Garden Pond Volume & Pump Size Calculator

Garden Pond Guide

What should I know about Pond Volume Calculation?

Rectangular: V = length × width × average depth. Circular: V = π × (diameter/2)² × depth. Oval: V = π × (length/2) × (width/2) × depth × 0.8. Irregular: approximate as rectangle × 0.7. Convert to litres: multiply m³ by 1000. A common garden pond (2.5m × 1.5m × 0.6m average depth): V = 2.5 × 1.5 × 0.6 = 2.25m³ = 2,250 litres. Minimum pond depth: 60cm minimum recommended (prevents freezing completely in UK winters). 90cm+ in coldest areas. Koi ponds: minimum 1.5m deep (1m working depth + marginal shelf zone for planting), since koi need deeper water to avoid summer overheating and to give herons and other predators less opportunity to reach them. Irregular ponds can be estimated by breaking the shape into rectangular and circular sections, calculating each separately, and summing the volumes.

What's the key thing to understand about Pump Sizing?

The pump should turn over the full pond volume once every 2 hours (for ornamental ponds). Every 1-2 hours for fish ponds. Every hour for heavily stocked koi ponds. Formula: minimum pump LPH = pond volume in litres / 2. For a 2,250 litre pond: 2250/2 = 1,125 LPH minimum. Choose a pump rated at 1,500-2,000 LPH to allow for head loss (pipe height reduces pump output). Filter: the biological filter must process the full pond volume at least once per 2 hours. Under-filtering is the most common cause of cloudy water and algae blooms in garden ponds, so it's worth sizing the filter to the pond's full volume (or slightly above) rather than to the pump's flow rate alone. UV clarifiers, where fitted, should be sized to match the filter's flow rate for effective algae control.

What do I need to know about Fish Stocking Capacity?

General guidelines (excluding filtration allowances): Koi: allow 500-1,000 litres per koi (adult koi can reach 60-90cm). A 2,250 litre pond can hold 2-4 small/medium koi with a good filter. Goldfish: 50 litres per goldfish (adult = 20-30cm). 2,250 litres = up to 45 goldfish. In practice, allow more space for fish health and comfort. Wildlife pond (no fish): far higher biodiversity — frogs, newts, dragonflies, and native aquatic plants cannot coexist easily with koi (which eat frog spawn and vege

What do I need to know about Pond Liner Sizing?

Liner size: length = pond length + (2 × max depth) + 0.6m overlap. Width = pond width + (2 × max depth) + 0.6m overlap. Example: 2.5m × 1.5m × 0.8m deep pond: liner = (2.5 + 1.6 + 0.6) × (1.5 + 1.6 + 0.6) = 4.7m × 3.7m. Always buy slightly larger — liner cannot be joined easily in the middle of a pond. EPDM rubber liner: most durable (25-50 year lifespan). Flexible and easy to work with. Butyl rubber: professional grade, highly flexible, expensive. PVC liner: budget option — less durable (10-20 year lifespan) and less flexible than EPDM or butyl, but significantly cheaper — a reasonable choice for a smaller, simpler pond shape where the reduced lifespan is an acceptable trade-off. Whatever liner is chosen, use an underlay (old carpet or purpose-made pond fleece) beneath it to protect against punctures from stones and roots in the excavated hole.