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Hot tubs & spas

Hot Tub Volume Calculator

Get the maximum your hot tub could hold from its inside measurements, and how to find its true capacity for dosing.

What you'll need (3)
  • A tape measure
  • Inside length and width (or diameter) at the water line
  • Water depth to the deepest point
Units
Shape
Inside measurements

Measure inside the shell at the water line, not the outside cabinet.

Water surface, wall to wall. Example: 78.
Water surface, wall to wall. Example: 78.
From the water line to the deepest floor (footwell). Example: 34.

Your result

Calculated

Your result will appear here

Enter your hot tub's inside measurements and water depth.

What this number means

Measure inside the shell at the water line. The calculator multiplies the surface area by the water depth. That gives the volume of an empty box the size of your tub, which is the most it could possibly hold.

Real hot tubs are full of molded seats, loungers and a footwell, and these displace a large, model-specific share of that box. So the true capacity is less than this number, often by a lot.

The best way to get your capacity

  1. Owner’s manual or spec sheet. Manufacturers list the capacity in gallons, usually on the product page too.
  2. Measure while refilling. At your next drain-and-refill, a hose-end water meter from a hardware store gives the exact number.

Once you know it, our hot tub chemical calculator remembers it in your browser.

Assumptions and limitations

  • Gives the empty-shell maximum. Seats, loungers and the footwell reduce the real capacity by an amount that varies by model, so we don't guess it.
  • For chemical dosing, use the capacity from your owner's manual or the manufacturer's spec sheet.

Common questions

How many gallons does a typical hot tub hold?

It varies widely with size and seating layout. Check your owner's manual or the manufacturer's spec sheet, which almost always lists capacity in gallons. If you don't have it, a hose-end water meter used while refilling gives an exact number.

Why doesn't the calculator subtract the seats?

Seat and footwell shapes differ so much between models that any fixed allowance would be a guess. We show the empty-shell maximum, which is a reliable upper limit, and recommend the manufacturer's figure for dosing.

Sources and methodology references

  1. NIST SP 811 unit conversions, National Institute of Standards and Technology

Published October 2, 2026. Results are calculated directly from your inputs. See how our calculators work.

Spotted a wrong result or an outdated source? Report a problem with this calculator.

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