How to Calculate How Many Gallons Fill a Hot Tub
Before you add chemicals or cleaning solutions to your hot tub, you will need to know how many gallons of water it takes to fill it in order to add the right amount to the water. If you have just purchased a hot tub or intend to purchase a hot tub and you want to calculate the amount of water you will use to estimate the increase to your water bill, you can estimate how many gallons it will hold using its dimensions, or you can track how much time it takes to fill to calculate a more accurate figure.
Using Outside Dimensions

Measure the depth, length and width of a square or rectangular hot tub in inches with a tape measure. Measure the width of round hot tubs across the center of the tub and its depth.

Multiply the width by the depth by the length of a square or rectangular tub, and divide the resulting figure by 1,728. Multiply the result by 2.4 if the hot tub has seats, or 4.8 if the hot tub doesn't have seats. The result is the approximate amount of gallons the square or rectangular hot tub will hold.

Multiply the diameter by the diameter by the depth of a round hot tub. Divide the resulting figure by 1,728. Multiply the result by 2.4 if the hot tub has seats, or 4.8 if the hot tub doesn't have seats. The result is the approximate amount of gallons the round hot tub will hold.
Timing Filling

Turn on the water source that you're using to fill the hot tub and place the water hose inside the hot tub. Use a stopwatch to start timing how long it takes to fill the hot tub.

Write down the amount of time it takes to fill in minutes and seconds.

Fill a five gallon bucket with the same water source and hose. Use a stopwatch to time how long it takes to fill. Write down the result in minutes and seconds.

Multiply the number of minutes it took to fill the hot tub by 60. Add the number of seconds to the resulting figure for the total amount of seconds it took to fill the entire tub.

Multiply the number of minutes it took to fill the five gallon bucket by 60. Add the number of seconds to the resulting figure for the total amount of seconds it took to fill the entire bucket.

Divide the number of seconds to fill the hot tub by the number of seconds it took to fill the five gallon bucket. Multiply the resulting figure by five. This is the number of gallons that your hot tub holds.
Tape measure
Stopwatch
Five gallon bucket
Things You Will Need
References
Writer Bio
Amy Kingston has been a professional writer since 2001. She has written articles for various publications, including "Health" magazine, "Jackson Parenting" magazine, the "Bolivar Bulletin" newspaper and "A Musician's Pursuit." Kingston was also published in "Voices of Bipolar Disorder."