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How Many Gallons of Water in a Hot Tub? (Formula Included)

Knowing the water capacity for your hot tub can be useful in planning for your water bill, or when determining if your deck will hold a hot tub. But how many gallons are in a hot tub?

As a general rule, a small 2-3 person hot tub will hold approximately 300 gallons, a 4-5 person hot tub will have 275-375 gallons, whereas a large 6-8 person hot tub will hold up to 675 gallons or more.

Hot tubs range in both size, shape, and depth. All of those factors affect how many gallons they hold.

But there are so many different sizes, shapes, and styles of hot tubs. So below we’ll into a few different ways to accurately calculate yours. So let’s keep going!

There are plenty of good reasons to calculate how many gallons of water are in your hot tub.

Once you get an idea of how much it holds, you can figure out all sorts of useful information. You can learn how much money it should take to heat, the correct amount of purity chemicals or cleaning products to add, or the final weight of your hot tub.

We’ll go over two tried and true calculation methods in this article. First is the timed filling method, a slower but simpler way. And second is the dimension calculation method, an instant but slightly more complex way.

Let’s get started!

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Ready to Spend Less Time On Maintenance and More Time Enjoying Your Hot Tub?

Let’s face it. Balancing the water, cleaning filters, dealing with rashes, and trying to figure out which chemicals to buy and add can make you feel more like a chemist than someone who just wants to relax after a long hard day!

That’s exactly why The Hot Tub Handbook and Video Course is so valuable!

This is from Matt over at Swim University and he developed it for people looking to save money, time, and frustration. His tips on chemicals can save you $100/year just by making sure you buy only what you need.

So if you’re ready to stop being confused or frustrated with your hot tub and start spending more time in it, check out The Hot Tub Handbook and Video Course.

Just click that link to learn more on their website.

Manually calculating how many gallons are in your hot tub

Calculate the water capacity of your hot tub by multiplying the width, length, and depth of the hot tub in inches. Divide the result by 1,728. If the hot tub has seats, multiply the result by 2.4. If not, divide by 4.8.

But there are different calculations for non-standard-shaped hot tubs. As mentioned above, there are two methods for determining a hot tub’s approximate capacity.

There’s the timed fill method and the dimension calculation method. They both involve a bit of math but the timed fill method seems to be the most simple. So, let’s start there.

The Timed Fill Method For Calculating How Many Gallons Are in Your Hot Tub

We’ll use the following steps to easily figure out your tub’s water volume:

  1. Fill your empty hot tub from a hose
  2. Time how long it takes to fill
  3. Basic math!
  4. Figure out your hose flow rate
  5. More math!

You’ll need:

  • Empty hot tub
  • Garden hose
  • A 1-gallon jug
  • The timer app on your phone
  1. Turn on your hose to full or a set point and leave it running in your tub.
  2. Set a timer and wait until it’s filled. This could take some time so it might be a good time to grab an adult beverage with a nice novel and relax.
  3. Stop the timer when it’s full. Not much more to this step than being patient!
  4. Calculate. First, we need to convert to seconds. So multiply the hours by 60 and add the remaining minutes. Next, multiply the total minutes by 60 and add the remaining seconds. This is the total amount of time it took to fill your pool in seconds.
  5. Record this number somewhere and go to the next step.

If you want to know about how long your hot tub will take to fill up, check out my filling guide here for a rough estimate of how long you’ll be waiting.

Just click that link to read it on my site.

Example 1 for the timing filled method:

It took 1 hour and 30 minutes to fill the hot tub. 1 hour x 60 = (60 minutes + remaining 30 minutes) x 60 = 5,400 seconds.

Get your 1 gallon bucket and timer ready. Make sure your hose is set at the same pressure as it was to fill your hot tub. If there’s a variance here, it will mess up the final calculation.

  1. Start the timer and fill the jug.
  2. Stop the timer as soon as it’s full. Once again, convert this number into seconds if it’s over a minute. (Just multiply by 60 again, add the remaining seconds.) You now have the flow rate for 1 gallon of space in seconds. Record this number.
  3. Time for more math!  Divide the total time it took to fill your entire hot tub and divide it by the time it took to fill up the jug. This will give you the approximate number of gallons.

For the severely math impaired, (i.e. me) the timed fill method is the way to go. It will take a bit more time, but filling your hot tub is part of the required maintenance anyway. It’s a good opportunity to take advantage of this easier solution.

If you need an answer NOW and don’t mind a bit more complex math read on to learn the dimension calculation method.

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The Dimension Calculation Method For Calculating How Many Gallons Are in Your Hot Tub

We’ll use the following steps to calculate the volume of your hot tub:

  1. Measure the external dimensions of your tub
  2. Multiply!
  3. Divide!
  4. Multiply again!

You’ll need:

  • Tape measure
  • Calculator
  • Pen and paper

This method varies slightly depending on if you have a square or round hot tub. Let’s start with a square tub.

