How to size a pond pump
A pond pump has one main job: to pass all the water through the filter often enough to keep it clean. Sizing it takes two numbers.
1. The flow. How much water must move each hour.
| What lives in the pond | Full pass | A 2,000 gal (7,570 L) pond needs |
|---|---|---|
| Plants and a few goldfish | Once every 2 hours | 1,000 GPH (3,785 L/h) |
| Koi, or plenty of goldfish | Once an hour | 2,000 GPH (7,570 L/h) |
| Heavily stocked with koi | 1.5 times an hour | 3,000 GPH (11,355 L/h) |
2. The head. How hard the pump has to push. Every pump moves less water the higher it lifts it, so the flow on the box (measured with no lift at all) is never what you get.
Worked example. A 2,000 gallon koi pond. The filter inlet is 3 ft above the water, on 20 ft of pipe with two sharp bends.
- Flow: 2,000 ÷ 1 = 2,000 GPH
- Head: 3 + (20 ÷ 10) + 2 = 7 ft
Buy a pump whose flow chart shows at least 2,000 GPH at 7 ft of head. A pump sold as "2,000 GPH" will fall well short.
Reading a pump's flow chart
Every decent pump comes with a chart or table of flow against head height. Find your head on it and read across to the flow. If the flow there is at least the figure from this calculator, the pump is big enough.
Ignore the largest number on the box. It is the flow at zero head.
Waterfalls
A waterfall needs enough water to cover its lip. Guides differ a little: Cotswold Koi gives 1,000 L/h for every 100 mm of width, and Pond Trade Magazine gives about 100 GPH for every inch, which is half as much again. The calculator shows both, and uses the lower figure as the least the pump must deliver.
If the waterfall needs more flow than the pond does, check that your filter can take that much, or give the waterfall its own pump.
Why the pipe matters
Water rubbing along a pipe and turning corners costs the pump as much as lifting it. As a rule of thumb, every 10 ft (3 m) of pipe counts as 1 ft (0.3 m) of extra lift, and so does each sharp bend and each check valve. Wider pipe and sweeping bends cut that loss.
This is a rule of thumb for normal garden ponds. For long pipe runs or narrow hose, the real loss can be higher.
Common mistakes
- Buying on the flow printed on the box. That figure is for zero lift.
- Forgetting the pipe. Twenty feet of pipe with a few bends can double the head.
- Using the height of the waterfall alone when the filter sits higher. Measure to the highest point the water reaches.
- Sizing for the fish you have now. Koi grow, and so does the load on the filter.
- Picking a pump with no flow to spare. Filters clog and pumps wear; a valve can turn a bigger pump down.
Questions and answers
What size pump do I need for my pond?
One that moves the whole pond through the filter once an hour for koi, or once every two hours for a lightly stocked pond, at your head height. A 2,000 gallon koi pond needs 2,000 GPH delivered, not just printed on the box.
What is head height on a pond pump?
It is how high the pump must push the water, from the pond surface to where the water comes out, plus an allowance for the pipe and its bends. The higher the head, the less water a pump moves.
How many GPH do I need for a 1,000 gallon pond?
About 1,000 GPH for a koi pond and 500 GPH for a lightly stocked pond, both measured at your head height. Heavily stocked koi ponds are better at 1,500 GPH.
How much flow does a waterfall need?
About 1,000 to 1,500 liters an hour for every 10 cm of width, which is roughly 65 to 100 GPH for every inch. A 12 inch waterfall needs about 800 to 1,200 GPH.
Can a pond pump be too big?
A little too big is fine: fit a valve and turn it down. Far too big can push water through the filter faster than it is built for, so check the filter's rated flow.
Numbers for standard tanks
Have a standard tank? Its size, weight, heater and stocking are already worked out.
- 10 gallon tank
- 20 gallon long tank
- 29 gallon tank
- 40 gallon breeder tank
- 55 gallon tank
- 75 gallon tank
- 125 gallon tank
- All tank sizes
Sources
- All Pond Solutions, "How do I calculate the right pond filter and pump size": the whole pond through the filter once an hour for koi, at least once every two hours for lighter ponds
- Play It Koi, pond pump sizing guide: one turnover an hour, 1.5 to 2 for heavily stocked koi ponds
- Cotswold Koi, "What size pond pump do I need": 1,000 L/h for every 100 mm of waterfall width; lightly stocked ponds once every two hours
- Jeff Richardson, "Pond basics: pumps" (Play It Koi): 10 ft of tubing, a 90° fitting or a check valve each add 1 ft of head
- Pond Trade Magazine, "A contractor's guide to choosing the right pump": about 100 GPH for every inch of waterfall width