Backyard Pond Calculator
How many gallons a pond holds, how big a liner to buy, the pump flow to turn the water over, a safe fish load, and how deep it must be for fish to make it through winter. Sizing figures from aquaculture extension.
How the math works
Volume is surface area times average depth times seven and a half, the gallons in a cubic foot. A freeform pond with sloped sides holds about eighty percent of its bounding box, so we trim for that. Liner size is the pond plus twice the depth on each axis plus a two-foot overlap all around. Pumps are sized to turn the whole pond over about once every two hours, or once an hour with koi, plus extra for a waterfall, roughly a hundred to a hundred and fifty gallons an hour per inch of spillway. Buy that flow rate at your lift height, not the pump’s headline rating.
Fish, honestly
Stocking rules are deliberately conservative, and water quality matters more than gallons. A biofilter is sized to how much you feed, not the pond size, so under-stock and add fish slowly. In freezing climates, fish survive only if part of the surface stays open, with a de-icer or aerator keeping a hole for gases to escape; sealing the pond under ice and snow is what causes winter fish kills.
Sources
Volume and the 7.48 factor: SRAC Publication 103 and Michigan State University Extension. Stocking, depth, aeration and filtration: Texas A&M AgriLife Extension and University of Florida IFAS. Winter fish kills and keeping ice open: Penn State Extension. Liner, turnover, and waterfall flow are long-standing water-garden industry conventions; fish numbers are shown as conservative ranges, not hard limits.