Spa Volume Calculator

How to Calculate Hot Tub Volume in Gallons

Proper water treatment and hot tub maintenance are dependent on the size of your hot tub, so it is important to know the capacity of your hot tub in terms of gallons. Use the following formulas based on the shape of your hot tub as a guide to approximate your hot tub’s volume. Click here for a printable version.

Method #1 – Fill Time

The most accurate way to determine the volume of a hot tub, without fancy equipment, is by fill time. First, using a stopwatch or wristwatch time how long it takes to fill your hot tub. Convert minutes and seconds to total seconds, and write it down. Then, using the same hose and pressure, time how many seconds it takes to fill a one gallon bucket.

Now divide the number of seconds it took to fill your hot tub, by the number of seconds it took to fill the one gallon bucket, and you will have the approximate water volume! For example: You filled your hot tub in 30 minutes (1800 seconds) and filled the one gallon bucket in 6 seconds. 1800 divided by 6 = 300 gallons.

Using a five gallon bucket will result in an even smaller margin of error. Using the above example with a five gallon bucket: You filled your hot tub in 1800 seconds, and filled the five gallon bucket in 30 seconds. 1800 divided by 30 = 60 five gallon buckets, or 300 gallons.

Method #2 – Dimensional

The dimensional method is not as accurate as the fill-time method, but is a quicker way to determine the approximate volume of any hot tub.

Portable Acrylic-Fiberglass Hot Tubs:

(Take all measurements in inches)

Our formulas make it easy because you use the outside dimensions of your portable hot tub cabinet.

  • For Square or Rectangular portable acrylic hot tubs:
  • Formula for Volume of Square or Rectangular Portable Acrylic-Fiberglass Spas
  • For Round or Multi-sided portable acrylic hot tubs:
  • Formula for Volume of Round or Multi-sided Portable Acrylic-Fiberglass Spas

* Results are estimated volume only. Consult with manufacturer.

In-Ground Hot Tubs, Soft Hot Tubs, & Wooden Hot Tubs:

(Take all measurements in inches)

These types have fewer interior water-displacing features, so this formula takes that into account.

  • For Square or Rectangular types:
  • Formula for Volume of Square or Rectangular In-Ground Spas, Soft Spas, or Wooden Hot Tubs
  • For Round, Irregular, or Multi-sided types:
  • Formula for Volume of Round, Irregular, or Multi-sided In-Ground Spas, Soft Spas, or Wooden Hot Tubs

* Results are estimated volume only. Consult with manufacturer.