Outdoor ponds don't follow the same stocking guidelines as indoor aquariums — variable filtration, algae, temperature swings, and predation all change the math. This calculator applies the standard surface-area-based koi and goldfish stocking rules, plus a depth cross-check for safe overwintering, so you get a sanity check before adding fish.
How the Pond Fish Stocking Calculator works
The calculator estimates your pond's water surface area from its volume and average depth (or uses your directly measured surface area, if you have it), then divides that surface area by a per-inch density factor that depends on fish type. Koi use roughly 1 square foot of surface per inch of adult length — a much lower density than indoor tank rules, since outdoor ponds have more variable filtration and bioload conditions to manage. Goldfish are smaller-bodied and hardier, so they're allowed roughly double the density (about 0.5 sq ft per adult inch). A mixed koi/goldfish pond uses a blended 0.75 sq ft per adult inch.
Inputs and what they mean
Pond size in gallons and average depth in feet are the primary inputs — together they estimate your water's surface area (gallons ÷ 7.48 gallons-per-cubic-foot ÷ depth). If you've measured your pond's actual surface area (length × width at the waterline, in square feet), switch to "I know my surface area" and enter it directly for a more precise result — you'll still enter average depth so the calculator can cross-check overwintering depth. Fish type determines the density factor: Koi, Goldfish, or Mixed.
Limits and edge cases
This is a rule-of-thumb starting point, not a hard limit. Strong filtration and aeration (waterfalls, fountains, air stones) can safely support more fish than the base rule suggests, while heavy plant cover or minimal filtration should stay well under it. The depth cross-check only checks average depth against a general overwintering guideline — it doesn't know your specific climate or whether part of your pond has a deeper shelter zone. Always plan stocking around adult fish size, not the size at purchase, since koi in particular grow dramatically over a few years.