How Many Gallons of Water are in a Hot Tub?

Knowing how much water your hot tub can hold is crucial to many aspects of pool maintenance, including getting the right hardware attachments and maintaining the tub’s chemical balance. But not all hot tubs come with specifications slapped across the exterior, which might make you think about how you can calculate the gallons of water in the hot tub.

There are 320 to 700 gallons of water in a hot tub, proportional to the number of people who can simultaneously be in it. On average, a hot tub will hold 80 to 100 gallons of water per person, which means that a 3-person tub carries 300 gallons of water, whereas a 7-person hot tub holds 700 gallons.

In this article, you will learn different ways of knowing how many gallons are in a hot tub can help you save money, time, and energy. You will also find out how to figure out how big of a tub your family needs and the steps you can take to calculate how many gallons there are in your hot tub, including how to:

  • Figure out the time it takes to fill one gallon
  • Calculate the time it takes to fill the tub
  • Divide the time it takes to fill the tub with the time taken to fill a gallon
  • Round the number to the closest hundred for accuracy

Why You Should Know How Much Water Your Hot Tub Holds

Whether you’re testing to see how much baking soda you can add to your hot tub or simply want to know how much your new tub will run up your water tab, you might not be considering all the advantages of this information. 

Moreover, finding out the exact gallon capacity of your tub is hard, so if you are to follow through with the steps in this article, you must be sufficiently motivated. This section highlights the advantages of knowing how much water there is in your hot tub.

You Don’t Overspend on Pool Chemicals

Do you know that you can save hundreds of dollars in pool chemicals every year just by being smart with quantities? Most people following the online advice regarding chemical balancing are looking at swimming pool resources. 

Though this blog covers a lot of swimming pool content, articles like this one are written specifically for hot tubs. When it comes to chemical balancing, you might overspend following general internet advice. But if you know how many gallons of water your hot tub has, you can adjust any advice you receive by knowing how much muriatic acid, baking soda, or chlorine you need to add for every gallon of water.

You Don’t Undershoot or Overshoot on Hardware

Yes, variable speed pumps save significant money for those who have large pools, but if you get one that’s too powerful for your hot tub and tame it down, you’ll spend at least a year using it before you make back just what you paid unnecessarily getting it. 

This applies to everything from saltwater chlorinators to heat pumps and filters. Each piece of hot tub equipment is quantity-pegged. Put simply, a pump that’s good for 1,200 gallons of water is too powerful for a 300-gallon tub.

Similarly, a pump that works for a 300-gallon tub might not be ideal for a larger tub twice its size. If you undershoot, you end up having to replace or buy new equipment. If you overshoot, you sit with the losses. Either way, the situation is not ideal and is entirely avoidable if you know how many gallons your hot tub holds.

It Allows You to Budget and Plan Around Your Water Bill

A hot tub can be expensive to maintain, especially if you need to drain it often. Homeowners often have to decide between backing yet another filtration cycle or replacing the water at different times throughout the year. If you know what each tub-draining costs, you can make a more informed decision and can even choose between the filtration and water replacement once you see if the trade-off is worth it.

You Know Which Hot Tub to Get

In an ironic twist, you might actually need to know the gallons of water in your hot tub even before you have a hot tub. The last thing you want is to spend money setting up a hot tub only to realize that it’s too small and that one person has to awkwardly stand outside. 

Instead of facing this situation and having to decide who your least favorite family member is, it is much easier to know how many gallons of water your tub must be able to hold in order for it to be big enough for your whole family. As mentioned elsewhere in this post, a hot tub usually has 80 to 100 gallons for every person. 

This can vary depending on the tub’s depth. If the tub is only three feet wide, it can have a thousand gallons worth of water but will only hold one person. But most hot tubs aren’t tubular like that, and you do get an additional person’s space per hundred-gallon capacity.

How to Measure Water in a Hot Tub

For most decisions regarding hot tubs, including the kind of filter, chlorinator, and heat pump to install, you can go by the 100-gallon per person assumption and just see how many people can easily sit in your hot tub at once. 

However, when it comes to things like chemical balance and additives, it is crucial to get the exact amount of water right. For this, you will need a one-gallon container like Pinnacle Mercantile Plastic Jug as this will help you get as precise as possible. Follow the method below to find out how many gallons are in your hot tub,

Measure the Time It Takes to Fill a One-Gallon Container

In this step, you’re going to use the same water source as your hot tub and set it at the same water flow as you use to fill the hot tub. If you change the water flow rate for the one-gallon container, you will get an inaccurate reading. It also helps to drain your hot tub and rely on recent memory to set the same degree of water flow for both the one-gallon container as well as the hot tub.

