 # Units and Unit Conversions

Units are a very important part of scientific values and calculations. Without units, the numbers scientists talk about have no meaning. For example, saying "it is 20 outside today" doesn’t mean anything unless you attach a unit of measurement. "It is 20 degrees Fahrenheit outside today," means much more.

The units that most people from the United States are familiar with are the English Units. For instance, you know that a football field is 100 yards long, or that the speed limit on the highway is 65 miles per hour. Here are the common English units:

 Most Common English Units Physical Quantity Name of Unit Abbreviation Weight pound lb Length foot ft Time second s Temperature Fahrenheit ° F Amount of Substance mole mol

There are common examples of each of these in everyone's kitchen. For instance, weight is used to describe a whole chicken - the chicken weighs three pounds. A simple scale can be used to measure this. An example of length would be how a chocolate chip cookie recipe tells you to place the dough an inch apart from each other on the cookie sheet because the cookies tend to spread out when they bake. Time is used in the kitchen when you set the egg timer for 35 minutes when baking a cake. Temperature is also important when cooking because you need to know what temperature to set the oven at when baking. For example, when baking a potato, the oven should be set at 350 degrees Farenheit. The amount of a substance is also used in that when you add two cups of water to a cake mix, there are so many moles in two cups. You can measure two cups using a measuring cup. Items like teaspoons and measuring cups measure volume.These are all examples of how these physical quantities are used in the average kitchen.

Most people from other countries however, are not familiar with the English System, instead they know the Metric System. All scientists had to decide on which units they were going to use so that they could communicate. There is a standard unit that scientists attach to all the major types of measurement. The International System (or le Système International in French) is the standard of units that scientists use. For short, this is called the SI system. The following table shows the fundamental units:

 The Fundamental SI Units Physical Quantity Name of Unit Abbreviation Mass kilogram kg Length meter m Time second s Temperature Kelvin K Amount of Substance mole mol

Every measurement in science can be categorized into one of these physical quantities. Mass is how much something weighs. Length is how long something is; the distance an object covers. Time is the relationship between past, present, and future. Temperature is how hot or cold something is. The amount of a substance is how much of something there is.

Prefixes can be used in addition to the fundamental units given above. These prefixes make the amount of something smaller or larger. A table of these is given below:

 Prefixes for the SI System Prefix Symbol Meaning mega M 1,000,000 kilo k 1,000 hecto h 100 deka da 10 deci d 0.1 centi c 0.01 milli m 0.001 micro m 0.000001 nano n 0.000000001

Unit conversions are an important part of scientific calculations. Many different conversions are possible. For instance, you can convert from small SI units to large SI units, or from large English Units to small English Units, or from English Units to SI Units, and so on. Here are some examples.

Converting from small SI Units to large SI Units:

Say that you have traveled 8975 meters and you want to know how many kilometers that is. Here is how you would do it.

 8975 m  x 1 km----------1000 m = 8.975 km

You want the units to cancel out as is shown in the above example. Meters in the numerator cancel with meters in the denominator, leaving only kilometers in the numerator. You have just converted meters into kilometers!

Converting from large SI Units to small SI Units:

Let’s say that you determine your mass is 65 kilograms and you want to know how many grams that is. Here’s how you would do it.

 65 kg  x 1000 g----------1 kg = 65,000 g

You have just converted kilograms into grams! This is getting easy!

Converting from small English Units to large English Units:

Say that you are 72 inches tall, and you want to know how many feet that is. Here is how you’d do it.

 72 in  x 1 ft----------12 in = 6 ft

English Units are not as easy to work with as the SI Units, because you cannot use the metric prefixes. With the metric prefixes, everything is a multiple of 10, and therefore they are easy to manipulate. Even so, the idea is the same for English Units as it was metric units.

Converting from large English Units to small English Units:

You know that a football field is 100 yards long and you want to know how many inches that is. Here’s how you’d do it.

 100 yds  x 3 ft----------1 yd x 12 in----------1 ft = 3600 in

This example required an intermediate step, converting from yards to feet and then from feet to inches. Other than that, you use the same approach as described above.

Converting from English Units to SI Units:

Let’s say that you know it is 45 miles from Champaign to Bloomington and you want to know how many kilometers that is. Here’s how you would do it.

 45 miles  x 5280 ft----------1 mile x 12 in----------1 ft x 2.54 cm----------1 in x 1 m----------100 cm x 1 km----------1000 m = 72.42 km

In this example, there are a lot of intermediate steps, but this just goes to show that starting from any unit of measurement, you can go to any other unit of measurement; just do out all the steps!

Now that you have learned how to convert units, here is an automatic unit converter that will do it for you!

Automatic Unit Converter