Koi Pond Stocking Calculator

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 that produce a fraction of the waste.

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. That is perhaps 5 juvenile koi at 13 inches each, or 3 mature koi at 22 inches.

Notice which of those numbers is the trap. Five 13-inch koi fit today. In six years those same five fish are 24 inches each, which is 120 inches of fish in a pond rated for 67. Nobody buys nine koi for a small pond, but plenty of people buy five and then find themselves rehoming four of them, or losing all five in one August week.

Stock for the adult size, not the shop size

The single most useful habit in koi keeping is to decide the adult length first and work backwards. A pond that holds three adults holds three fish, full stop. Buying five because they are small and cheap now is buying a problem for the year the filter cannot keep up.

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 at 25C, waste production and oxygen demand are much higher than in spring at 15C. Overstocking that seems fine in cool weather creates crisis conditions in July, and the fish that die are usually the largest ones, because oxygen demand scales with body mass.

Depth is a separate requirement from volume

Koi need a minimum depth of 60-90 cm (24-36 inches). This has nothing to do with how many gallons the pond holds. Depth is what buys you a stable temperature layer in a heatwave, a zone that does not freeze solid in winter, and water a heron cannot stand in.

A wide shallow pond can hit 2,000 gallons at 40 cm deep and still be unsuitable for koi. If you are in a climate with real winters or a summer over 30C, treat 90 cm as the target rather than the ceiling.