Use a stopwatch on your smartphone and hit the “start” button just as you turn the faucet or the hose and start filling the one-gallon container. As soon as the water reaches the brim, hit the stop button. Do not go by the instinctive response of trying to turn off the water because each millisecond counts. You’ll be scaling the results to hundreds of gallons, so even a single second’s delay can cause a significant divergence in the result. 

As long as you hit stop or pause on the stopwatch, you’ll have precise enough reading for chemical balancing. Write down the amount of time, in minutes and seconds, it took for the container to get filled. Label this as “Time per Gallon.”

Drain Your Hot Tub

As mentioned earlier, your hot tub must be drained for this method to work because trying to dig out your long-term memory for how much you turn a faucet or the degree to which you let the water flow through the hose isn’t reliable. Whether you’ve drained the tub before getting the time per gallon reading or you do it after getting the reading, all that matters is that you fill the tub once you know the time per gallon.

Record the Total Time It Takes to Fill Your Hot Tub

This is where you calculate the total time for the same water flow as step one to fill up your whole hot tub. If you turned the faucet all the way up during step one, this step must be easier because you don’t have to pay a lot of attention to the nuance of the degree to which your tap knob must be turned. But even if you do not get the knob to the same degree, you can still get a precise enough reading if you achieve a close-enough waterflow.

Fortunately, faucets don’t change water flow too drastically with as small a difference as a millimeter. As long as you have the stopwatch running right as the water starts flowing into your hot tub, you will have a pretty close reading. But before that, you need to complete the time recording process. For that, you must fill the hot tub to the brim. 

Once again, you’ll have to go against your instinct to cut off the water once the tub is filled to the functional degree. That’s because we don’t usually fill tubs to the brim, accounting for water displacement.

Generally, a tub must have enough space for the water to rise but not overflow as we get into the tub. But that’s a problem we solve at a later step. For now, keep the faucet running until the hot tub is filled to the brim. 

Right then, hit pause or stop on the stopwatch before you even consider the hose. Get the time down on paper or your phone’s notes app before cutting off the water. You can also have a friend or a family member assist with cutting off the water as you hit pause on your stopwatch. Mark this reading as “Hot Tub Filling Time.”

Calculate the Gallons by Crunching the Water Flow Numbers

This is the most important stage of the process. But that doesn’t mean it is the hardest step. If anything, it’s the easiest part. Simply take whatever the bigger of the two readings you have taken is (the time it took to fill the tub) and divide it by the smaller reading (the time it took to fill a gallon). The result will be the closest reading of the number of gallons in your hot tub.

Read this paragraph if you’re interested in knowing how this works. If you don’t care and just want the result, skip to the next one and read about the nuances of this division. This works because you’ve found out the time it takes to fill up a single gallon and then found out how many times the same amount of time has passed when filling the hot tub. If it took 500 times the time, then you probably have a 500-gallon hot tub.

To get this reading right, you must make sure you’re converting both readings to seconds before you divide them. Whether you enter the times into a seconds converter like Metric Conversion Calculator or simply want to multiply the minutes by 60 and add them to the remaining seconds (2 minute 30 seconds is (60 x 2)+30), it works as long as both readings are in seconds. 

You might ask, “can’t I have them both in minutes?” You could, but the unit is too big to be precise for the one-gallon reading. You cannot divide 3.5 minutes with 6.3 minutes because in math, the decimal shifts at .99, whereas with time, the decimal shifts at .59).

Round Down or up to the Closest Hundredth Gallon

Finally, you will need to round up or down the result, so it reads in hundreds. That’s because you have made human errors in your calculation and are relying on the fact that hot tub companies make their products with capacity increasing or decreasing in hundreds. 

In other words, if you seem to have a 402-gallon hot tub, you can be sure it is a 400-gallon hot tub. Alternatively, if you have a 387-gallon hot tub, you’re likely to actually have a 400-gallon hot tub.

Bonus Step: Remove Water From the Hot Tub to Make Room for People

Since you’ve filled the tub to its brim during this experiment, you need to drain some of the water. You can go by how you feel drain water until it reaches the level you think is right. Or you can get more specific and remove 9 gallons of water from the hot tub for every adult and 6 gallons for every child in your family. That’s because this is the amount of water humans displace when they enter any container filled to the brim with liquid.

Final Thoughts

Knowing how many gallons of water are in a hot tub is helpful whether you’re thinking about getting one with the right capacity for your family or simply want to know how much baking soda or chlorine to add to your current tub. To do so effectively, figuring out the time it takes to fill up a gallon is the first step, after which you should find out the time it takes to fill the entire tub. Dividing the larger number by the smaller, you will get your answer in gallons. Just make sure both numbers are converted to seconds before you divide the two.

Jed Arnold

Jed spent every year from the ages of 15 - 22 as a Lifeguard (Red Cross) and ages of 17 - 22 as a Certified Pool Operator (CPO). Between working for over a dozen facilities and owning a pool, he carries over a decade of pool experience.

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