Koi Pond Stocking Calculator
Calculate how many koi can live in your pond from water volume.
Applies the inch-per-gallon stocking guideline with filtration quality adjustment.
Koi grow large — a well-kept koi in a good pond will reach 60-90 cm (24-36 inches) over 10-15 years. The common aquarium rule of “1 inch of fish per gallon” fails badly for koi because it was designed for small tropical fish.
The adjusted koi rule
A conservative and widely accepted starting point is 1 inch of fish (measured as body length, not including tail) per 10-20 gallons of pond water, depending on filtration quality.
With basic filtration: max_inches = pond_gallons / 20 With good biological filtration: max_inches = pond_gallons / 15 With high-quality filter and UV clarifier: max_inches = pond_gallons / 10
For a 1,000-gallon pond with good filtration, you can keep about 67 inches of total fish — perhaps 5 juvenile koi (13 inches each) or 3 mature koi (22 inches each).
Why koi are demanding
Koi produce far more waste relative to their size than smaller fish. They are also messy eaters. Ammonia and nitrite spikes from overstocking cause gill damage within hours and kill fish within days. A good biofilter (capable of converting ammonia to nitrate) and water changes of 10-20% per week are not optional.
Temperature matters. Koi metabolism rises with water temperature. In a warm summer pond (25C), waste production and oxygen demand are much higher than in spring (15C). Overstocking that seems fine in cool weather creates crisis conditions in July.
Koi also need minimum depth (60-90 cm) to avoid temperature extremes and predators. Shallow decorative ponds are not suitable for koi regardless of volume.