At the very core of most ponds lies a submersible pump. The pump typically operates 24 hours a day throughout the water gardening season. Not only does the pump aerate and circulate water, but it is the power behind such features as a fountain, stream, waterfall, filter and ultra violet clarifier.
 
A submersible pump can have a direct or magnetic drive. A direct-drive pump is more powerful in terms of the height and length it can push water. This type of pump is suited to a large pond, waterfall, filter system, or stream.
 
A magnetic-drive pump uses far less wattage, making it energy efficient. It lacks the power of a direct-drive pump. This type of pump is suited to a medium sized pond, fountain, waterfall, or simple water circulation and aeration.
 
There are two methods used to determine the right sized pump for a pond:
 
1. one is based on circulation and the number of fish in the pond; and
2. the other is based on the power required for a water feature.
 
Use the following formula to calculate pond volume in US gallons. For all shapes, the dimensions should be in feet.
 
Rectangular pond = length x width x average depth x 7.48
Circular pond = diameter x diameter x average depth x 5.9
Irregular pond = length x width x average depth x 6.9
 
1. If the only function of the pump is to circulate and aerate water for a large number of fish, then calculate the volume of your pond, and select a pump that will circulate 50% of the volume per hour.
 
Example:
 
VOLUME of a rectangular pond (8ft x 7ft x 2.5ft):
 
8 x 7 x 2.5 x 7.48 (rectangular multiplier) = 1,047 US gallons
 
CORRECT PUMP SIZE (allow 50% circulation) = 500-600gph
 
2. If the function of the pump is to power a waterfall or stream as well as circulate the water, then select a pump powerful enough to pump the desired flow rate at a particular height (vertical lift or head) AND to circulate 50-100% of the volume per hour.
 
To determine the correct pump size for a waterfall or stream:
 
measure the width at the top of the waterfall and/or stream in inches;
select the desired water thickness from the chart (see page 11); and
apply the appropriate formula.
 

Sizing a Waterfall/Stream Pump
Gentle Falls ¼“ water thickness	allow 50gph per inch of waterfall width, allow for the vertical lift from the pond, and allow 1ft head for every 10ft of hose used
Standard Falls ½“ water thickness	allow 100gph per inch of waterfall width, allow for the vertical lift from the pond, and allow 1ft head for every 10ft of hose used
Splashing Falls 1“ water thickness	allow 150gph per inch of waterfall width, allow for the vertical lift from the pond, and allow 1ft head for every 10ft of hose used
Standard Stream	allow 100gph per inch of stream width, allow for the vertical lift from the pond, and allow 1ft head for every 10ft of hose use

Tip: Try not to circulate more than the pond’s volume in one hour.
 
At first glance, the above chart can seem confusing. The table below illustrates it better.
 
Example Pond for Waterfall Flow Rates:
 
Pond Volume = 3,105 US gallons
Waterfall = 3ft high x 30 inches wide
Vertical Lift Allowance = 3ft (waterfall height) + 2ft (20ft of hose) + 1ft (filter) = 6ft
 

		GPH @ TOP OF WATERFALL
Volume	Lift	Gentle Falls	Standard Falls	Splashing Falls
3,105 gal	6ft	1,500 gph	3,000 gph	4,500 gph
Pump Size @ 6ft Lift		2,000-2,500gph	3,500-4,000gph	**

** The volume would have to be turned over 1.5 times in an hour to achieve a one inch water thickness. The maximum turn over rate should be the pond volume per hour. In this case, splashing falls would not be recommended.
 
Example Stream for Stream Flow Rate:
 
1st Pond Volume = 1,795 US gallons, 2nd Pond Volume = 531 US gallons = 2,326 US gallons
Stream = 15ft long x 18 inches wide x 1ft incline
Formula = 100gph x 18 inches (stream width) = 1,800 gph
Vertical Lift Allowance = 1ft (vertical incline) + 3ft (for 30ft of hose) = 4ft
Total is 1,800gph @ 4ft (vertical lift)
 
Correct Pump Size = 2,000 - 2,400 gph
 
Tip: When buying a pump, it is better to oversize than to undersize.
 

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Web Administrator - Isa Webb.
Last Updated: Wednesday February 24, 2010