Volume is measured in different ways in different applications. The flow of rivers is frequently measured in cubic feet per second. Water flow in homes is often measured in gallons per minute. Your water bill will state the amount of water used in the last month in terms of cubic feet, while statistics on domestic water usage will be given in gallons.

## Using the Conversion Ratio

Write the conversion ratio including the units: 1 gallon / 0.134 cubic feet = 1 gal / 0.134 cf = 1. Alternatively, 1 cubic foot equals 7.48 gallons.

Given a number, G, of gallons, use the conversion ratio to cancel gallons leaving cubic feet. Write: (G gal) x (0.134 cf / 1 gal). Now cancel the gallons and you will have a number with the units of cubic feet. Suppose you want to build a container to hold 100 gallons of water. The container will need to be 100 gal x (0.134 cf / 1 gal) = 134 cubic feet.

Given a number C of cubic feet, use the conversion ratio to cancel cubic feet leaving gallons. Write (C cf) x ( 1 gallon / 0.134 cf), and cancel cubic feet. For example, the inside dimensions of a long semi trailer are 52.0 feet long, 9.19 feet high, and 8.25 feet wide. Multiplying these together gives a volume of 3940 cubic feet. This volume is equivalent to roughly 3940 cf x ( 1 gal / 0.134 cf ) = 29,400 gallons. So, a 52 foot long semi trailer can carry at most 29,400 gallons of milk.

Use Google Calculator. In a Google search field, type “3940 cubic feet in gallons” and hit enter or click the magnifying glass icon. Google will give you 29473.2. The previous number suffers from rounding error due to preserving significant digits.

## Geometric Calculation

Use Google Calculator for easy unit conversions.

Measure the length, width and height of a 1/2 gallon milk carton in inches. Assume the milk comes just up to the line where the carton makes a triangle, and measure the height to that level.

Convert each of these measurements of inches into feet by dividing by 12. Multiply the length, width and height to get the volume in cubic feet. The formula is volume = length_width_height. The result is the volume of one half gallon in cubic feet, or in other words, the number of cubic feet in a half of a gallon.

Multiply the volume you obtained for a half-gallon by 2 to get the number of cubic feet in one gallon. You should get a number close to the actual conversion ratio.

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