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Swimming Pool Calculations

When purchasing a new swimming pool or swimming pool product there are several calculations that can be helpful. Below are a few calculations that may help determine the size of your swimming pool, heater, filter, or many other products. Please select from the following calculations...

Surface Area
The surface area of a swimming pool can help determine the BTU size of pool heaters.

Pool Shape Formula

Rectangular		 Surface Area = L X W

Round		 Surface Area = 3.14 X (D/2) x (D/2)

Kidney		 Surface Area = .45 X (A + B) L

Gallons of Water
Determining the gallons of water in your swimming pool can help determine such thing as the size of your pump and filter. The gallons of water depends on the shape of your pool.

Gallons of Water calculator


aD = Average Depth

Pool Shape Formula

Rectangular		 Gallons = Length x Width x Average Depth x 7.5

Round		 Gallons = Diameter x Diameter x Average Depth x 5.9

Oval		 Gallons = Long Diameter x Short Diameter x Average depth x 5.9

The Horse Power of Inground Pool Pumps

The three best manufacturers of pool pumps are Hayward, Pentair, and Sta-Rite. Each manufacturer sells different models of pumps. These models range in performance, and are rated by their Horse Power (HP). There are two important factors when trying to figure out which HP pump, the gallons per minute (GPM) and the Amount of head in feet. In order to figure out what ideal GPM you need for your pool pump follow these steps.

  • To figure out the amount of gallons in your pool use this calculator
  • A good rule of thumb is you should be able to clear your swimming pool in an 8 hour period of time. Take the number of gallons and divide that by 8.

Ideal Gallons Per Hour (GPH) = Gallons of pool water / 8

  • Next take the GPH and divide that by 60

Ideal GPM = GPH / 60

Now that you have the ideal GPM you need to figure out the Feet of head of your pump. A good estimate is to take the amount of feet from where your suction lines are (skimmers or main drain) back to where your pump will be located. The total amount of return piping should give you a good idea on how many feet of head your pump will have.

Many of the Swimming pool pumps will have charts showing the Horse Power of each pump. These charts are usually shown with the GPM vs. The Feet of Head.

For example, if you have a 20,000 Gallon pool.

GPH = 20,000 / 8 = 2,500

GPM = 2,500 / 60 = 41.7

Now that you have your GPM, we need to make a couple more assumptions. Let's say you choose the popular Hayward Super Pump and you have found your total return piping to be a total of 30 feet. N ext look at the chart listed on Hayward Super pump description page (see below)

  Pump Output (GPM) vs. Total Resistance To Flow (Feet of Head)
Model No. Max Rated 20 ft 30 ft 40 ft 50 ft 60 ft 70 ft
SP2600X5 55 45 29 -- -- --
SP2605X7 67 58 47 31 -- --
SP2607X10 85 76 65 50 27 --
SP2610X15 97 90 80 67 50 10
SP2615X20 116 111 99 85 70 51
SP2621X25 109 109 104 95 84 69

With 30 feet of head and 46.7 GPH we would use the SP2605X7 (always go to the higher size GPH, 58 in the example). The SP2605X7 is a 3/4 HP pump.

swimming pool pumps

The Size of Inground Filters

The most important factor of the size of the Swimming Pool Filter is the GPM your Pool Pump produces. Filters have a designed flow rate. This designed flow rate should be higher than the size of your GPM your pool pump produces.

In the example above we were using the Hayward Super pump that produces a 58 GPM at 30 feet of Head. If we were to choose the Hayward Star Clear Plus as our chosen filter we would use the chart found on the description page

Performance Data--Residential

  Turnover (In Gallons)
Model Number Effective Filtration Area Design Flow Rate* 8 Hours 10 Hours
C7512 75 ft2 75 GPM 36,000 45,000
C9002 90 ft2 90 GPM 43,200 54,000
C12002 120 ft2 120 GPM 57,600 72,000
C17502 175 ft2 120 GPM 57,600 72,000

The size of the filter that could handle the 58 GPM Super Pump would be the C7512.

swimming pool filters

The Size of the Above Ground Pump and Filter System

The size of your system is mostly determined on the size of your above ground pool. The gallons found inside your pool will give you the horse power needed for your pump. A simple rule to follow is you should be able to clear all of the gallons of your pool in an 8 hour period of time. You can use the following as a guideline.

• Above Ground Pool up to 24 Ft. Round = 1 HP pump
• Above Ground Pool over 24 Ft. Round = 1 ½ HP pump

Determine the BTU Size for Swimming Pool Heaters

The Surface area will help determine the BTU size of your pool heater. Decide on the temperature rise you want per hour Then consult this chart. Heat rise will vary slightly when dealing with different heaters. We usually recommend an increase of 1 - 1 1/2 degree per hour.

  Pool Size -Square Feet
BTU Size 500 Sq. Ft. 650 Sq. Ft 800 Sq. Ft.
200 1 degree 1 -
250 1 1/2 1 1/2 -
350 2 1 3/4 1
400 2 1/2 2 1 1/4

pool heaters