http://www.haywardnet.com/inground/products/energysolutions/calculator/index.cfm?CFID=28795594&CFTOKEN=35914519
Step 1) Length, Width & Depth
Firstly measure your pool’s width, length and average depth. Write
the measurements down in feet or meters, using decimals to approximate
fractions of feet. (In other words, 20 feet, 3 inches would be 20.25
feet.)
Step 2) Calculate Pool Surface Area
Multiply the length of your pool by its width to determine its surface area.
Example: 10m x 5 = 50 m2 or 33ft x 16.5ft = 544.5 ft2
The surface area of a swimming pool can help determine the BTU size of pool heaters.
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 
Step 3) Pool Surface Area x Pool Depth
Multiply the pool’s surface area by its depth.
Example: 50 m2 x 1.2m (depth) = 60 or 544.5 ft2 x 4 (depth) = 2178
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.
aD = Average Depth
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 
Step 4) Calculate Pool Volume
If you have measured your pool in feet multiply the figure calculated
in Step 3 by 6.25 to determine how many gallons of water your pool
holds. This formula will work only if your pool is perfectly
rectangular. If you have measured your pool in meters, multiply the
figure calculated in Step 3 by 1.0 to determine how many cubic meters of
water your pool holds.
Example: 60 x 1.0 = 60 Cubic Meters or 2178 x 6.25 = 13612.50 UK Gallons
Circular Pools
Diameter (ft.) X Diameter (ft.) X Average Depth (ft.) X 5.9 = Total Pool Capacity in Gallons
Oval Pools
Length (ft.) X Width (ft.) X Average Depth (ft.) X 6.7 = Total Pool Capacity in Gallons
Rectangular Pools
Length (ft.) X Width (ft.) X Average Depth (ft.) X 7.5 = Total Pool Capacity in Gallons
Step 5) Convert UK Gallons to Cubic Meters
If you already have your pool volume in cubic meter then disregard this step.
As most swimming pool pump charts display cubic meters and not UK
gallons you must do this conversion now. To convert UK gallons to cubic
meters, divide the number of gallons you calculated in Step 4 by 220.
Example: 13612.50 Gallons divided by 220 = 61.88 Cubic Meters of Water
Choose a Swimming Pool Pump
Step 6) Approximate Pool Turnover Rate
A pool turnover rate is the amount of hours it takes to circulate the
whole of your pool water through your swimming pool filter. Please
select from the options below, the operation that relates to your
swimming pool.
Outdoor Domestic Pool – Heavy Use – 4 Hours
Outdoor Domestic Pool – Light Use – 6 Hours
Indoor Domestic Pool – 8 Hours
Commercial Pool – 2.5 Hours
Hydrotherapy Pool – 1 Hours
Step 7) Minimum Flow Rate
Divide the number of cubic meters of water your pool holds (Step 4/5) by the number of hours you selected in Step 6.
For example if you have an indoor pool measuring 10m x 5m x 1.5m you
would have calculated your pool to have 75 cubic meters of water (10 x 5
x 1.2 x 1.0m3).
60 cubic meters divided by 8 hours = 7.5 Cubic Meters / Per Hour
This figure will determine the minimum flow rate you need use to ensure the water in your pool can make a complete circulation.
Flow rate is simply the number of gallons the pump moves per
minute. Turnover is the minimum amount of time it takes to circulate all
the water one time through your pool's filter. In general, an average
turnover of every 6 to 8 hours is sufficient for most pools.
Use this formula to determine your turnover rate:

Step 8 ) Pipework Maximum Flow Rate
Determine the maximum flow rate your pool can handle by looking at
its plumbing system. Most domestic pools use 1.5 inch pipes, which can
handle about 9 cubic meters per hour on the suction side and 15 cubic
meters per hour on the return side.
A 2 inch pipe can handle more than this with about 15 cubic meters
per hour on the suction side and 25 cubic meters per hour on the return
side, while a 3inch pipe can handle more than 34 cubic meters per hour.
Use a slightly lower flow rate if your plumbing has a lot of
intricate turns and connections that could be stressed by too much
rushing water.
Step 9) What is your Pool Filters Maximum Flow Rate?
Solving for pump flow rate.
You need to check at this point to make sure your filter’s maximum
flow rate, specified by the manufacturer, will accommodate the size of
your pump you are going to choose.
If you do not have a swimming pool filter
you need to choose one of a larger size if you want the cleanest water
with the lowest amount of maintenance. You will need to clean the filter
when it becomes clogged with debris. A larger filter will take longer
to clog.
Example: Your pool has a minimum flow rate of 7.5m3 per hour and you
use 1.5 inch pipes which will give you a maximum head of 15 cubic meters
per hour but you have a 20 inch filter that has a maximum flow rate of
10 cubic meters an hour then 10m3 is the highest flow rate you should be
using. If you have a 30 inch filter that has a maximum flow rate of 21
cubic meters an hour than you are able to push this amount of water
through. However as you only have 1.5 inch pipework there is no point
going over 15 cubic meters an hour as there is only so much water you
can get through the pipe at any one time.
The size of your pool's pipes determines the maximum flow
rate in your pool. Count your pool's number of intake lines and refer
to the common pipe sizes below:
 For each 1.5" intake line, the maximum flow rate is 42 gpm.