Square Tub Method For Calculating How Many Gallons Are in Your Hot Tub

  1. Use your tape measure to check the width, length, and depth of your tub.
  2. Write down all the numbers in inches,
  3. Multiply the length by the width by the depth.
  4. Take the result and divide by 1,728 (this is the number that converts cubic inches to cubic feet).
  5. Multiply again. If your hot tub has seats, multiply the result by 2.4.
  6. If it does not have seats, multiply by 4.8. The resulting figure is the total capacity in gallons.

This method also works for a rectangular hot tub. Next, let’s check out the round tub method.

Round Tub Method For Calculating How Many Gallons Are in Your Hot Tub

  1. Measure the diameter of the tub (center your tape measure straight across the middle of the tub). Next, take the depth and record these numbers.
  2. Multiply the diameter by the diameter by the depth.
  3. Divide by 1,728 to get the capacity in cubic feet.
  4. Multiply again by 2.4 if your tub has seats or 4.8 if it doesn’t.

Math is hard! How much water is in a hot tub roughly?

You’ll find a few rough estimates below. But you absolutely should measure yours accurately to get the approximate amount of gallons.

For more on measuring hot tub weight, along with estimated weights empty, filled with water, and occupied with people check out my full guide here.

It’s full of quick answers in case you need to move a hot tub, or if you’re placing one on a wooden deck and aren’t sure if it can take the weight. After all, the average weight of a hot tub full of water and people, even a small tub or rectangular tub, will still be well into the thousands of pounds.

Just click that link to read it on my site.

How many gallons is a 2-person hot tub?

Smaller models like a 4-person hot tub contain about 150-220 gallons of water.

How many gallons is a 3-person hot tub?

Small hot tubs like a 4-person hot tub contain about 200-300 gallons of water.

How many gallons is a 4-person hot tub?

Small hot tubs like a 4-person hot tub contain about 275-350 gallons of water.

How many gallons is a 5-person hot tub?

Small hot tubs like a 5-person hot tub contain about 300-375 gallons of water.

How many gallons is a 6-person hot tub?

A 6-person hot tub is similar in size to a 5-person tub. So the amount of water is about 325-475 gallons in a 6-person hot tub.

How many gallons does a 7×7 hot tub hold?

Medium-large hot tubs like a 7×7 hot tub typically hold about 370 gallons of water. This is the the size of the average hot tub.

How many gallons is an 8 x 8 hot tub?

Large hot tubs like an 8 x 8 hot tub can hold anywhere from 550 to 625 gallons of water. This is the most typical hot tub.

How many gallons does an 8-person hot tub hold?

An 8-person hot tub can hold about 615-675 gallons of water. These larger capacity hot tubs can weigh as much as 7800 pounds when filled! So think twice before installing one of these bad boys on your deck.

I have a full article about safely putting a hot tub on a deck. I get into how to easily calculate whether your deck can hold one, but also how to retrofit your deck to take the extra weight.

Just click that link to read it on my site.

Final thoughts

Using the above estimates might be okay temporarily.

But we do hope you’ll consider measuring your hot tub’s capacity accurately. As you can see, it’s not very difficult or time-consuming. Doing it the right way will set you up to enjoy your hot tub exactly as it was meant to.

You’ll also get plenty of valuable insights like your hot tub’s weight, electricity costs, and proper chemical amounts when cleaning time comes around.

Try the timed-filled method next time you change your water for the most painless way to go. Or grab a calculator and use the dimension measuring method if you need an answer now.

It’s not too hard. I promise!

Frequently Asked Questions

Does water evaporate from hot tubs?

Water does indeed evaporate from hot tubs.

This is one important reason to know precisely how many gallons your hot tub holds. You’ll have to add water accordingly from time to time. When you do, make sure to check the chemical levels to prevent bacteria growth.

Keep in mind that there are a lot of factors that determine how quickly water will evaporate from your tub. Some of those include:

  • The hot tub’s temperature setting
  • The outside temperature (colder air means faster evaporation)
  • Frequency of use
  • The quality of your tub’s cover
  • Humidity

Want to learn more? Check out a full write-up here about water loss in hot tubs on one of my other websites.

Is there a hot tub volume calculator?

Yes, there is one available. It still requires some of the measurements I talked about above. But it does make it quick and easy to calculate and takes some of the math headaches out for you.

Just go to AquaChek to see their calculator and use it now. Totally free.

How long does it take to fill a hot tub?

Typically, a standard garden hose releases water at around 5-7 gallons per minute.

Given this, if you’re filling a small 250-gallon hot tub, it’ll take roughly 35 to 50 minutes. For a larger tub with a 500-gallon capacity, you’re looking at 70 to 100 minutes. Don’t forget external factors, like water pressure in your area, which can speed up or slow down the process.

To ensure optimal operation and avoid potential damage, it’s crucial to keep track of the fill time. Overfilling or underfilling might cause complications. If you’re in a hurry, try using two hoses!

It’s a nifty trick to cut down on wait time.

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Photo credits that require attribution:

Redwood Tub by David Goehring is licensed under CC2.0

Jeff Campbell