Fish Stocking Density Calculator
Enter your pond volume, pick goldfish or koi, and add up the expected adult length of every fish to see how close your pond is to a comfortable stocking density.
Stocking check
How the stocking check works
The calculator divides your pond volume by the gallons-per-inch guideline for your fish type — about 10 gallons per inch for goldfish, about 20 for koi — to get a maximum recommended total fish length. Your entered total is then compared against that maximum as a percentage.
Below 70% of that figure, the pond is comfortably understocked with room to add more. From 70–100% it's in a well-stocked range. Above 100% it's near or over the recommended capacity, which is worth addressing with a bigger filter, more frequent water changes, or fewer fish.
Frequently Asked Questions
Where does the gallons-per-inch guideline come from?
It's a long-standing pond-keeping rule of thumb: goldfish are generally stocked at around 10 gallons per inch of adult fish length, and koi at around 20 gallons per inch, since koi grow larger and produce more waste per inch of body length. It's a planning guideline, not a hard biological limit.
Should I use my fish's current size or their adult size?
Plan for adult size. A 3-inch goldfish fingerling can become an 8-to-12-inch adult, and a koi can grow well beyond that, so stocking a pond for today's small fish leads to overcrowding a year or two later.
What happens if a pond is overstocked?
More fish means more waste, which means more ammonia and nitrite for the filter to process. An overstocked pond is more prone to poor water quality, algae blooms, and stressed fish, even if the filter and water changes are otherwise well managed.
Can better filtration let me stock more heavily?
A stronger filter and more frequent water changes can support a somewhat higher stocking density than this guideline, but the gallons-per-inch rule is a good starting point for most garden ponds — check the filter turnover calculator too if you plan to stock near or above the recommended density.
Planning guideline only, not a guarantee of water quality or fish health. Every pond's filtration, plant load, and maintenance schedule differ — if a fish looks unwell, consult a qualified aquatic veterinarian rather than relying on a stocking estimate.