The right pump can be a crucial part of a garden pond's ecosystem. By circulating the water in your pond, a pump helps keep fish healthy by maintaining safe levels of dissolved oxygen. Keeping the water moving also helps to promote aeration and nutrients throughout the pond and discourages the growth of pests like mosquitos and algae. Equipment like filters, waterfalls, and fountains each need specifically sized pumps to function well.

Most pumps are either submersible, meaning they sit on the bottom of the pond, or external, meaning they're positioned above the water line. Pumps come in a variety of sizes and strengths, and the type you choose should depend on what you need it for. You wouldn't use the same engine to power a diesel truck and a compact car. In the same way, you'd need a different pump to circulate water in a large, heavily stocked pond than you would to supply a small waterfall.

To size a pump, you need to consider two important numbers: flow rate and head. Together, they tell you how much water a pump can move and how much resistance it can overcome.

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Pump Head vs Flow Rate - What's The Difference? (Flow Definitions)

1) Flow Rate & Head Height

The first step in sizing a pump is finding the flow rate you need. Flow rate refers to the volume of water moved per unit of time. Depending on where you live, flow rate is usually given in gallons per hour (GPH) or liters per hour (LPH). The more energy your pump can supply, the more water it can move at one time.

Once you know the flow rate you need, the next step is to estimate your setup's head height. Pumps don't supply the same flow rate in every situation. Instead, a pump's flow rate varies based on a quantity called head height, or just head. Head is an engineering term that refers to the height at which a pump lifts water above the surface of a pond. As head increases, a pump's flow rate decreases. That's because the higher the water travels, the more gravity and friction resist its flow. Stronger pumps can supply larger flow rates at higher amounts of head. Your pump should provide your desired flow rate at your system's head height.

2) Total Dynamic Head (TDH) & Friction Head

The overall head height of a pond configuration is called its Total Dynamic Head (TDH). Precise TDH calculations are fairly complicated and can involve a lot of variables. In terms of a backyard pond, though, it's easy to simplify. Every foot of vertical distance between the surface of the pond and the highest point the water is pumped to counts as 1 foot of head. This distance is sometimes called static head.

True TDH calculations can also involve friction head and pressure head. Friction head describes how pipe size and material affect resistance to flow. Friction head usually doesn't make a big difference for garden ponds, and a detailed explanation is beyond the scope of this article. Still, there are a couple of easy ways that all pond owners can limit friction head. When choosing a pump, always plan to use the manufacturer's suggested pipe size. Smaller pipes mean less room for water and therefore less flow rate. Using a thinner pipe is a waste of your pump's power and can even damage certain setups. You can also account for friction by adding 1 foot of head for every 10 feet of horizontal piping between the pump and your filter or feature. Every 90° turn in the piping contributes 1 foot of head as well. For more exact calculations, you can use a friction loss chart. Keep in mind that these are often intended for professional landscapers.

3) Pressure Head & Max Head

Pressure head is the resistance from devices that use pressure to operate. You may need to include pressure head if your pump is going to power a pressurized filter, UV light, or spray nozzle. (Pressure head is easy to estimate, but if you happen to know a device's actual pressure, you can calculate it exactly using the conversion factor 2.31 feet = 1 psi.)

After adding up all your sources of head, you'll need to find a pump that supplies the right flow rate at that height. If you need 3,000 GPH at 10 feet of head, a pump that can give only you that flow rate at 5 feet won't be strong enough. Manufacturers usually list the flow rates their pumps provide at several head heights. One of these will be the max head height, which is the furthest distance a pump can lift water. At the max head height, the flow rate is basically zero. You can use pumps' max head heights to compare how strong they are.

These general guidelines are useful for understanding what flow rate and head height can tell you about a pump. The next step is applying them to your particular setup. Below, we've laid out some specific details about sizing pumps for water features, and for filtration and circulation.

What Size Pump For Water Features?

The size of your pump directly affects the appearance of water features like fountains and waterfalls. For simplicity, we'll stick to setups where the water feature is the only thing connected to the pump. If you want to include other components, like extra filters, you'll need to account for them as well.

