By circulating the water in a pond, a pump helps keep it clean and healthy for fish and plants. Poor water circulation can result in stagnation, algae buildup, mosquitoes and other problems. Using an adequately sized pump helps prevent these problems. These tips can help you choose the right water pump size for a fish pond, aquatic garden or other pond.

Pond Size

To get the right size pond pump, you’ll need to know how much water the pond holds.

The most accurate way to figure this out is to use a flow meter that shows total volume on the water supply line when you’re filling the pond.

If you can’t do that, there are ways to estimate the volume of water in your pond. These formulas work with ponds that have simple shapes and straight-up-and-down sides:

• For a square or rectangular pond, multiply the pond length by the pond width by the average depth by 7.5 to estimate the volume in gallons.
  Formula: pond length x pond width x pond average depth x 7.5
• For a round pond, figure out the distance across the pond, which is the diameter. Divide the diameter by two to get the radius, then multiply the radius by itself to get the radius squared. Multiply the radius squared by 3.14 to get the surface area of the pond. Multiply the surface area by the depth by 7.5 to estimate the volume in gallons.
  Formula: pond radius x pond radius x 3.14 x pond depth x 7.5
• For an oval pond, measure the width at its widest point, the length at its longest point and the depth at its deepest point. Multiply the width by the length by the depth by 6.7.
  Formula: largest pond width x largest pond length x deepest pond depth x 6.7

System Head

To get the right size pump, you’ll also need to know how much gravity and friction the pump will be working against. This is the “head” of the system, an indication of how hard the pump will need to work to move water through it.

Start by taking the height in feet of the waterfall or fountain outlet above the water level.

Then, add another one foot of head for every 10 feet of horizontal pipe or tubing between the pump and the water feature or outlet. (If there's less than 20 feet of tubing, you can skip this step.)

Add one foot of head for every 90 degree angle in the line.

Add an additional foot of head for every filter, UV light or other component in the line. You can also check with the manufacturer about how much head these components add.

Circulation Rate and Flow

For ponds that are well stocked with fish and plants, experts recommend circulating the full volume at least once per hour. For ponds with few or no fish, it may be enough to circulate the water once every two hours. If you're using a pressurized filter, that may also lower your circulation requirements.

Ponds that are larger than 4,000 gallons may not need to have the water circulated as frequently, but they should have a water feature to help oxygenate the water.

Once you’ve decided on the circulation rate, use the pond volume to figure out the necessary flow.

For example, if you want to circulate the water in a 1,000-gallon pond once per hour, you’ll need to achieve a flow rate of 1,000 gallons per hour (gph) or 16.7 gallons per minute (gpm).

Sizing the Pump

Now it’s time to look at pump ratings.

Pumps are rated by their flow at a certain head. The head, or lift, is a measurement of how high the discharge point is above the surface of the water. As the head increases, the flow of the pump decreases, because it’s harder to push the water against gravity. Pumps are also rated by maximum head, which is the maximum height to which they can theoretically lift water.

For example, here’s how a pump that has a maximum head of 16 feet might be rated:

• 10 gpm of flow at 5 feet of head
• 2 gpm at 10 feet of head
• Less than one gpm at 15 feet of head

To get the right pump for your pond, look for a pump that can achieve your desired flow rate at the head of your system. It's better to choose a pump that's a little more powerful than you think you'll need. If the pump is too powerful, you can always reduce the output – but if the pump doesn't have enough power, you can't do anything about it. 

Other Considerations

Keep in mind that pumps have different cord lengths. Make sure the cord is long enough plug in far away from the water. Some electrical codes specify that the outlet for water features must be at least 6 feet from the water. Make sure to use an outlet designed for outdoor use, with a ground fault circuit interrupter (GFCI) that will immediately shut off if there’s an overload.

It's also important to use the correct tubing size. This has a direct effect on the pond pump’s maximum lift capability. If you use smaller tubing than specified, you’ll limit the pump’s maximum lift and the amount of water circulated.

Submersible vs. Non-submersible Pumps

There are two basic kinds of pond pumps: submersible and non-submersible.

Submersible pumps are designed to be submerged underwater at the deepest part of the pond. They go directly into your pond, or in a skimmer box or pond vault. Submersible pumps range in flow rate from 50 to 5,000 gph. They’re easy to install and are sometime a more economical solution for smaller ponds (up to 1,000 gallons of water). They are also quiet and can also be used to drain your pond. If you have fish or other aquatic life in your pond, you may want to consider a model that doesn’t use oil because there’s a danger of the pump seal breaking and oil coolant leaking into the water.

Non-submersible pond pumps are a reliable, energy-efficient option. They’re installed in a dry location near your pond. Non-submersible pond water pumps are suitable for larger ponds (over 1,000 gallons). They’re typically louder and more complicated to install than a submersible pump, but they’re also easier to maintain.