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Important Formulas

Effective Pond Volume

When selecting the proper size equipment for your pond, actual volume alone is not enough. You must determine the effective volume of your pond which is influenced by various environmental factors. Determine if your pond is affected by direct exposure to sunlight, shallow depth, or climate conditions, and add to the actual volume by the factors listed below:

Average pond water depth is less than 2' 6": + 25 %
Pond is located in full sunshine: + 25 %
Pond is located in subtropical climate (e.g. Florida): + 35 %
Pond is located in temperate climate (e.g. Eastern Seaboard, Southern U.S.): + 15 %
Pond is located in Northern climate: + 0 %

Example 1
If you have a 1,500 gallon pond, 2' deep and exposed to full sunshine in Kentucky, your pump and/or pond filtration equipment would need to be increased by 65 % (25 + 25 + 15). You would therefore base your selection on a pond volume of 2,475 gallons.

Example 2
If you have a 2,500 gallon pond, 4' deep, located in full sunshine and you live in a Northern climate, the effective volume of your pond is 3,125 gallons (2,500 + 25 %).

Allowance for Fish Stocking Level

The information listed above is based on a fish stocking level of not more than 100" of fish per 1000 gallons of pond volume. Any variations in stocking level above this number will require a pro-rata increase in the size of all equipment. Thus, a 2000 gallon pond stocked at 150" of fish per 1000 gallons will require equipment appropriate for a 3000 gallon pond, i.e. 50 % more fish requires 50 % greater equipment capacity.

Total Head

The higher the pump must push the water, the less water will be pumped. The terms "head" or "lift" are used to indicate the rise, measuring how high the water must be pumped for a particular application.

Pumping water through tubing (to a waterfall, for example) adds resistance. Please calculate friction loss as per the chart below.

Friction Loss Chart

Add the allowance for friction loss to the vertical distance (in feet) that you will be pumping the water.  The vertical lift is measured from the surface of the pond. The resulting sum will be the TOTAL head that the pump will be required to pump. You should compare the amount of flow that you require to the flow rate that the pump provides at this specific head.

Vertical distance between pond surface level and the top of the waterfall is 3 feet. You have 20 feet of tubing between the pump and the waterfall. Using a 3,200 GPH pump with 1-1/2" tubing, your total head is approx. 5.4 feet (3 ft. + 2 x 14.4 in.). Using a 3,200 GPH pump with 2" tubing, your total head is approx. 3.8 feet (3 ft. + 2 x 4.8 in.).

Tubing Flow Rates

The tubing size running from the pump is determined by the maximum flow rate of the pump you select. Pick the tubing diameter that is most appropriate for the volume of water coming from the pump. A hose adapter or a combination of adapters is required to attach the hose to most pumps. Following are the maximum flow rates in GPH for various tubing diameters:


Maximum Flow
Required Tubing Size
(Inside Diameter)
300 1/2"
720 3/4"
1,200 1"
2,000 1-1/4"
3,000 1-1/2"
4,800 2"
6,000 2-1/2"
9,000 3"
12,000 4"

If your pump does not deliver the amount of water it is rated for, perhaps you are using the wrong size tubing.

Recommended tubing sizes are listed for each pump that we sell. Please click on the pump model number for more details. The requirements of different pumps can vary and the above chart is meant as a guideline only.

Waterfall Weir Rates

When operating a waterfall, an important consideration is appearance. The volume of water required to achieve different effects (a robust waterfall or just a trickle)  will depend upon the width of the waterfall lip (weir) or stream and the material that it is constructed from. The chart below will tell you how much water is required PER inch of waterfall width to achieve different thicknesses of water over the entire width of the waterfall weir. Once you have calculated the total head of the waterfall, it becomes quite easy to determine which pump to use.


Desired Water Thickness
Sharp Metal Weir
Desired Water Thickness
Stone Weir
6" to 11" Wide
Desired Water Thickness
Stone Weir
12" or Wider
Required GPH per Inch of Waterfall Width
1/4" 3/16" 1/8" 30
3/8" 5/16" 1/4" 50
1/2" 3/8" 5/16" 75
3/4" 9/16" 7/16" 140
1" 3/4" 5/8" 200
1-1/4" 1" 3/4" 275
1-1/2" 1-1/4" 1" 375

You have a 3' tall (above the surface of the pond) waterfall and will have 15' of tubing run between the pump and the top of the waterfall. The total head is thus 4.5'. To achieve a 3/4" water thickness over the width of an 8" wide stone waterfall weir, you would require a pump that would produce 1600 GPH (200 GPH per inch x 8 inches total width) at a total head of 4.5'.

You can also use this chart to predict the effect that you will get from different volumes of water.

If you use the same 3' tall and 8" wide waterfall as above with 15' tubing run and you have a pump that only supplies 500 GPH at 4.5' total head, you would expect to get 62.5 GPH per inch (500 divided by 8) over the 8" width of the waterfall weir. This would result in a little less than a 3/8" water thickness.

How to Measure a Flow Rate

This formula can be used to measure the flow rate of your pump.

Take a container of known volume (i.e. a 5-gallon bucket) and time how long it takes to fill it (in seconds) at the flow that you have. Then divide 3600 by the number of seconds it takes to fill the container and multiply by the volume (gallons) of the container. The result will be the flow rate in gallons per hour.

It takes 10 seconds to fill a 5-gallon bucket.
3600 : 10 seconds x 5 gallons = 1,800 GPH

You can also use this formula to decide how much flow you would like over a waterfall.  Simply place a garden hose at the top of the waterfall and adjust the volume of water until you find the flow that you like. Measure this flow and you will have an idea of the volume required to get the effect you desire.

Useful Conversions

You may find the following helpful:

To calculate power consumption: Volts x Amps = Watts
To calculate yearly cost of operation: Watts divided by 1000 x the price of electricity in $ per kilowatt hour x 24 hours x 365 days

One US gallon = approx. 0.834 Imperial gallons
One US gallon = approx. 3.78 litres
One Imperial gallon = approx. 1.20 US gallons
In most cases the flow rates for pumps in North America are given in US gallons

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