Sizing Your Pump

Pump Selection

By Brian Morgan

To size a pump for your pond, first calculate the pond volume using this formula.

Avg. length x avg. width x avg. Depth x 7.5

This will give you a close estimate of the gallons. For small ponds less than 2000 gallons I like to circulate the water twice every hour, Larger ponds once an hour may be sufficient. This is the filtration flow rate.

The next factor to consider is the lift head loss. Head is the height water must be pumped above water level to the highest point. Friction head is the loss of flow due to plumbing. The larger the diameter hose, the less loss. Generally speaking 10’ of horizontal run equals 1’ of head. A 90 degree elbow equals 10’ of hose. See attached chart for hose size factors. Find flow rate and hose size and multiply the factor by the length of hose.

The final factor to consider is the waterfalls. How much water is required. 100 gph will produce a flow 1” wide 3/16 – 1/4 “ deep. A 12” waterfall will require about 1200 gph.

Lets do an example, say your pond will be 10’ x 15’ x 1.5’ deep with a 15” wide waterfalls 6’ high, you are using 25’ of 1.5” hose and 1 90 degree elbow.

10’ x 15’ x 1.5 x 7.5 =1687.5 gal. ( Double this for the filtration flow rate )
Filtration flow rate = 3400 gph
Waterfall flow rate = 1500 gph
Select the higher of the two
Next calculate the head ( add lift head and friction head) 6’ high falls (lift head) + 25’ of 1.5” hose, add 10' of hose for elbow (.18 from chart x 35 = 6.3’ friction head) = 12.3’ of head.

You should select a pump capable of producing between 2000 - 3500 gph @ 12' head.

Three types of pumps are suitable for most pond applications. The first two are submersible pumps and are placed in the pond or skimmer. The third is an external pump.

The first are mag-drive or magnetically driven. Mag-drives are energy efficient, competitively priced, most are rebuildable and have a descent life span, However mag-drive pumps suffer greater flow loss as the head increases.

Next come the waterfall or sump pump style. These pumps tend to have higher flow rates, will push water higher, move more debris and have more pressure than a mag-drive pump. They work great for external filters. The down side to these pumps is the operating cost. Some can cost twice what a comparable mag-drive will cost to run.

External pumps are generally used in large pond applications requiring substantial water flow, height or both. These pumps are located along side the pond and draw water in through an intake hose or bottom drain, through a leaf basket then into the pump. Cost and hiding the pump are two disadvantages.

Go Back