How to Calculate the Gallons Per Cubic Foot

One cubic foot equals about 7.48 gallons.
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Compared to the SI (metric) system of measurement, the imperial system is a hodgepodge of different units. People in the United States of the future will hopefully recognize the elegance of the metric system and relegate the imperial system to the annals of history, but until that happens, you have to know how to convert between imperial units.

When it comes to gallons, the situation is complicated by the fact that there are three different ones. The U.S. liquid gallon and the U.S. dry gallon, which were discarded by the British when they standardized the imperial system in 1824, are both smaller than the British imperial gallon.

Having switched to the metric system in 1965, the British don't even measure in gallons anymore, so unless otherwise specified, a "gallon" usually refers to a U.S. liquid gallon. However, it's good to know the conversion of gallons to cubic feet for all gallons.

So What is a Gallon, Anyway?

The history of the gallon goes back to the turn of the 18th century, during the rule of Queen Ann. In those days, wine was more important than fuel, and a ​wine gallon​ was established to standardize the measurement of this precious commodity. It was defined to be equal to 231 cubic inches.

There were separate gallons for measuring ale and corn or wheat, and believe it or not, wine gallons and corn gallons are still actual units of measurement in the United States!

The ​corn gallon​ is known as a dry gallon, and the conversion from cubic feet to dry gallons is different than the conversion from cubic feet to liquid gallons. To make the conversion, you must know that a dry gallon is defined to be equal to 268.8 cubic inches, which makes it about 16 percent bigger than a wine gallon.

In 1824, when the British passed the Weights and Measures Act, they sought to create a single gallon for all measurements. They based it on the volume occupied by 10 pounds avoirdupois of water at 62 degrees Fahrenheit and atmospheric pressure of 30 inches. This made one ​imperial gallon​ equal to 277.421 cubic inches. The conversion factors, rounding to whole numbers, are as follows:

  • 1 U.S. liquid gallon = 231 cubic inches
  • 1 U.S dry gallon = 269 cubic inches
  • 1 Imperial gallon = 277 cubic inches 

Finding Gallons per Cubic Foot

Once you know the definitions of the various gallons in terms of cubic inches, it's easy to express them in cubic feet. One cubic foot is equal to (12 × 12 × 12) = 1,728 cubic inches, Therefore, one cubic inch = 1 / 1,728 = 5.8 × 10 −4 cubic feet.

The conversion factors then become:

  • 1 U.S. liquid gallon = 0.134 cubic feet
  • 1 U.S. dry gallon = 0.156 cubic feet 
  • 1 Imperial gallon = 0.16 cubic feet

Conversely, 1 cubic foot equals:

  • 7.48 U.S. liquid gallons 
  • 6.48 U.S. dry gallons
  • 6.23 Imperial gallons

Converting Gallons to Cubic Feet for Compressed Gases

Both propane and natural gas are used as motor vehicle fuels in the United States, and each has its own gallon measurement. The conversion factor for compressed natural gas, established by the U.S Department of Revenue Services, is:

  • 1 U.S liquid gallon natural gas = 126.67 cubic feet
  • 1 cubic foot natural gas = 0.00789 U.S. liquid gallons

The U.S. government does not establish a conversion factor for propane, leaving that to the states. The propane cubic feet to gallons calculator used by the State of Connecticut is typical. At at 14.73 psi and 60 degrees Fahrenheit, the conversion is

  • 1 U.S. liquid gallon compressed propane = 35.97 cubic feet propane
  • 1 cubic foot compressed propane = 0.0278 U.S. liquid gallons