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How long is a piece of string?
But really, to answer a question like “how many gallons are in a hot tub,” you need to know which tub you’re talking about and which factors can change the water capacity of hot tubs.
Fortunately for you, we’ve got time to go through these factors. It’s up to you to determine exactly how many gallons are in that hot tub but, through the use of averages and what we know about most hot tub manufacturing practices, we can give you the means to find your answer.
We’ll do this by describing the factors that affect the water capacity of hot tubs so you can understand in a general sense how to gauge the gallon count of your spa model.
Then we’ll give you some tried and true concrete methods you can use to measure the volume of your hot tub.
What Determines Hot Tub Water Capacity
By knowing the factors that change a hot tub’s typical water capacity, you can better understand how many gallons your own hot tub can hold.
It’s important to know these factors before attempting a more specific method of calculating the gallon count of your tub since the considerations below are part of that equation.
Volume to Gallons of Water
By size, we mean the length and footprint of the hot tub which, to an extent, determines the person count of the tub. You should know how long each side of the tub is as well as the radius, or corner area if your tub is square.
We shouldn’t need to tell you that a larger tub, with all else being equal, will hold more gallons of water.
The person count of a tub is usually an arbitrary and brand-specific descriptor, so there’s no objective way that gallon capacity corresponds with a person count.
That said, it’s a reality that hot tubs with larger person counts will themselves be larger, and so can hold more water, so let’s go through some very general things to keep in mind.
A two-person tub tends to hold as much as 150 to 220 gallons of water, a three-person tub can usually support 200 to 300 gallons, and a six-person tub can have as much as 320 to 475 gallons of water in it.
These are general numbers, of course, so the exact numbers may vary, but we think it’s handy to know these general water capacity trends before calculating a more exact figure.
Hot Tub Sizes
A smaller but very deep hot tub will hold more than a large but shallow tub, so you can’t ignore the depth of your model.
Thankfully, the depth of hot tubs doesn’t vary that much.
The overwhelming majority of commercial hot tubs have a depth between thirty-one and forty-two inches, the optimal comfort zone for most people’s heights when seated.
You can add or subtract a few inches depending on how tall the above-water lip of the tub is, too.
The exceptions here are custom tubs that can easily exceed forty-two inches since, if you’re forking out to have a custom tub installed where the installation alone comes at a cost, then you’re probably treating yourself to a larger hot tub than the standard commercial types.
Rectangular vs. rounded hot tub water capacity
Hot tubs come in a variety of shapes that will change how many gallons they can hold. Besides the typical fact that the shape influences the size of the tub, it also changes the seating arrangement below the water level, fundamentally changing the depth capacity too.
This is why it should be one of the three main considerations you have when trying to find out how many gallons your hot tub can hold.
Rounded hot tubs tend to be smaller, especially when they have lost that volume at the corners that a square hot tub would have.
They tend to be the smaller models since they’re more intimate and maximize the space you have in your backyard. Triangular hot tubs exist too and tend to be very snug, sometimes even smaller than round hot tubs.
However, when most of us think of hot tubs, a square or rectangular model is likely what comes to mind. The seating and jet placement tend to be equalized throughout the tub, making for a more consistent depth that exceeds the capacity of rounder models.
How to Calculate the Volume of Water
There are two main ways that customers, without the use of specialized equipment, can determine the gallon count of your hot tub.
The first one is more of an equation you can use to get a reliable figure whilst the second method is much more practical and can be done to keep you occupied as you fill your hot tub.
Hot Tub Dimensions
Surprise, the first method involves an equation where you use the specs discussed in the first half of this piece to discover the accurate gallon count of your tub.
You should measure the length, width, and depth of your tub if it’s a square or rectangular model. If you have a round hot tub, measure the diameter of the model to get your width measurement.
For the square and rectangular tubs, multiply the width by the depth and then the length, and then divide the resulting figure by 1,728.
If your tub has seats, multiply the result by 2.4, or 4.8 if there aren’t any seats. For ease of reading, see the equation below.
Width x Depth x Length = X
X ÷ 1,728 = Y
Y x 2.4 (if seats) or 4.8 (if no seats) = Your square tub’s approximate gallon count.
With rounded hot tubs, multiply the diameter by the depth and divide the figure you get by 1,728.
Do the same as with squared tubs, multiplying the result by 2.4 or 4.8 depending on whether you have seats or not. Once again, have an equation.
Diameter x Depth = X
X ÷ 1,728 = Y
Y x 2.4 (if seats) or 4.8 (if no seats) = Your round tub’s approximate gallon count.
How Much Time to Fill a Hot Tub
Here is the fun one that you can use when first filling or just refilling your hot tub. Grab a stopwatch, turn on the water, and watch it flow while timing how long it takes to fill.
Take note of that time when it’s full, and try not to forget it since resetting this method is costly and time-consuming.
Using the same water source and the hose or device you used to fill the tub, fill a five-gallon bucket, and time how long that takes. Again, write down the results. Can you see what we’re doing here?
Equalize each time measurement by multiplying how long, in minutes, it took you to fill the tub by sixty. Add the seconds as needed, too, to get the number of seconds it took to fill the tub.
Do the same for the five-gallon bucket, divide every minute by sixty, and add the seconds in, getting a measurement that’s measured in seconds.
Divide the number of seconds it took to fill the tub by the seconds it took to fill the five-gallon bucket and then multiply the result by five.
This should be the number of gallons your tub can hold. Have another equation, for the road.
Hot tub fill time in minutes x 60 = X
X + Seconds = Hot tub fill time in seconds
Five-gallon bucket fill time in minutes x 60 = Y
Y + Seconds = Five-gallon bucket fill time in seconds
Hot tub fill time in seconds ÷ Five-gallon bucket time in seconds = Z
Z x 5 = Your hot tub’s approximate gallon count.