Maintaining Desired Temperature

You will require about 10 BTU per hour per square foot of pool surface for each degree your pool temperature is above the air temperature. If we look at our 20' x 40' pool we have a surface area of 800 sq feet. If the air temperature is 60⁰F and you wish to keep your pool at 80⁰F then we have 80 - 60 = 20 x area of 800 = 16,000 x BTU per sq foot of 10 = 160,000 BTU per hour. With the addition of a pool cover this will drop dramatically.

Take the oval pool above for a second example. We have a surface area of 405 sq feet so with a temperature 20⁰F above the unheated temperature we have 405 x 10 x 20 = 81,000 BTU per hour. Again, drop this figure 50-80% with the addition of a pool cover.

And, of course, here's the calculator. Plug in the surface area of your pool, the temperature you want to maintain and a rough air temperature and you will find the number of BTUs per hour it will take to maintain your desired water temperature.

And that is pretty well it. When all is said and done, a bigger capacity heater is always better than a lower capacity one. The energy cost difference is slight but wear and tear is significant. For a pool which is not used often and will be heated from cold rather than maintained at a constant temperature, gas heaters are the best choice. Use the figures you obtained through your calculations from above as a minimum rating for a heater.
Heating Your Pool

Now you have the volume of your pool we can get to the nitty gritty of how much heating capacity you will need. There are two heating conditions you will need to be aware of  -  heating from cold and maintaining the desired temperature.

Heating from cold - this is the initial heating phase when you will be bringing your pool from cold to your desired temperature. If you will be using your pool continuously this will not need to happen often as you will reach your temperature and then maintain that temperature. If you will be using your pool occasionally then this will be done more often - gas heaters are usually better for this. Now back to the maths class!

If you are not totally fascinated by all the maths stuff and the theory behind it   (no? really?) just scroll down a bit and you will come to another calculator.

Fistly, you need to know 1 BTU is the heat energy required to raise 1 pound of water by 1⁰F and there are 8.33 pounds of water in 1 gallon.

So   Volume x 8.33 = total pounds of water then divide by 24 to give BTUs per hour required to heat your pool 1 degree F in a day.

For example,    30,000 x 8.33 = 249,900 then divide by 24 = 10,412 BTUs per hour for a 1 degree rise in 24 hours.

Now you need to measure the temperature of your pool. This you will subtract from your desired temperature (usually around 80-84⁰F) to find the rise in temperature required. Suppose your pool is at 60⁰F and you want it up to 80⁰F then 80 - 60 = 20⁰F  rise.

Now multiply BTUs for a 1⁰F rise by the rise required to get BTUs per hour.

For the 30,000 (20' x 40' pool) gallon example:  10,412 x 20 = 208,240 BTUs per hour.

Another example, if your pool is 15,000 gallons: 15,000 x 8.33 = 124,950 divide by 24 = 5,206

Pool is at 55⁰F and required temp is 82⁰F so rise is 82 - 55 = 27.

BTUs per hour required are: 27 x 5,206 = 140,562 BTUs.

It is a good idea to add an extra 15-20% to the heating capacity you arrive at to make up for inefficiencies in the system. Remember, the BTU figure we have arrived at here is to achieve the temperature rise over one day. If you are going to do it in two days the per hour requirement is halved and if you want to get there in half a day then it is doubled. Also remember your pool will be losing some of the heat it has gained due to natural losses such as evaporation so the desired temperature may take a bit longer. A pool cover will help stop this heat loss by about 50-80% so the benefits of a pool cover are obvious.

And here's the calculator. You do need a couple of bits of information to plug in here. You need the starting temperature of your pool in ⁰F and the temperature you want it to rise to, also in ⁰F. You also need to know how long you want to wait for the pool to rise to your desired temperature in hours. Be a bit practical here - you aren't going to heat an olympic-sized pool in a couple of hours!

- half pool width x half pool width x 3.14 (this gives surface area) x  average depth x 7.5

For example, your pool is 30' wide, 4' at shallowest and 6' at deepest.

Your average depth is 5' (4 + 6 = 10 divided by 2 = 5)

So you have 30/2  x 30/2  x 3.14 (gives 706 sq feet area) x 5 x 7.5 = 26,494 gallons (we will round these figures so 26,500 gallons is good).

Use the following calculator with your pool width and average depth in feet to work out the volume in gallons.


- length x width x average depth (giving area) x 7.5 x 0.9

The 0.9 will help give the approximate volume - we don't need to be exact.

Your pool is 30' long, 15' wide, 3' at shallowest point and 6' at deepest.

Average depth is 3 + 6 = 9 divided by 2 = 4.5'

So we get to 30 x 15 (x 0.9 = 405 sq ft surface area) x 4.5 x 7.5 x 0.9 = 13,669 gallons (13,700).