For a fountain, the pump's strength determines how high and forcefully the water climbs. To figure out the flow rate you'll need, measure the diameter of your fountain's outlet. For every inch, add 100 GPH to your flow rate.

Next, find your fountain's head height by measuring the vertical distance from the outlet to the spot where the pump will lie. Remember that if your pump's max head height equals this distance, the water leaving the fountain will be only a dribble (if that!). A pump with a max head height that's about 1.5 times greater than this distance can supply a healthy stream of water to the top of the fountain. For example, if your fountain's head height is 22 inches and its outlet diameter is 3 inches, you'll want a pump that provides at least 300 GPH (3 * 100 = 300 GPH) and has a max head height of about 33 to 35 inches (22 * 1.5 = 33).

Sizing a pump for a waterfall is similar, but there are a couple of twists. To find the head height, measure the vertical distance from the surface of the water to the top of the waterfall. Remember to add 1 foot of head for every 10 feet of pipe that will connect the pump and waterfall. Next, measure the width of the waterfall where the water will spill over.

To create a moderate flow, look for a pump that supplies a flow rate of 150 GPH for every inch of width. To make the flow gentler, adjust this figure to 100 GPH; to make it more powerful, adjust to 200 GPH per inch. For example, if your waterfall is 20 inches wide, you'll need a flow rate of about 3,000 GPH for a moderate flow (20 * 150 = 3,000 GPH). If your waterfall is 6 feet tall from the pond surface to the top and is connected with 20 feet of hose, your pump will need to provide that 3,000 GPH at 8 feet of head height (6 + 20/10 = 8 feet).

What Size Pump For Fish Ponds?

When sizing a pump for circulation, the most important factors are the volume of your pond and the amount of fish you have. If your pond is densely stocked, your pump should circulate the entire volume of water every hour. That means that if your pond is 1,200 gallons, your pump should supply a flow rate of 1,200 GPH at the head height of the outlet. For ponds with lighter stocks (or no fish at all), pumps that move half the volume may be enough. To estimate volume in gallons, multiply your pond's length * width * depth * 7.5 (measure in feet for this formula). If your pond is irregularly shaped, use the deepest, widest, and longest points. Too little circulation can hurt your pond's ecosystem, so it’s usually better to overestimate than underestimate volume.

Flow rate is equally important for pumps powering filters. If the flow is too slow, the filter won't clean the water fast enough, and water quality will suffer. Conversely, if flow is too fast, there might not be enough time for biological filters and UV clarifiers to effectively process the water. To find the right balance, look for a pump that comes close to, but doesn't exceed, your filter's maximum suggested flow rate.

Your filter may also add pressure head to your system. Most pressurized filters add 3 – 5 feet of head. A few models add up to 10 feet, which you'll usually see in the user manual. UV clarifiers add 1 – 2 feet. Make sure to clean your filter regularly, too. As filters become dirtier or clogged, less water can pass through them, increasing the pressure.

Pump Size for an Entire System

In most cases, we don't recommend using a single pump for both a filter and a water feature. There are multiple reasons why it's better to use separate pumps for these applications. Filters need to run all day, but you might want more freedom to turn off a water feature. Also, many filters can't handle the flow rates that waterfalls and fountains require. Using separate pumps can save you in energy costs, give you more flexibility, and help to keep water features and filters working well.

Occasionally, though, you may be able to make one pump work for an entire system like this. For those intent on doing this, we've provided two formulas, one for a fountain and one for a waterfall below. There are three things to note about these formulas, though:

1. These formulas only work for systems with one filter and one water feature. This is because water features release water directly back into the pond, meaning there's no way to connect more than one along the same path.
2. The formulas provide only rough estimates. Your calculation may need additional adjustments depending on your specific situation.
3. The formulas don't account for pond volume. The flow rate you need for your water feature and the flow rate needed to properly circulate the pond water need to match at least somewhat closely. If they don't, your pond water may not circulate often enough, or your filter and water feature may not function properly.