Pond Pump Size Calculator
Estimate your pond volume, the flow to aim for and the head your pump must overcome, then take those numbers to a pump curve.
Pond pump size calculator
Estimate your pond volume, the flow you should aim for, and the head your pump has to overcome. The result is a planning estimate to take to a pump curve, not a guaranteed specification.
How this calculator works, and its limits
- Volume is length × width × average depth. Ovals and circles use the ellipse formula. An irregular pond is treated as filling 70% of its length × width rectangle, which is an assumption: freeform ponds usually fall between 60% and 80%.
- Target flow is a turnover range, not one number: about half to once the pond volume per hour for a planted pond with no fish, once to one and a half times for goldfish, and once to twice for koi. These are widely used rules of thumb and manufacturers differ, so follow your filter manual where it is more specific.
- Waterfall flow uses roughly 50, 100 or 150 US gallons per hour for each inch of spillway width for a gentle, even or strong flow.
- Head is your vertical lift plus pipe friction from the Hazen-Williams equation at the target flow, plus an equivalent pipe length of 30 pipe diameters for each 90° elbow. Filters, UV clarifiers, valves and nozzles add more, and only their manuals can tell you how much.
- What it cannot do: predict what a specific pump will deliver. That depends on the pump’s own performance curve. Use the flow and head shown here to read a manufacturer’s curve, and never exceed the maximum flow of your filter or UV clarifier.
What to do with your result
- Note the flow range and the head. These two numbers describe what your system needs.
- Find a pump curve. Look in the manual or specification sheet of each pump you are considering for the chart or table of flow against head.
- Read the flow at your head. Choose a pump whose flow at that head falls inside your range.
- Check the limits. Make sure that flow does not exceed the maximum for your filter or UV clarifier.
- Compare running costs. Between suitable pumps, the one that uses fewer watts will usually cost less to own.
Learn the method
- What size pond pump do I need?The full sizing method with a worked example.
- Head height explainedWhat counts as head and how to read a pump curve.
- Match a pump to a filterFind the flow window your filter and UV allow.
- Compare pond pumpsPublished figures for three pumps side by side.
Electrical safety
Use outdoor-rated equipment on a GFCI (RCD) protected supply, follow the manufacturer’s manual and your local electrical code, and use a qualified electrician for fixed outdoor wiring.