Once the “where” is decided, we need to determine size and depth.
An ideal pond for the average hobbyist is between 23 and 25 feet long by 12 to 13 feet wide and 3 to 4 feet deep. (The 1% or less of koi hobbyists who want to grow jumbo koi or keep fish primarily to compete in shows with, will want much deeper ponds of 6 to 8 feet). This size pond can accommodate 15 to 20 mature fish (24 to 28 inches) giving them plenty of room to exercise and, will not look overstocked and crowded. Here’s how we calculate the volume of water in this pond: Length x width x depth = cubic feet x 7.5 gallons per cubic foot = volume in gallons. So, 25 x 13 x 3 x 7.5 = 7312 gallons. However, this is only an approximate figure as the pond will not normally be a perfect rectangle and the shape may be more freeform. We will only know the exact volume of pond and filtration system upon filling the feature and metering the input. And it’s extremely important to know the exact volume.
Keeping the width of the pond to less than 13 feet is so that when we have to catch a fish, for whatever reason (and there will be reasons), we need to be able to extend the net from one side to the other and any net and handle longer than about 12 feet is quite unmanageable. (See section on catching fish).