Garden Pond Size & Fish Stocking Calculator

Garden Pond Guide

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.

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 excellen

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 waterfal

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, too easy to ov