Garden Pond Size & Fish Stocking Calculator

Garden Pond Guide

What should I know about Pond Volume Calculation?

Rectangular pond: length × width × average depth = volume in m³. Multiply by 1,000 for litres. Circular pond: π × (diameter/2)² × depth. Oval pond: π × (length/2) × (width/2) × depth. For irregular shapes: use the 0.8 multiplier for a conservative estimate: length × width × depth × 0.8. Average depth: most ponds have shelves — a pond with a 1m deep central area and 0.3m shelves has an average depth of perhaps 0.6m. Measure or estimate the average depth across the entire area. A 3×2m pond with 0.

What should I know about Fish Stocking Levels?

Goldfish: maximum 25cm of fish per 1,000 litres (1 small goldfish is approximately 10-15cm). Better guidance: 50-100 litres per goldfish (allowing for growth). Example: 3,600 litre pond can support 36-72 goldfish by the 100L rule — but 20-30 is far better practice. Koi carp: minimum 1,000 litres per koi. Better: 2,000-5,000 litres per koi for healthy water quality. Koi grow to 50-90cm — they need very large, well-filtered ponds. Dense stocking: any stocking above minimum levels requires excellent filtration and more frequent water testing to keep ammonia and nitrite safely under control.

What's the key thing to understand about Pump and Filter Requirements?

Rule of thumb: pump should turn over the pond volume once per hour minimum. For a 3,600 litre pond: pump rated at minimum 3,600 L/hour. With a biological filter: pump must also push water through the filter resistance — buy a pump rated at 50% more than the pond volume per hour to account for filter head loss. UV clarifier: sized by pond volume — a 3,600L pond needs a UV unit rated for 4,000L minimum. UV kills suspended algae (green water), not blanketweed. Aeration: add an air stone or waterfall feature to keep oxygen levels healthy, particularly important in summer when warmer water holds less dissolved oxygen.

What do I need to know about Pond Depth and Wildlife?

Wildlife ponds (no fish): minimum 60cm deep for overwintering insects and amphibians. Gently sloping edges at one end for amphibian access. No fish — they eat frog spawn, tadpoles, and aquatic invertebrates. Plant 2/3 of the surface with native aquatic plants (not invasive species). Fish pond: minimum 80cm depth to prevent freeze-through in winter. A deeper section (1.2m+) gives fish a refuge in cold weather. Avoid plastic ponds below 900 litres — too small for a stable ecosystem, and too easy to overstock or overheat compared to a larger, more forgiving volume of water.