Calculate the exact liquid capacity for rectangular tanks, swimming pools, aquariums, and cylindrical pipes. Instantly convert cubic feet to US Gallons and Liters.
Whether you are filling a backyard swimming pool, setting up a saltwater aquarium, or calculating the water capacity of a rural storage tank, you need to know exactly how many gallons of water the space holds. This is critical for buying the correct amount of chemicals, chlorine, or water conditioners.
For rectangular pools and tanks, the formula is simple: Length × Width × Depth = Cubic Feet. Then, multiply the cubic feet by 7.48 to get US Gallons.
For cylindrical pipes and round tanks, you must calculate the area of the circle first using Pi (3.14159) × Radius², multiply by the height, and then convert to gallons.
This is one of the most common questions plumbers and pool builders get. There are exactly 7.48052 US Gallons in 1 Cubic Foot of liquid.
Why such a weird number? A US Gallon is legally defined as exactly 231 cubic inches. A cubic foot is 12" × 12" × 12" (1,728 cubic inches). If you divide 1,728 by 231, you get 7.480519... which contractors round to 7.48.
When flushing a plumbing system or calculating the weight of water in a vertical stack, you need to know the volume of the pipes. Water is heavy—it weighs 8.34 pounds per gallon (or 62.4 lbs per cubic foot). A large vertical pipe filled with water can add hundreds of pounds of structural load to a building.
Use the "Cylindrical" toggle on our calculator above. Enter the pipe's inner diameter in inches and the length in feet to find the exact liquid volume.
A pool that is 10 feet wide, 20 feet long, and 4 feet deep holds exactly 800 cubic feet of water. Multiply 800 by 7.48, and the pool holds 5,984 US Gallons.
Measure the diameter across the center of the tank and divide by 2 to get the radius. Square the radius, multiply by Pi (3.14159), and multiply by the height of the water. This gives you cubic feet. Multiply that by 7.48 to get gallons.
A standard 5/8-inch garden hose holds about 0.16 gallons of water per foot. Therefore, a 50-foot hose holds roughly 8 gallons of water when fully pressurized.
To convert gallons back to cubic feet, divide by 7.48. A standard 40-gallon water heater occupies roughly 5.34 cubic feet of physical space.