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4.5.1 The Metric System
Learning Objective(s) 1 Describe the general relationship between the U.S. customary units and metric units of length, weight/mass, and volume. 2 Define the metric prefixes and use them to perform basic conversions among metric units.
Introduction
In the United States, both the U.S. customary measurement system and the metric system are used, especially in medical, scientific, and technical fields. In most other countries, the metric system is the primary system of measurement. If you travel to other countries, you will see that road signs list distances in kilometers and milk is sold in liters. People in many countries use words like “kilometer,” “liter,” and “milligram” to measure the length, volume, and weight of different objects. These measurement units are part of the metric system.
Unlike the U.S. customary system of measurement, the metric system is based on 10s. For example, a liter is 10 times larger than a deciliter, and a centigram is 10 times larger than a milligram. This idea of “10” is not present in the U.S. customary system—there are 12 inches in a foot, and 3 feet in a yard…and 5,280 feet in a mile!
So, what if you have to find out how many milligrams are in a decigram? Or, what if you want to convert meters to kilometers? Understanding how the metric system works is a good start.
What is Metric?
The metric system uses units such as meter , liter , and gram to measure length, liquid volume, and mass, just as the U.S. customary system uses feet, quarts, and ounces to measure these.
In addition to the difference in the basic units, the metric system is based on 10s, and different measures for length include kilometer, meter, decimeter, centimeter, and millimeter. Notice that the word “meter” is part of all of these units.
The metric system also applies the idea that units within the system get larger or smaller by a power of 10. This means that a meter is 100 times larger than a centimeter, and a kilogram is 1,000 times heavier than a gram. You will explore this idea a bit later. For now, notice how this idea of “getting bigger or smaller by 10” is very different than the relationship between units in the U.S. customary system, where 3 feet equals 1 yard, and 16 ounces equals 1 pound.
Length, Mass, and Volume
The table below shows the basic units of the metric system. Note that the names of all metric units follow from these three basic units.
Objective 1
Length Mass Volume basic units meter gram liter other units you may see kilometer kilogram dekaliter centimeter centigram centiliter millimeter milligram milliliter
In the metric system, the basic unit of length is the meter. A meter is slightly larger than a yardstick, or just over three feet.
The basic metric unit of mass is the gram. A regular-sized paperclip has a mass of about 1 gram.
Among scientists, one gram is defined as the mass of water that would fill a 1-centimeter cube. You may notice that the word “mass” is used here instead of “weight.” In the sciences and technical fields, a distinction is made between weight and mass. Weight is a measure of the pull of gravity on an object. For this reason, an object’s weight would be different if it was weighed on Earth or on the moon because of the difference in the gravitational forces. However, the object’s mass would remain the same in both places because mass measures the amount of substance in an object. As long as you are planning on only measuring objects on Earth, you can use mass/weight fairly interchangeably—but it is worth noting that there is a difference!
Finally, the basic metric unit of volume is the liter. A liter is slightly larger than a quart.
The handle of a shovel is about 1 meter.
A paperclip weighs about 1 gram.
A medium-sized container of milk is about 1 liter.
Though it is rarely necessary to convert between the customary and metric systems, sometimes it helps to have a mental image of how large or small some units are. The table below shows the relationship between some common units in both systems.
Common Measurements in Customary and Metric Systems Length 1 centimeter is a little less than half an inch. 1.6 kilometers is about 1 mile. 1 meter is about 3 inches longer than 1 yard. Mass 1 kilogram is a little more than 2 pounds. 28 grams is about the same as 1 ounce. Volume 1 liter is a little more than 1 quart. 4 liters is a little more than 1 gallon.
Converting Units Up and Down the Metric Scale
Converting between metric units of measure requires knowledge of the metric prefixes and an understanding of the decimal system—that’s about it.
For instance, you can figure out how many centigrams are in one dekagram by using the table above. One dekagram is larger than one centigram, so you expect that one dekagram will equal many centigrams.
In the table, each unit is 10 times larger than the one to its immediate right. This means that 1 dekagram = 10 grams; 10 grams = 100 decigrams; and 100 decigrams = 1, centigrams. So, 1 dekagram = 1,000 centigrams.
Example
Problem How many milligrams are in one decigram?
kg hg dag g dg cg mg ^ ^
Identify locations of milligrams and decigrams.
