Determining the Perfect Heater Size for Your Pool or Hot Tub

Determining the Perfect Heater Size for Your Pool or Hot Tub

Picture yourself on a warm summer day, ready to plunge into your refreshing pool or soak in the soothing waters of your hot tub. The anticipation builds, but suddenly, you realize that the water is far from the ideal temperature you envisioned. Whether you’re a pool owner seeking to extend the swimming season or a hot tub enthusiast yearning for consistent warmth, one crucial question arises: What size heater do you need to maintain the perfect temperature?


Choosing the right heater size for your pool or hot tub is a critical aspect of ensuring optimal comfort and enjoyment. A heater that’s too small might struggle to raise the temperature adequately, leaving you shivering and disappointed. Conversely, an oversized heater can lead to excessive energy consumption, wasting resources and unnecessarily inflating your utility bills.


In this comprehensive guide, I will delve into the intricacies of determining the ideal heater size for your pool or hot tub. I’ll explore the factors to consider, the calculations involved, and the best practices to ensure you make an informed decision.


So, let’s dive in and unlock the key to achieving the perfect balance between warmth and efficiency in your aquatic oasis.


Don’t forget I have a YouTube Channel Too!

Pool and Hot Tub Heating Calculator

Below you will find the calculator to help you size the heater for your pool or hot tub correctly. 

I must stress that this is a “lab calculation” – what I mean by this is that external factors are ignored. Things like good insulation, a good cover, ambient temperature all have an impact on the heating of your hot tub or pool.

The numbers that the calculator generates, do not include the above. Therefore, I would expect the heat up time stated to be slower in the “real world” than it is on the calculator. 

That said, it should give you a good indication.

What do you need to consider?

When choosing the right heater size for your hot tub or pool, you really need to consider your own use. Don’t go with what your friends have or what you have read online, think about your own use.

How quick do you want to heat it up?

What temperature do you want to hold it at?

These are key factors that you can use the calculator below to work out the right size of heater for you pool or hot tub.

How does the Pool and Hot Tub Calculator Work?

In order to calculate the heat needed to raise the temperature of the pool or hot tub then we need to convert the volume into a physical weight.

To determine the weight of water in your pool, multiply the number of gallons by 8.34. This gives you the weight in pounds.

Next, divide the weight of water by the BTU size of your heater to find the time required to increase the temperature by one degree Fahrenheit.

To find the total time required to raise the temperature by a specific number of degrees, multiply the desired temperature increase by the time it takes to raise it by one degree.

For instance, suppose you have a 20,000-gallon pool and a 200,000 BTU heater.

Multiply 20,000 by 8.34 to obtain 166,800 pounds of water.

Divide 166,800 pounds by 200,000 BTUs to get 0.834 hours per degree F

If you wish to increase the pool temperature by 10 degrees, multiply 10 degrees by 0.834 hours per degree, resulting in 8.34 hours.

Pretty straight forward, but the calculator below will do this for you so just input the variables that you need.

Pool and Hot Tub Heater Calculator


Pool Heat Time Calculator

Can I Help You?

If I can help you in any way I would love to hear from you. You can get in touch using the form below.

Thanks - Andi


Hi, Andi here. I own and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Today, I've helped over 900 DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi

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