Knowing your hot tub's capacity can help you estimate the amount of money you'll have to spend on energy to keep the tub warm for guests. In addition, the capacity also tells you the quantity of chemicals you need to put in the tub to keep it clean. The easiest way to figure your hot tub's capacity is by fill rate, but you can also estimate the volume using the dimensions of the hot tub.
Things You'll Need
 Measuring tape
 Stop watch
Fill Rate Method

Use the stop watch to time how long it takes to fill your hot tub. If more than one minute, multiply the number of minutes by 60 and then add the number of seconds to convert to seconds. For example, if it takes 28 minutes 30 seconds, multiply 28 by 60 to get 1,680 and then add 30 to get 1,710 seconds. Leave the water running.

Time how long it takes to fill a gallon of water from the same source. This is the flow rate. If more than one minute, multiply the number of minutes by 60 and then add the number of seconds to convert to seconds.

Divide the time it takes to fill your hot tube in seconds by the time it takes to fill a gallon in seconds. For example, if it takes 1,710 second to fill your hot tub and 12 seconds to fill a gallon, divide 1,710 by 12 to find your hot tub has a capacity of 142.5 gallons.
Dimensions Method

Use the tape measure to measure the dimensions of the hot tub in inches. If you have a round hot tub, measure the diameter, or distance across the pool through the center, and the depth. If you have a rectangular hot tub, measure the length, width and depth.

Multiply length by width by depth for rectangular tubs or multiply diameter by diameter by depth for round hot tubs. For example, if you have a round hot tube that measured 60 inches across and 48 inches deep, multiply 60 by 60 by 48 to get 172,800.

Divide the result by 1,728. In the example, divide 172,800 by 1,728 to get 100.

Multiply the result by 2.4 if your hot tub has seating inside or 4.8 if it has no seating inside to find the number of gallons in the hot tub. Completing the example, if the hot tub has no seating inside, multiply 4.8 by 100 to find the hot tub capacity equals 480 gallons.
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