the pond. Knowing this measurement will make the rest of the pond planning process much easier. Below are a couple of links to tested pond calculators that we use on a regular basis. Under those will be a few formulas for finding various measurements important to pond building, if you wish to do the calculations yourself instead of relying on an online calculator, we strongly recommend learning to use these formulas.
For pump sizing and head calculations, refer to "Choosing the Perfect Pump" by Atlantic Water Gardens.
Liner Length: Max. Pond Length + (2 x Max Pond Depth) + 2 = Min. Liner Length
Liner Width: Max Pond Width + (2 x Max Pond Depth) + 2 = Min. Liner Width
*Please note liners do experience shrinking over time please take this into account when doing these calculations
Rectangle Pond Volume: Length x Width x Depth x 7.5 = Total Volume in Gallons
Oval Pond Volume: Length x Width x Depth x 6.7 = Total Volume in Gallons
Circle Pond Volume: Length x Width x Depth x 5.9 = Total Volume in Gallons
Other Pond Volume: Avg Length x Avg Width x Avg Depth x 7.5 = Total Volume in Gallons
*Please note these formulas are not 100% accurate and are more for quick calculations. More complex, accurate, formulas are available but for practical purposes these are not necessary and more often lead to confusion.