Join the Rest of the World With the International System of Units

Author: Colin D. Bennett <>
Date: 29 May 2006

The United States is by all accounts the last to catch up to the standard international system of measurement. It is the only country on the planet still using miles, feet, inches, pounds, and ounces to describe measurements. Every other country now uses the International System of Units, which you may know as the metric system. In 1960, the name of the metric system was officially changed to le Système International d'Unités (abbreviated SI), which translates from the French name as the International System of Units (Shattuck). The SI is a more organized and consistent system of measurement than the current U.S. system. The United States should quickly and completely adopt the International System of Units. As a result of using SI units, life will be more convenient for everyone.

Every other country has adopted the SI except for the United States. The official system of units in the U.S. is called U.S. customary units but is commonly called English units. Oddly, we're more stubborn about holding on to our old units than even the English were about the system they created. As Shattuck writes, “both Canada and Britain officially converted to SI units in the early 1980s.” Now that the U.S. is the last holdout, other countries are finding that producing and packaging products specially for the United States is an undesirable expense. “Even European Union countries that have long been good U.S. customers no longer want our nonmetric products. In turn, they don't want to supply the U.S. with nonmetric products anymore because of the additional cost” (Lipsett). The rest of the world is unified in their system of measurement, and the United States is going to be left out if they don't join in. By the end of the year 2009, the European Union will prohibit use of any non-metric units to describe products (Meissner 7). In order to remain a key player in the world economy, the United States needs to standardize on the same measurement system.

Not only is standardizing on units important for international commerce, having a single system of units is important for domestic purposes. Currently in the United States, U.S. customary units are used side by side with metric units. Even though Americans use mostly U.S. units such as inches and pounds in their everyday lives, scientists almost exclusively use the SI. As a result, turning scientific research into products requires a change in measurement systems. Sometimes this can be problematic. In 1999, NASA launched a $125 million spacecraft intended to explore the surface of Mars. But on approach to Mars, disaster struck. Because of a failure to properly convert critical data between metric units and U.S. units, the spacecraft veered off course (Oliver and Nichols). Communication with the craft was lost, so no one knows whether it crashed into the surface of the planet or glanced off into the depths of space (Lipsett). The lack of a standard measurement system magnifies the importance of keeping units attached to numbers. Knowing that the force of a thruster is 12.5 tells you nothing unless you know the units. Extreme care is required in computer programs such as spacecraft navigation systems. Since computer programs usually deal purely with numbers, the units must be assumed by the program. Clearly, an incorrect assumption can have disastrous results. Additionally, as Lipsett points out, “This is not an isolated example, simply a reported one.” Maintaining two completely different systems of measurement is wasteful in a variety of ways and is costly to the country.

The SI was designed to be natural to use. One of the primary advantages SI has over the U.S. customary system is that everything is in multiples of ten. Instead of 5280 feet per mile, 16 ounces per pound, and 12 inches per foot, it has 1000 meters per kilometer, 1000 grams per kilogram, and 100 centimeters per meter. You probably know how to quickly multiply and divide by ten. This is possible because our number system, using the digits 0 through 9, is a base ten number system. The magic of base ten is that dividing and multiplying by multiples of ten are as easy as moving the decimal point left or right. As a convenience, the SI has established prefixes as shorthand for common multiples of ten. It's easier to say that a coin is 1 millimeter thick than it is to say it is 0.001 meters thick, for instance. The prefixes represent a scale factor in a compact manner. You probably already recognize the most common prefixes such as kilo, mega, centi, and milli. This consistent system makes it convenient to talk about quantities of any magnitude. Working with fractional units is particularly easier in the SI than the U.S. system. Try adding 5/32 of an inch to 1/8 of an inch in your head. Which quantity is larger? Even if you are good at doing arithmetic in your head, it still takes some effort. Now give it a try in SI units: Add 4 millimeters and 3 millimeters. Now can you tell which quantity is larger? By using the International System of Units, simple tasks such as adding or comparing quantities becomes easier.

