If you grew up in the United States, chances are excellent that the word "gallon" was a regular feature in your household's vocabulary. If you were raised in Canada, Europe or practically anywhere else, you may view the gallon the way the general U.S. public sees pay telephones nowadays: You can find them, but only if you look.
The gallon is a unit of volume. Until the late 20th century, most American supermarkets sold certain beverages, such as milk, in quantities that stepped in size from 16 oz. to 32 oz. to 64 oz. to gallon size – units better known as the pint, the quart and the half-gallon. These sizes have been almost entirely supplanted by metric units except in the case of milk and gasoline.
What Is a Unit of Volume?
Volume in physics and chemistry is a unit that describes three-dimensional space. It is derived from units of length (one dimension). The basic SI (international system) unit of volume is the cubic meter (m3) but a more common unit used in the metric system is the liter L.
The numerical whims of the liter have allowed for a somewhat painless transition from imperial to metric labeling on things like beverages in the U.S., because it happens to be very similar to a quart.
Volume can describe the amount of a liquid or gas ("a half-gallon of chocolate milk") or the capacity of something ("A 50-gallon tank").
The U.S. Gallon vs. the Imperial Gallon
A point of confusion with units among students is exactly what the names of the different systems refer to. "U.S. system" and "British imperial system" (or just "imperial system") don't always refer to the same thing.
Importantly, the U.S. and imperial systems use the same names for volume quantities that are not identical. A U.S. gallon is 128 fluid ounces, or exactly four quarts; an imperial gallon is 160 fluid ounces, or exactly five quarts.
Thus although you may hear people correctly refer to the non-metric system that has persisted into the 21st century in the U.S. as "imperial," volume units are an exception – though these are obviously not metric, either.
The Gallon and Other Volume Units
You can always refer to a table and use a calculator to convert between the many volume units in use today (see Resources for an example), but it's helpful to have a sense of how the most commonly encountered ones relate to each other.
A gallon includes eight pints, and a pint has long been a convenient unit of sale for beverages. (If you drink one pint, or 16 oz., of water after exercising, you replace about one pound of lost fluid.) As it happens, one half a liter, or 500 milliliters (500 mL) equals 16.9 fluid ounces, so many bottling companies now use this quantity instead.
Similarly, 1 L = 33.8 fluid ounces, making this volume relatable to a quart.
Gas Prices Throughout History
The price per gallon of gasoline has risen over the years just like virtually everything else. In reality, though, excepting fluctuations, when inflation is factored out, gas costs only a little more in 2015 than it did in 1929. This reflects great advances in the technology used to extract fossil fuels from far underground, keeping pace with exploding worldwide demand for fossil fuels since the post-World War II era.
A better summary of gas prices per gallon is that they fluctuate markedly in the face of geopolitical events, but have apparently borne little relationship to anything else over time. It should be interesting to see how renewable energy development affects this situation.
- Quirky fact: You get a volume discount when purchasing milk (that is, a half-gallon might cost $2 and a full gallon $3) but consumers have access only to a flat price-per-gallon scenario at the gasoline pumps.