To calculate the volume of a container in gallons, you usually need to do so by converting the volume from another unit of measure. You can calculate gallons from any other unit of measure used to describe volume.
Steps
Part 1 of 3: Calculate Gallons from Other Imperial System Units
Step 1. Convert cubic inches to gallons
There are 231 cubic inches in a gallon. To calculate the volume of a container in gallons when you have the volume in cubic inches, divide the number of cubic inches by 231.

Example: the volume of a tank is 3042 cubic inches. Find the volume in gallons.
 3042 cubic inches * (1 gallon / 231 cubic inches) = 3042/231 = 13.17 gallons
 Final answer: 13.17 gallons
Step 2. Find gallons from cubic feet
There are 7.48 gallons in 1 cubic foot. To find the volume of a container in gallons if you have that volume in cubic feet, multiply the number of cubic feet by 7.48.

Example: the volume of a tank is 3.89 cubic feet. Find the volume in gallons.
 3.89 cubic feet * (7.48 gallons / 1 cubic foot) = 3.89 * 7.48 = 29.1 gallons
 Final answer: 29.1 gallons
Step 3. Use cubic yards to find gallons
There are 201.97 gallons in 1 cubic yard. If you have the volume of a container in cubic yards and you need to find that volume in gallons, multiply the number of cubic yards by 201.97.

Example: the volume of a swimming pool is 1.86 cubic yards. Find the volume in gallons.
 1.86 cubic yards * (201.97 gallons / 1 cubic yard) = 1.86 * 201.97 = 375.66 gallons
 Final answer: 375.66 gallons
Step 4. Find gallons from ounces
There are 128 ounces in 1 gallon. If you have the volume of a container in ounces, find the number of gallons in the container by dividing the number of ounces by 128.

Example: the volume of a tank is 931 ounces. Calculate the volume of the same tank in gallons.
 931 oz * (1 gallon / 128 oz) = 931/128 = 7.27 gallons
 Final answer: 7.27 gallons
Step 5. Convert drachmas to gallons
There are 1023.99 drachmas for each gallon. If you have the volume of a container in drachmas but need to know the volume in gallons, convert the drachmas to gallons by dividing the number of drachmas by 1023.99.

Example: the volume of a tank is 3841.1 drachmas. Convert this volume to gallons.
 3841.1 drams * (1 gallon / 1023.99 drams) = 3841.1 / 1023.99 = 3.75 gallons
 Final answer: 3.75 gallons
Step 6. Find gallons from cups
There are 16 cups in each gallon. To calculate the number of gallons from a number of cups, just divide the number of cups by 16.

Example: the volume of a container is 86.23 cups. Find the number of gallons.
 86.23 cups * (1 gallon / 16 cups) = 86.23 / 16 = 5.39 gallons
 Final answer: 5.39 gallons
Step 7. Convert pints to gallons
There are 8 pints in each gallon. To find the volume of a container in gallons when you have the volume in pints, divide the volume in pints by 8.

Example: the volume of a tank is 72.99 pints. Find the number of gallons.
 72.99 pints = (1 gallon / 8 pints) = 72.99 / 8 = 9.12 gallons
 Final answer: 9.12 gallons
Step 8. Use quarts to find gallons
There are 4 quarts in each gallon. Calculate the volume of a container in gallons when you have the volume in quarts by dividing the volume in quarts by 4.

Example: the volume of a tank is 54.31 quarts. Convert this amount into gallons.
 54.31 quarts * (1 gallon / 4 quarts) = 54.31 / 4 = 13.58 gallons
 Final answer: 13.58 gallons
Part 2 of 3: Calculate Gallons from Metric Decimal Units
Step 1. Calculate gallons from cubic meters
There are 264.17 gallons for every cubic meter. If you have the volume of a container in cubic meters, you can calculate the volume of that container by multiplying the volume in cubic meters by 264.17.

Example: the volume of a swimming pool is 8.12 cubic meters. Find the volume in gallons.
 8.12 cubic meters * (264.17 gallons / 1 cubic meter) = 8.12 * 264.17 = 2145.06 gallons
 Final answer: 2145.06 gallons
Step 2. Convert cubic centimeters to gallons
There are 3785.41 cubic centimeters in a gallon. To find the volume of a container in gallons when you have the volume in cubic centimeters, divide the number of cubic centimeters by 3,785.41.