Decigrams (dg) are larger than milligrams (mg), so you expect there to be many mg in one dg. ×10 × kg hg dag g dg cg mg ^ ^
Dg is 10 times larger than a cg, and a cg is 10 times larger than a mg.
Since you are going from a larger unit to a smaller unit, multiply.
1 dg • 10 • 10 = 100 mg Multiply: 1 • 10 • 10, to find the number of milligrams in one decigram.
Answer There are 100 milligrams (mg) in 1 decigram (dg).
Example
Problem Convert 1 centimeter to kilometers.
km hm dam m dm cm mm ^ ^
Identify locations of kilometers and centimeters.
Kilometers (km) are larger than centimeters (cm), so you expect there to be less than one km in a cm.
÷ 10 ÷ 10 ÷ 10 ÷ 10 ÷ 10 km hm dam m dm cm mm ^ ^
Cm is 10 times smaller than a dm; a dm is 10 times smaller than a m, etc.
Since you are going from a smaller unit to a larger unit, divide.
1 cm ÷ 10 ÷ 10 ÷ 10 ÷ 10 ÷ 10 = 0.00001 km
Divide: 1 ÷ 10 ÷ 10 ÷ 10 ÷ 10 ÷ 10, to find the number of kilometers in one centimeter.
Answer 1 centimeter (cm) = 0.00001 kilometers (km).
Once you begin to understand the metric system, you can use a shortcut to convert among different metric units. The size of metric units increases tenfold as you go up the metric scale. The decimal system works the same way: a tenth is 10 times larger than a hundredth; a hundredth is 10 times larger than a thousandth, etc. By applying what you know about decimals to the metric system, converting among units is as simple as moving decimal points.
Here is the first problem from above: How many milligrams are in one decigram? You can recreate the order of the metric units as shown below:
(^1 )
kg hg dag g d g c g m g
This question asks you to start with 1 decigram and convert that to milligrams. As shown above, milligrams is two places to the right of decigrams. You can just move the decimal
point two places to the right to convert decigrams to milligrams:
1 2
1 dg = 100. mg.
The same method works when you are converting from a smaller to a larger unit, as in the problem: Convert 1 centimeter to kilometers.
5 4 3 2 1
k m h m d am m d m c m mm
Note that instead of moving to the right, you are now moving to the left—so the decimal
point must do the same:
5 4 3 2 1
1 cm = 0.00001 km.
Self Check B
How many milliliters are in 1 liter?
Self Check C
Convert 3,085 milligrams to grams.
Example
Problem Convert 7,225 centimeters to meters.
7,225 cm = ___ m Meters is larger than centimeters, so you expect your answer to be less than 7,225.
cm m
m
cm
Using the factor label method, write 7,225 cm as a fraction and use unit fractions to convert it to m.
7,225 cm 1
m
cm
=___ m
m
= m
m
= m
Cancel similar units, multiply, and simplify.
Answer 7,225 centimeters = 72.25 meters
Self Check D
Convert 32.5 kilometers to meters.
Now that you have seen how to convert among metric measurements in multiple ways, let’s revisit the problem posed earlier.
Example
Problem If you have a prescription for 5,000 mg of medicine, and upon getting it filled, the dosage reads 5 g of medicine, did the pharmacist make a mistake?
5,000 mg = ___ g? Need to convert mg to g.
mg g
g
mg
5,000 mg 1
g
mg
=___ g
g g
g
= g
Answer 5 g = 5,000 mg, so the pharmacist did not make a mistake.
Summary
The metric system is an alternative system of measurement used in most countries, as well as in the United States. The metric system is based on joining one of a series of prefixes, including kilo-, hecto-, deka-, deci-, centi-, and milli-, with a base unit of measurement, such as meter, liter, or gram. Units in the metric system are all related by a power of 10, which means that each successive unit is 10 times larger than the previous one.
This makes converting one metric measurement to another a straightforward process, and is often as simple as moving a decimal point. It is always important, though, to consider the direction of the conversion. If you are converting a smaller unit to a larger unit, then the decimal point has to move to the left (making your number smaller); if you are converting a larger unit to a smaller unit, then the decimal point has to move to the right (making your number larger).
The factor label method can also be applied to conversions within the metric system. To use the factor label method, you multiply the original measurement by unit fractions; this allows you to represent the original measurement in a different measurement unit.
