Choosing the correct pool pump can seem like a daunting task. Consumers are faced with new terms like "flow rate" and "head pressure" and have no real idea how to figure these rates. Using a few simple calculations, consumers can get a good idea of what pump will suit them best.

The first calculation that needs to be figured is called flow rate. This tells the amount of water that can be moved within a certain time period and is usually measured in gallons per minute (gpm). To figure this, consumers first have to know how many gallons of water are in their pool.

To figure the gallons in the pool, first multiply the length and width of the pool by the depth. For example, for a pool that is 10 feet (3.0 meters) wide and 20 feet (6.1 meters) long and 5 feet (1.5 meters) deep, multiply 10 times 20 times 5 to get a total of 1000 cubic feet (28.3 cubic meters). There are about 7.5 gallons (28.4 liters) in one cubic foot of space, so now multiply 7.5 by 1000. The swimming pool in this example has approximately 7,500 gallons (28,390 liters).

The average pool needs to circulate the equivalent of all of the water in the pool about every four to six hours at least once or twice a day. This does not mean that all the water will be circulated, but it ensures that a large portion of it will be. To figure out the flow rate, the user must decide how often they want the water to fully circulate.

For example, for the 7,500 gallon (28,390 liter) pool explained above, a consumer decides they would like the pool to fully circulate every six hours. Divide the total gallons by six. That means there are 1,250 gallons (4,731 liters) per hour that need to circulate. Divide that number by 60, the number of minutes in an hour, to find the gpm. For the example here, the flow rate would be right around 20 gpm.

Some will stop here and assume that they simply need a pool pump with a flow rate of 20gpm. This calculation does not take into account the resistance to flow that occurs from the drag created within piping and through filtration systems. This resistance is called head pressure. It is calculated in feet of head.

To determine the exact head pressure is a difficult and mathematically challenging task. Instead, pool owners can use a quick at-home estimation. First, figure out how many feet of piping there is between the pump and the area where water is put back into the pool. This is the initial pressure and the starting figure for head pressure.

Now, if a filter is used to draw and return water, divide the number in half since that assistance will lessen the difficulty of transporting water. For any areas where the diameter of the return line is changed at, add a number equal to the percentage of change. For example, if using a 1 inch (2.54 cm) pipe for 10 feet (3 meters) that changes to 3/4 inch (1.9 cm) pipe, add 25% more, or 2.5 feet (.7 meters), to the calculation.

Every 90 degree turn adds another one to two feet of head. Any inline filters will add two to ten feet of head. If the filter uses a course material, estimate on the lower end. The finer the filter type, the more feet added.

If the water is returned at the water surface of the pool or above, that is the end of calculations. If it is below the surface, add one feet of head for each foot under the surface. Generally, an in-ground pool will have 50-60 feet of head pressure, while an above ground will have around 30 feet of head pressure.

After the head loss has been calculated in feet, check the manufacturer's charts to see what pool pump will work best for the particular setup. Each pool pump can handle various flow rates and head losses with different horsepowers.

It is important not to choose a pool pump larger than needed. A too-large pump uses extra energy and contributes to higher than necessary energy bills. Using these simple calculations, pool owners can enter a shop prepared to purchase the correct pump for their individual setup.