Example: the volume of a tank is 7982.09 cubic centimeters. Convert this amount into gallons.
 7982.09 cubic centimeters * (1 gallon / 3785.41 cubic centimeters) = 7982.09 / 3785.41 = 2.11 gallons
 Final answer: 2.11 gallons
Step 3. Find gallons from liters
There are 3.79 liters in each gallon. If you have the volume of a container in liters but want to convert that amount into gallons, do so by dividing the number of liters by 3.79.

Example: the volume of a tank is 8.3 liters. Calculate the volume of the same tank in gallons.
 8.3 liters * (1 gallon / 3.79 liters) = 8.3 / 3.79 = 2.19 gallons
 Final answer: 2.19 gallons
Step 4. Calculate gallons from milliliters
There are 3785.41 milliliters in each gallon. Find the volume of a container in gallons by dividing the number of milliliters by 3,785.41.
 Note that the number of milliliters in a gallon is the same as the number of cubic centimeters in a gallon.

Example: the volume of a tank is 9877.1 milliliters. Find the volume in gallons.
 9877.1 milliliters * (1 gallon / 3785.31 milliliters) = 9877.1 / 3785.31 = 2.61 gallons
 Final answer: 2.61 gallons
Part 3 of 3: Calculate the volume
Step 1. Find the volume of a rectangular prism
The volume of a rectangular prism can be calculated by multiplying the length of a container by its width and by its height. Convert the final answer to gallons using the instructions in this article.

Example: A rectangular tank has a length of 104 inches, a width of 52 inches, and a height of 98 inches. Find the volume in gallons.
 Volume = L * W * H
 Volume = 104 inches * 52 inches * 98 inches
 Volume = 529984 cubic inches
 529984 cubic inches * (1 gallon / 231 cubic inches) = 529984/231 = 2294.3 gallons
 Final answer: 2294.3 gallons
Step 2. Find the volume of a cylinder
The volume of a cylinder can be calculated by multiplying the value of "pi" by the radius squared of the base and by the height of the container. After getting an answer, you need to convert the amount into gallons.

Example: a cylindrical tank has a height of 893 cm and a base with a radius of 271 cm. Find the volume in gallons.
 Volume = π * R^{2} * H
 Volume = 3.14 * 271^{2} * 893 cm
 Volume = 3.14 * 73441 square centimeters * 893 cm
 Volume = 205930032.82 cubic centimeters
 205930032.82 cubic centimeters * (1 gallon / 3785.41 cubic centimeters) = 205930032.82 / 3785.41 = 54400.88 gallons
 Final answer: 54400.88 gallons
Step 3. Find the volume of a cone
Find the volume of a cone by multiplying the value of "pi" by the height of the cone and the radius squared of its base. Divide this value by 3. When finished, convert your final answer to gallons.

Example: A conical container has a height of 2.3 feet and a base with a radius of 1.8 feet. Find the volume in gallons.
 Volume = (1/3) * π * R^{2} * Height
 Volume = (1/3) * 3.14 * 1.8 ^{2} ft * 2.3 feet
 Volume = (1/3) * 3.14 * 3.24 square feet * 2.3 feet
 Volume = 7.8 cubic feet
 7.8 cubic feet * (7.48 gallons / 1 cubic foot) = 7.8 * 7.48 = 58.34 gallons
 Final answer: 58.34 gallons
Step 4. Find the volume of a regular pyramid
Find the volume of a regular pyramid by multiplying the area of the base by the height of the pyramid. Divide this value by 3. When you calculated your final value in cubic units, convert the answer to gallons.

Example: a pyramidal container has a height of 1.92 meters and a base with a length of 2.3 meters and a width of 2.1 meters. Find the volume in gallons.
 Volume = # ** Volume = (1/3) * Base * Height
 Volume = (1/3) * (Length * Width) * Height
 Volume = (1/3) * (2.3 m * 2.1 m) * 1.92 m
 Volume = 3.09 cubic meters
 3.09 cubic meters * (264.17 gallons / 1 cubic meter) = 3.09 * 264.17 = 816.29 gallons
 Final answer: 816.29 gallons