You can use these pond calculators to calculate the volume of water in your pond, the surface area of your pond, minimum recommended pumps size or the size of liner you need to build your pond.
It is important to know the details about your pond such as the volume. Any time you add a water treatment or medication for your koi or goldfish you need to know the volume so that you can to add the correct amount of treatment. If you under-dose with a treatment it will work poorly or not at all. If you overdose, you could kill the fish in your pond. It is also important to calculate the volume of you pond to properly size your pump and filter.