4.5.1 Self Check Solutions
Self Check A
Which of the following sets of three units are all metric measurements of length?
kilometer, centimeter, millimeter Correct. All of these measurements are from the metric system. You can tell they are measurements of length because they all contain the word “meter.”
Self Check B
How many milliliters are in 1 liter?
There are 10 milliliters in a centiliter, 10 centiliters in a deciliter, and 10 deciliters in a liter. Multiply: 10 • 10 • 10, to find the number of milliliters in a liter, 1,000.
4.5.2 Using Metric Conversions to Solve Problems
Learning Objective(s) 1 Solve application problems involving metric units of length, mass, and volume.
Introduction
Learning how to solve real-world problems using metric conversions is as important as learning how to do the conversions themselves. Mathematicians, scientists, nurses, and even athletes are often confronted with situations where they are presented with information using metric measurements, and must then make informed decisions based on that data.
To solve these problems effectively, you need to understand the context of a problem, perform conversions, and then check the reasonableness of your answer. Do all three of these steps and you will succeed in whatever measurement system you find yourself using.
Understanding Context and Performing Conversions
The first step in solving any real-world problem is to understand its context. This will help you figure out what kinds of solutions are reasonable (and the problem itself may give you clues about what types of conversions are necessary). Here is an example.
Example
Problem Marcus bought at 2 meter board, and cut off a piece 1 meter and 35 cm long. How much board is left?
2 meters – 1 meter 35 cm To answer this question, we will need to subtract.
First convert all measurements to one unit. Here we will convert to centimeters.
• ___
m cm
cm
m
Use the factor label method and unit fractions to convert from meters to centimeters.
2 m 100
cm
m
= ___ cm
cm
cm
Cancel, multiply, and solve.
Objective 1
1 meter + 35 cm 100 cm + 35 cm 135 cm
200 cm – 135 cm 65 cm
Convert the 1 meter to centimeters, and combine with the additional 35 centimeters.
Subtract the cut length from the original board length. Answer There is 65 cm of board left.
An example with a different context, but still requiring conversions, is shown below.
Example
Problem A faucet drips 10 ml every minute. How much water will be wasted in a week?
10 60 minute 24 hours 7 days
1 minute 1 hour 1 day 1 week
ml Start by calculating how
much water will be used in a week using the factor label method to convert the time units.
1 minute
ml 60 minute
1 hour
24 hours
1 day
7 days
1 week
Cancel, multiply and solve.
1111 week
ml
1 week
ml To give a more useable
answer, convert this into liters.
100800 ml 1 L
1 week 1000 ml
100800 ml
1 L
1 week 1000 ml
100800 L
1000 week
L
week
Cancel, multiply and solve.
Answer The faucet wastes about 100.8 liters each week.
This problem asked for the difference between two quantities. The easiest way to find this is to convert one quantity so that both quantities are measured in the same unit, and then subtract one from the other.
Example
Problem A bottle contains 1.5 liters of a beverage. How many 250 mL servings can be made from that bottle?
250 mL = ___ L You need to convert 250 mL to liters ÷ 10 ÷ 10 ÷ 10 kL hL daL L dL cL mL ^ ^
- mL → 0.250 L
On the chart, L is three places to the left of mL.
Move the decimal point three places to the left in 250 mL
1.5 L ÷ 250 mL =
025 L
1 .5L
250 mL
1 .5L
Now divide as was done in the last example
025 L
1 .5 L
Answer The bottle holds 6 servings.
The initial answer checks out — the bottle holds 6 servings. Checking one conversion with another method is a good practice for catching any errors in scale.
Summary
Understanding the context of real-life application problems is important. Look for words within the problem that help you identify what operations are needed, and then apply the correct unit conversions. Checking your final answer by using another conversion method (such as the “move the decimal” method, if you have used the factor label method to solve the problem) can cut down on errors in your calculations.
4.5.2 Self Check Solutions
Self Check A
A bread recipe calls for 600 g of flour. How many kilograms of flour would you need to make 5 loaves?
Multiplying 600 g per loaf by the 5 loaves, 600g ∙ 5 = 3000 g Using factor labels or the “move the decimal” method, convert this to 3 kilograms You will need 3 kg of flour to make 5 loaves.