  - pretty close to an oval pool - width x length (x 0.95 = surface area) x average depth x 7.5 x 0.95

For example, you have a width (edge to edge) of 15' and length of 30'.

You have a shallow of 4' and a deepest point at 6' so average depth is 4 + 6 = 10 divided by 2 = 5'

Then we go 30  x 15 ( x 0.95 = 427 sq ft area) x 5 x 7.5 x 0.95 = 16,031 gallons.
Irregular Shapes

- sometimes you will be able to come to a reasonable estimate by looking at the shape and breaking it down into a few shapes joined together and then work out a volume for each section and total them up.

The best way would be to approach the builder of your pool as they should have a record and be able to inform you of the volume.
Beautiful swimming pool
Swimming Pool Heater Guide

What Size Heater Do You Need?

~ Who else wants to extend their swim season? ~

Rectangular or Square

-  pool length (in feet) x pool width (in feet) (this gives surface area of pool) x average depth (in feet) x 7.5

The average depth will be close enough if you add the shallowest depth to the deepest depth then divide by 2. The 7.5 gives us the answer in gallons instead of cubic feet (1 cubic foot = 7.48 gallons US).

So, for example,  your pool is 20' x 40' and is 4' in the shallow end and 6' in the deep end.

You have an average depth of 5' (4 + 6 = 10 divided by 2 = 5).

So 40 x 20 (= surface area of 800 sq ft) x 5 x 7.5 = 30,000 gallons.

Enter your pool length, width and average depth (in feet) into the calculator below to obtain the volume of your pool in gallons and the surface area in square feet.

Oval Swimming Pool
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And the surface area is: square feet



And the surface area is: square feet




And the surface area is: square feet




And the surface area is: square feet

Pool Volume:

Starting Temperature:

Desired Temperature:

Hours to Reach Desired Temperature:

per Hour

An extra 20% gives us: per Hour

Pool Area:

Desired Pool Temperature:

Outdoor Air Temperature:

per Hour

Before you rush off madly spending money, it is a good idea to figure out what heating capacity you need for your swimming pool. Following the instructions following on this page you will be able to calculate the volume of a pool and work out a rough minimum heating capacity for your pool.

Going for a larger capacity heater will result in a shorter heating time and less wear and tear on the heater. The energy consumption will be little changed so don't opt for a smaller heater to save dollars - it doesn't work that way. A larger heater overall costs little more to run than a smaller one.

So into the nasty mathematical stuff. A BTU or British Thermal Unit is the standard for measuring heating capacity and most pool heaters will be described in terms of their BTU output. What we will do first is work out the number of BTUs you will need to increase the water temperature of your pool by 1⁰F in one day. From that we can easily work out the rest. So we have some things to work out.

Pool Volume in Gallons

If you already know the capacity of your pool that's great, if you don't it's not hard to get a rough idea. Since pools are not all the same shape we will have to work things a little differently for the different shapes. If you can contact the pool builder, they should be able to give you an accurate volume for your pool and this is the best way to go.

When you have your pool volume, record it as you will need it in the next step.
Picking a Heater

Looking at heaters, if we take the 20' x 40' pool from above then we need to find a heater which is capable of  at least 210,000 BTU per hour. Remember that a higher capacity heater will take less time to do the same job and not use a great deal more fuel. To maintain 80⁰F the 210,000 BTU heater would be running almost full time where a heater around 400,000 BTU per hour would run for about 30 minutes per hour.

With heat pumps, the normal maximum output is about 130,000 BTU per hour and the cost is between $3,000 and $4,000 for the unit. The big advantage with heat pumps is they cost about a third to a half of the cost of a gas heater to operate. Some heat pumps are set up like reverse cycle airconditioners - that is, they can even cool your pool on those days when you jump in and get hotter! These models generally cost about $1,000 more than the plain heating models. A heat pump with capacity of 130,000 BTU/hour will take about 48 hours to heat the 30,000 gallon (20'  x 40') pool in the examples above and would need a cover to be able to maintain the temperature.

Solar pool heaters are a different kettle of fish. Some manufacturers will quote a BTU per hour rating but there are too many variables to find an accurate result. Most solar pool systems will recommend about 50-80% of the pools surface area in solar panel area. So the 20' x 40' pool with 800 sq feet will need about 400-640 sq feet of solar panel. If the panels are 4' x 10' this means about 10 to 16 panels. Solar systems are best suited to extending your swimming season rather than trying to heat a cold pool to a comfortable temperature. If you want to spend some money to save some money, a combination of solar and either gas or heat pump would give a good year round result.
Irregular-Shaped Swimming Pool
Kidney-Shaped Swimming Pool
Round Swimming Pool
Rectangular Swimming Pool