Such simple tasks are the basis for many important activities. Baking is one example of an everyday activity complicated by the disorder of the U.S. system. For instance, most consumer cookbooks call for quantities of ingredients by volume as opposed to weight. However, in the U.S. customary system, volume units are classified as either “fluid” or “dry” measures. The division of fluid and dry measures is purely a historical wart, and there is no technical reason why a distinction should be made between fluid volume and dry volume. In practice, dry measures are rarely used and include the dry pint, dry quart, dry gallon, and the bushel. Fluid measures such as cups and tablespoons are usually used for dry ingredients as well as liquid ingredients. The distinction between fluid and dry measures is particularly distressing in the cases where there is a dry measure having the same name as a fluid measure such as with pints, quarts, and gallons. For instance, a dry pint is about 16% larger than a fluid pint, and a dry quart is 14% larger than a fluid quart. Then there is the fluid ounce and the “weight” ounce. I have often seen measurements listed as “12 oz” or “20 oz” on measuring cups, which should be specified as fl oz to indicate that it is a “fluid” measure. Recall that a fluid measure is a measure of volume, not weight. Don't be led to believe that you can measure a pound of sugar by filling a measuring cup with it to the “16 oz” line. Even specifying fluid ounces leaves some ambiguity because the U.S. fluid ounce is not the same as the British Imperial fluid ounce. In the SI, there is never confusion about this sort of thing. The SI was designed to be complete and straightforward without any unnecessary redundancy. The disorder in the U.S. customary system is primarily a result of adoption of redundant units throughout the history of the system. To demonstrate the difficulty of using even the most common measures in cooking, consider this: Do you even know how many teaspoons there are in a tablespoon, how many tablespoons per cup, or how many cups in a quart? There are three teaspoons per tablespoon, sixteen tablespoons per cup, and four cups per quart. There really is no consistency in the units and, unlike the SI, they are not organized in a way to make common tasks easy. Because ingredients are often specified in fractions of a measure such as 3/4 tablespoon, 1/3 cup, and so on, doubling or tripling a recipe means you have to deal with fractions in addition to the inconsistent measures. Even when shopping for groceries, the U.S. customary system of units gets in the way. Many grocery stores conveniently display the “unit price” on price tags. This can help you save money as a consumer by making it easier to see which brand of a particular product is the most cost-efficient. However, I've encountered the situation where one brand's unit price is listed in dollars per pound and another brand right next to it has its unit price listed in cents per ounce. Can you tell if $4.27 per pound is a better deal than 31¢ per ounce? Converting between dollars and cents is easy, similar to converting between centimeters and meters, but converting dollars per ounce to dollars per pound is not as simple. If the products were all listed in dollars per kilogram, comparison shopping would be a great deal easier. By using SI units, everyday tasks such as cooking and shopping for food will become easier.

Although most Americans feel comfortable with the U.S. system of units, they have also accepted SI units in many areas. As proof that we can adjust to metric units, consider the two liter soda bottle. You never hear anyone object to the use of metric units to measure their Mountain Dew. Another everyday use of SI units is on Nutrition Facts labels. The labels list fat, protein, cholesterol, and other elements in grams or milligrams only. After a moment's consideration, the reason is apparent. Since the ounce is the smallest measure of weight in the U.S. system of units, a product containing 90 milligrams of sodium per serving would have to be described as having 0.003 ounces of sodium per serving. Because using SI units allows more compact representation of quantities, the SI was the clear choice for use on Nutrition Facts labels. Adoption of SI units for certain purposes has become accepted in the United States, but the decisions have had to be made by specific decision-making groups such as the beverage bottling industry or the Food and Drug Administration. Any movement toward the use of metric units should be considered progress, but individual Americans can experience the greatest advantage by personally applying metric units in their lives. Without the cooperation of people like you and me, the U.S. will not succeed in adopting the world's standard measurement system.

It's important for individuals to see how much easier it is to work with SI units than U.S. units. It will take time to become familiar and comfortable with the SI units, but you will see benefits quickly. One of my classmates spent a year in Europe and found that learning the metric system was really not difficult. Most importantly, you need to get a feeling for the approximate size of the new units. The length of a foot or the size of a gallon feels natural to most Americans. However, you can visualize a two liter bottle and have an immediate feel for the magnitude of a liter. The main obstacle in personally adopting SI units is that most people around you in the U.S. are not yet using SI units. But for many purposes, it is possible and useful to begin using SI units yourself. Just thinking about things in metric units can help. For instance, you could begin weighing yourself in kilograms. You'll begin to gain an instinctual feel for the magnitude of kilograms. You may also consider having a two-digit weight to be a pleasant side benefit. Personally adopting the metric system, even to a small degree, is a big step forward toward better understanding of the world's standard measurement system.

There are three options for measurement systems in the United States. We can use only the U.S. customary system, use only the SI, or continue to casually mix the two systems. The first option is clearly not feasible because the SI has become a permanent part of science, and the rest of the world uses it. So either we exclusively adopt the International System of Units or live with two systems. Maintaining two completely different systems of measurement causes wasted time and effort in converting between systems and results in increased costs to produce goods. While it will take effort at the individual level to learn the new system and become familiar with its units, the payoff will come quickly.

Works Cited

Lipsett, Robert. “It's Time to Go Metric.”
Design News 4 Apr. 2005: 24. ProQuest Direct. UMI. 11 May 2006.
Meissner, Donald. “Whatever Happened to the Metric System?”
Science Scope: 29.1 (Sep. 2005): 6-7. ProQuest Direct. UMI. 13 May 2006.
Oliver, J. Steve and B. Kim Nichols. “Early Days: Metric Revisited.”
School Science and Mathematics 102.2 (Feb. 2002): 94-97. ProQuest Direct. UMI. 11 May 2006.
Shattuck, Deborah. “SI Units Revisited.”
Journal of the American Dietetic Association. 102.4 (2002): 466-467. ProQuest Direct. UMI. 11 May 2006.