How to use the gallons of water in a hot tub calculator
- Choose the shape: round, oval or rectangular, and whether you are measuring in feet or inches.
- Enter the diameter for a round tub, or the length and width for the others. Measure inside the tub at the water line.
- Enter the water depth from the floor of the tub to the water line.
- If you know how much of the box the seats and footwell take up, enter it as a percent. Read the gallons, liters and the weight of the water.
Formula
Round: area = pi / 4 x diameter^2
Oval: area = pi / 4 x length x width
Rectangle: area = length x width
Volume = area x depth, then gallons = cubic feet x 1,728 / 231
A US gallon is 231 cubic inches, so 1 cubic foot is 1,728 / 231 = 7.4805 gallons. Water weighs about 62.4 pounds per cubic foot.
A 7 foot round tub filled to 3 feet
The area is pi / 4 x 7 x 7 = 38.48 square feet. At 3 feet deep the volume is 115.45 cubic feet, which is 115.45 x 7.4805 = 863.7 US gallons, about 3,269 liters. The water weighs about 7,204 pounds.
An oval tub of 8 by 7 feet at 3 feet holds 987.0 gallons by the same method, and a rectangular one of the same size holds 1,256.7. If seats and footwells take up 25 percent of the round tub's box, it holds about 647.7 gallons.
A hot tub is not a simple shape
The formulas give the volume of a plain cylinder, ellipse or box. A real tub has seats, a footwell, steps and equipment that take up part of that volume, and rounded corners or sloped walls that change it further. That is why the allowance field exists. Leave it at zero to see the geometric maximum, or enter a percent from your manual.
For a purchase or for chemical dosing, the manufacturer's stated capacity is the best figure, because it already accounts for the shape. Use this tool to check that figure or when the tub's capacity is unknown.
Why the weight matters
A tub full of water is very heavy, and the load sits on a small footprint. The weight shown is the water alone and does not include the tub, the cover or the people in it. If you are placing a tub on a deck or a pad, ask the manufacturer for the full filled weight and check that the structure can carry it.
For other containers and tanks, use the tank volume calculator or the pool volume calculator. The volume calculator handles other solids, and the electricity cost calculator can estimate the cost of heating.
Assumptions and limits
- The tub is a plain cylinder, ellipse or box with straight walls and a flat floor.
- Measurements are inside the tub, and the depth is the water depth.
- The seat and footwell allowance is optional and starts at 0 percent.
- Water weighs 62.4 pounds per cubic foot, and the weight shown is water only.
Frequently asked questions
How many gallons of water are in a hot tub?
It depends on the size and shape. A 7 foot round tub with 3 feet of water is about 864 gallons as a plain cylinder, and a real tub holds less after seats and footwells are taken out.
How do I find the volume of an oval hot tub?
Multiply pi divided by 4 by the length and the width to get the surface area, then multiply by the water depth. Convert cubic feet to gallons by multiplying by 7.4805.
How much does the water in a hot tub weigh?
About 8.3 pounds per gallon, or 62.4 pounds per cubic foot. A tub with 864 gallons holds roughly 7,200 pounds of water.
Should I use this number for chemical dosing?
Use the manufacturer's stated capacity if you have it, since it reflects the real shape. Otherwise use a result with an allowance for the seats, and follow the instructions on the product you are dosing with.