Estimate the fill capacity of common tank shapes with our easytouse calculator. We’ll also provide you with manual calculations to determine the number of US gallons your tank will hold.
To find the total tank volume for a rectangular tank, enter the tank’s length, width, and depth in feet. The calculator will automatically compute the volume in cubic feet and the number of US gallons your tank will hold.
Tank volume in cubic feet = length (ft) x height (ft) x depth (ft)
Note: If your dimensions are in inches, divide the number of inches by 12 to convert to feet. For example, if your tank has a length of 44 inches, 44/12 equals 3.67 feet.
The formula to compute volume for a cylindrical tank is different. You’ll need to know the tank’s radius, which is the diameter divided by two. You’ll also need the height for a vertical cylinder or length for a horizontal cylinder.
Vertical cylindrical tank volume in cubic feet = 3.14 x radius (ft) x radius (ft) x height (ft)
Horizontal cylindrical tank volume in cubic feet = 3.14 x radius (ft) x radius (ft) x length (ft)
Since there are about 7.5 gallons in one cubic foot, you can convert volume to gallons by multiplying by 7.5.
US Gallons = Cubic Footage x 7.5
Examples of How to Calculate Tank Volume
In our first example, we’ll calculate the volume of a rectangular tank with the dimensions 3’ x 2’ x 2.5’.

 To find the volume in cubic feet, we’ll multiply length x width x depth
 Cubic feet = 3 x 2 x 2.5

 To determine the number of gallons our tank will hold, we’ll multiply cubic feet by 7.5
 US Gallons = 15 x 7.5
In our second example, we’ll compute the volume of a vertical cylindrical tank with a 6’ diameter and 4’ height.

 To find the volume in cubic feet, multiply the pi x the radius x radius x height.
 Our radius is 3’, which is half of our diameter. We’ll use 3.14 to represent pi.
 Cubic feet = 3.14 x 3 x 3 x 4

 To determine gallon capacity, multiply cu ft by 7.5.
 US Gallons = 113.04 x 7.5
How to Calculate the Filled Volume of a Tank
If your tank is partially filled, and you want to know how many gallons it’s holding, use one of these formulas, ensuring all measurements are in feet.
Filled volume for a rectangular tank:
Volume = length x width x filled depth
Multiply the volume total by 7.5 to determine how many gallons your tank currently holds.
Filled volumed for a vertical cylindrical tank:
Volume = 3.14 x radius x radius x filled depth
Multiply the volume total by 7.5 to determine the number of US gallons in the tank.
Frequently Asked Questions (FAQ)FAQ
How much will it cost to fill my tank with water?
If you know how many gallons of water your tank will hold, estimate costs by dividing the total by 1,000 and multiplying by $7. City water costs about $4 to $10 per 1,000 gallons, with $7 being the average.
How do you calculate gallons in a square tank?
To calculate gallons in a square tank, find the volume in cubic feet by multiplying the length x width x height in feet. Then multiply the total cu ft by 7.5.
How to convert gallons in a tank to liters?
If you know the number of US gallons your tank will hold, convert it to liters by multiplying by 3.78541.