Since you are basing your estimate of the pond size on dimensions, which are OK if the pond walls are vertical, and the corners are square, or the pond is a perfect circle, but are much less reliable if the pond has sloped walls, as the dimensions of the bottom are much less, ponds with shelves for plants, ponds with irregular shapes as curves cut off part of the top area of the rectangle. You can get a TDS meter from Amazon or other sites and take a measurement of the total dissolved solids, then add a measured amount of salt (use something close to 17 to 20 pounds, bag of salt for water softener, solar crystals, in water softener section of hardware is 40 pounds), and after a couple of days remeasure the TDS. The difference can then be used with the calculator located at the top of the page to determine volume. This method is much more accurate than dimensions. As accurate a measure as possible is important when it comes time to treat for parasites or other uses of chemicals as an under dose is ineffective and an overdose can kill the fish.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard