Koi are not big goldfish. They grow to 18–24 inches (45–60 cm) and more, live for decades, eat a lot and produce a lot of waste. A pond that is comfortable for a dozen goldfish can be dangerously small for three koi. Depth and volume are the two numbers that matter most, and both are much easier to get right before you dig than after.
How deep should a koi pond be?
The usual minimum is 3 ft (90 cm), and many keepers build 4–6 ft (1.2–1.8 m). Depth does several jobs:
- Temperature stability: deep water warms and cools slowly. In summer the bottom stays cooler and holds more oxygen; in winter the bottom stays unfrozen at about 4 °C (39 °F) while the surface freezes.
- Protection from predators: herons wade; they can’t fish water deeper than their legs. Steep sides at least 2–3 ft deep are a strong deterrent.
- Swimming room and body shape: koi are big, active fish. Deep water lets them swim vertically as well as horizontally, which breeders believe helps body shape.
Avoid marginal shelves around a koi pond’s whole edge — they are a heron’s ladder and koi uproot the plants anyway. If you want marginal plants, put them in a separate filtered planted area.
How many gallons does a koi pond need?
A common guideline is at least 1,000 US gallons (about 3,800 litres) for a koi pond, and about 250 US gallons (950 L) per adult koi. Serious hobbyists and show-fish keepers often give each fish far more — 500 to 1,000 gallons or more. The calculators on this site use the 1,000-gallon minimum and 250 gallons per fish as the upper stocking limit.
| Pond (rectangle) | Average depth | US gallons | Litres | Koi (max) |
|---|---|---|---|---|
| 10 × 6 ft | 3 ft | 1,346 | 5,097 | 5 |
| 10 × 8 ft | 4 ft | 2,394 | 9,061 | 9 |
| 12 × 8 ft | 4 ft | 2,873 | 10,874 | 11 |
| 15 × 10 ft | 4 ft | 4,488 | 16,990 | 17 |
| 20 × 12 ft | 5 ft | 8,977 | 33,980 | 35 |
| 25 × 15 ft | 5 ft | 14,026 | 53,094 | 56 |
Those figures assume adult size. Young koi bought at 4–6 inches grow fast — a 6-inch koi can be 18 inches in three or four years with good food and warm summers. Stock for the fish they will become.
Filtration and turnover
Koi ponds are usually built around their filtration. Rules of thumb:
- Turnover: the whole pond through the filter 1.5–2 times an hour — a 3,000-gallon pond needs 4,500–6,000 GPH of real flow at the working head. See the pump sizing guide.
- Bottom drain: a drain at the deepest point, gravity-fed to the filter, removes waste before it breaks down. It is far easier to install during construction than later.
- Mechanical then biological filtration: a settlement chamber, brushes, sieve or drum filter to remove solids, then biological media where bacteria convert ammonia to nitrite and then nitrate.
- UV clarifier: about 1 W per 50 US gallons for koi ponds. Use the UV calculator.
- Aeration: an air pump with diffusers keeps oxygen up on warm nights, when koi are most at risk.
Worked example
You have space for a 12 × 8 ft pond. At a flat 4 ft depth it holds 12 × 8 × 4 = 384 ft³ × 7.48 = 2,873 US gallons (10,874 L). By the 250-gallon rule that is up to 11 koi; by the more generous 500 gallons per fish, 5 or 6. The pump should deliver 4,300–5,700 GPH at the filter’s head, and the UV should be about 57 W — in practice a 75 W unit, the next common size up.
The same footprint dug to 3 ft holds 2,155 gallons: a quarter less water for the same surface area. Depth is usually the cheapest way to add volume, since digging deeper takes no more garden.
Shape
Simple shapes with smooth, rounded corners circulate better than complicated ones. Dead spots in tight corners collect waste. A rectangle or oval with the inlet at one end and the bottom drain near the middle lets the current carry waste to the drain.
Liner and construction
For a liner pond, size the liner from the full depth (liner sizing guide). For a 12 × 8 × 4 ft pond with 1 ft overlap, that is 22 × 18 ft. Many larger koi ponds are block-built and rendered or fibreglassed, which allows vertical walls and keeps herons out.
Water weight
A 3,000-gallon pond holds about 25,000 lb (11,400 kg) of water. In the ground that doesn’t matter; on a raised pond or a deck it is a structural load for an engineer to check, and raised pond walls must resist the outward pressure of the water.
Summary
- At least 3 ft (90 cm) deep; 4 ft or more is better.
- At least 1,000 US gallons (3,800 L), and about 250 gallons per adult koi as a maximum.
- Pump turnover 1.5–2 times an hour; bottom drain; mechanical plus biological filtration; UV about 1 W per 50 gallons; aeration.
- Stock for adult size and add fish slowly.