Hot Tub Volume Calculator

Why Volume Matters

Knowing your hot tub's actual water volume matters most for chemical dosing — sanitizer, pH adjusters, and shock treatments are all measured relative to gallons of water, and under- or over-dosing based on a wrong volume guess can leave water either under-sanitized or harsh on skin and equipment. Manufacturer spec sheets often list volume too, but measuring it yourself is a useful check, especially for older tubs or if you no longer have the original documentation.

How to Use This Calculator

Choose your hot tub's shape and enter its dimensions in feet, along with the average water depth (not the shell's total height — measure from the waterline down to the floor, accounting for any seating that reduces open water volume, which this calculator doesn't correct for).

How We Calculate It

Round: volume = π × (diameter ÷ 2)² × depth × 7.48 gallons per cubic foot. Oval: volume = π × (length ÷ 2) × (width ÷ 2) × depth × 7.48. Rectangular: volume = length × width × depth × 7.48. The 7.48 constant converts cubic feet to US gallons.

A Few Notes

This calculates open water volume based on a simple geometric shape — it doesn't subtract for built-in seats, jets, or other fixtures that displace water, which typically reduce actual volume by 10-15% below the geometric estimate for a seated hot tub. For precise chemical dosing, your manufacturer's stated capacity (if available) will usually be more accurate than a geometric estimate.