What This Calculator Does
The Hot Tub Volume Calculator estimates how much water your spa holds, in US gallons, liters and cubic feet. Knowing your tub's capacity is essential for correctly dosing chemicals like chlorine, bromine, pH adjusters and shock, since dosing instructions are almost always given "per gallons of water." It also helps you estimate fill time and heating costs.
How to Use It
Choose your tub shape — round or square/rectangular. For a round tub, enter the inside diameter across the top in feet. For a square or rectangular tub, enter the length and width. Then enter the average water depth (not the cabinet height — measure from the floor of the seating area to the typical fill line). The calculator returns the approximate water volume.
The Formula Explained
Volume in cubic feet is the surface area of the water times its depth. For a round tub the surface area is \(\pi \cdot r^2\) where \(r\) is half the diameter; for a rectangular tub it is \(\text{length} \times \text{width}\). Multiply cubic feet by 7.48 to convert to US gallons (one cubic foot holds 7.48 gallons):
Round: $$V = \pi \cdot r^2 \cdot d \cdot 7.48$$ Square: $$V = L \cdot W \cdot d \cdot 7.48$$
Worked Example
A round hot tub has a 7 ft diameter and an average water depth of 3 ft. The radius is 3.5 ft, so the surface area is \(\pi \cdot 3.5^2 \approx 38.48 \text{ ft}^2\). Multiplied by 3 ft of depth gives \(\approx 115.45 \text{ ft}^3\). Times 7.48 yields about 863.6 gallons. Most hot tubs hold less because seats and shells displace water — this gives a useful upper estimate.
FAQ
Why is my actual fill less than this number? Built-in seats, footwells and the curved shell displace water, so real capacity is typically 10–25% lower than the box calculation.
What depth should I use? Use the average water depth at your normal fill line, not the total cabinet height.
Round vs square? Round tubs use diameter (\(\pi \cdot r^2\)); square/rectangular tubs use \(\text{length} \times \text{width}\). Pick the shape that matches your spa.
