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TubWise

Hot Tub Volume Calculator

Estimate how many gallons or liters your hot tub holds using its interior dimensions and average water depth.

Spa shape

One unit applies to every measurement below.

Interior water length

Interior water width

Approximate vertical depth of the filled water, not the full exterior cabinet height

How the estimate works

The calculator converts your measurements into feet, then estimates volume in cubic feet. That cubic volume is multiplied by 7.48052 to get U.S. gallons, and gallons are converted to liters. Rectangular and square spas use length × width × average water depth. Round spas use π × (diameter ÷ 2)² × average water depth.

TubWise does not apply a universal correction factor for seats, footwells, rounded corners, or equipment displacement. No single adjustment fits every spa.

How to measure your hot tub

Use interior water dimensions when possible, not the full exterior cabinet size.

Rectangular or square tubs

  • Measure the interior water length
  • Measure the interior water width
  • Estimate the average filled-water depth to the typical waterline

Round tubs

  • Measure the interior diameter across the water surface
  • Estimate the average filled-water depth to the typical waterline

Average water depth means the approximate vertical depth of the filled water, not the full exterior cabinet height. A capacity listed in the owner's manual is more reliable than a calculated estimate when both are available.

Why actual capacity may differ

Your spa may hold less water than a simple geometric estimate because of:

  • Molded seats
  • Footwells
  • Loungers
  • Rounded corners
  • Varying water depth across the shell
  • Plumbing and equipment space
  • Measurements taken from the exterior rather than the interior

Why water volume matters

Knowing an approximate capacity can help you:

  • Compare your estimate with manufacturer specifications
  • Plan fill time
  • Interpret product-label directions that depend on water volume
  • Avoid relying on an assumed spa size when reading care instructions

This page does not calculate chemical doses. Follow the product label, your confirmed water volume, and current water-test results for any treatment decision.

Disclaimer

This calculator is a planning estimate only. It is not a substitute for your spa manufacturer's specifications, product labels, or professional advice. See the Disclaimer and Terms of Use for more.

Relevant TubWise guides

After you have a volume estimate, these guides help with testing, sanitizer selection, and related water care. Confirm capacity in your manual before adding any chemicals.