Pond Fish Stocking Calculator
Estimate how many koi or goldfish a garden pond can hold from its volume, using safe per-fish water rules.
How it works
max fish = pond gallons ÷ gallons-per-fish (koi ≈ 250, goldfish ≈ 50, ×1.4 if filtration is minimal)
Ponds are stocked by water volume per fish because oxygen and waste load — not swimming room — set the real limit. Adult koi grow large and dirty, so a common guideline is around 250 gallons each; goldfish are far smaller and need roughly 50. Strong filtration and aeration let you push toward these figures, while a lightly filtered pond should hold well under them. Stocking conservatively pays off: understocked ponds stay clearer, healthier and far more forgiving in summer heat.
Worked example
A 1,000-gallon pond of adult koi with good filtration allows 1,000 ÷ 250 = 4 koi. For long-term headroom, stocking 2–3 keeps the water quality easy to manage.
Frequently asked questions
How many koi can my pond hold?
A common rule is about 250 gallons per adult koi with good filtration. So a 1,000-gallon pond supports roughly 3–4 koi, and fewer is better for water quality.
Why stock by volume, not by fish size?
The limit is oxygen and the waste the filter must process, both of which scale with water volume. Crowding raises ammonia and drops oxygen, especially on hot nights, regardless of how much open water looks available.
Does filtration change the numbers?
Yes. Strong biological filtration and aeration let you approach the per-fish guidelines, while a pond with little filtration should be stocked well below them to stay safe.
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This calculator is for educational purposes. Double-check important results before acting on them.