 For each 2.0" intake line, the maximum flow rate is 73 gpm.
Example:
Two 1.5" intake lines = 84. The maximum flow rate is 84 gpm.
This number is important because your pool filter has a
maximum flow rate, which is measured in gpm. The pool pump's gpm rating
should be below the pool filter's maximum flow gpm rating. If the pool's
turnover rate is higher than the filter's maximum gpm, the filter is
undersized and will not work properly or become damaged. If the filter
is undersized, it should be replaced, or the pump should be undersized
to prevent damage to the filter.
Step 10) Flow Head
The total resistance to flow is measured in “meters of head”. As you
will notice from looking at the individual pump charts, the greater the
resistance to flow (expressed in meters of head), the lower the flow
rate. The greater the resistance to flow, the more powerful the pump
needs to be to overcome it.
As a general rule, most domestic pools have a head resistance of 10m.
Some installations do have a larger head and some may be smaller but we
take an assumption that 10m is the average.
Therefore when looking at the flow charts use 10m as the head.
Step 11) Selecting the Most Appropriate Pool Pump
Each pool pump will have a flow chart detailing which pumps have what
flow rate at what head. Select a pool pump with a flow rate between
your minimum and maximum allowance taking into consideration your
filters maximum flow rate.
Example: Your pool has a minimum flow rate of 7.5m3 per hour and you
use 1.5 inch pipes which will give you a maximum head of 15 cubic meters
per hour and your filter will allow you to go up to 21 cubic meters an
hour. Therefore an ideal level for you would be between 7.5 cubic meter
and 15 cubic meter per hour.
If you have a pool with no spa or other complications you can choose a
pump in the lower of the ideal range i.e. 7.5 to 9 cubic meters per
hour. If you have a pool/spa combination, you need to consider the flow
requirements of the spa jets. This will often push you towards a pump
on the higher end of the acceptable scale. In that case, we would look
for a pump in the 12 to 15 cubic meter range.
If you have a pool/spa combination, you might want to consider a 2
speed pump. It can run on low speed while it filters the pool, and then
switch to high speed when using the spa.
Example: As you can see from the following pump graph for the Espa Silen Swimming Pool Pump
range we have noted down the minimum flow of 7.5m3 per hour and the
maximum flow 15m3 per hour and also the head of 10m which we get from
step 10. We have drawn a line horizontally for the head and vertically
for the flow rate. As you will see the vertical lines of the min and max
flow rate meet the horizontal line of the head. Where they meet you
will see red curves relating to 5 pump sizes. It is here where we can
correctly size the pump for your pool. In the case of a pool with no spa
or other complications we indicated that you can choose a pump in the
lower of the ideal range i.e. 7.5 to 9 cubic meters per hour. Therefore
as the vertical line of 7.5m3 touches with the red curve of the 30
(0.33hp) model we can use this. However as it will be the minimum, you
may want to consider the 50 (0.5hp) model as you can see from the green
vertical line this version will pump 10.5 cubic meters an hour.
All pumps are of different qualities and power so be sure to check
the ratings before you decide on a pump. Please call us on 01895672515
to discuss this in further detail.
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 DataResidential
The size of the filter that could handle the 58 GPM Super Pump would be the C7512.
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 DataResidential
Turnover (In Gallons)  
Model Number  Effective Filtration Area  Design Flow Rate*  8 Hours  10 Hours 
C7512  75 ft^{2}  75 GPM  36,000  45,000 
C9002  90 ft^{2}  90 GPM  43,200  54,000 
C12002  120 ft^{2}  120 GPM  57,600  72,000 
C17502  175 ft^{2}  120 GPM  57,600  72,000 
Step 12): Calculating Resistance
Every piece of swimming pool equipment connected to your pool's
circulation and filtration creates resistance to water flow. This
includes the length and size of the pipe, the type of filter and
features like heaters and pool cleaners. The total amount of resistance
is called "feet of head"; the greater the feat of head, the stronger
your pump needs to be to overcome it. Even if you have an older pool
and don't know some of these factors, it's possible to determine the
pool's feet of head. Use a pressure meter and this formula:
 Check the pressure of water flowing into the filter tank and multiply that number by 2.31.
 Get a vacuum reading on the pump suction line and multiply that by 1.13.
 Add the two numbers together and the result is the total dynamic head.
Example:
If the water flowing into the filter tank is at 10 PSI, and the vacuum reading on the pump suction line is at 5 PSI:
Water flow into filter tank: 10 PSI x 2.31 =  23.10  
Vacuum reading on pump suction line: 5 PSI x 1.13 =  + 5.65  
Total resistance =  28.75  ft. of head  
Pool Accessories
Pool Cleaner: Vacuums
dirt and debris from pools. Models range from manual to robotic
cleaners that clean the pool automatically in as little as three hours.
Skimmer: These easytouse handheld nets make cleaning the surface of your pool a breeze.
Thermometer: Lets you check the pool's temperature before you take a dip.
Pool Test Kit: Check the chemicals, alkalinity and pH balance in your pool with a pool test kit to maintain safety and water clarity.
Safety Alarm: Sounds a loud alert when someone falls into the pool. Essential for families with small children and pets.
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