Metric System
Measures of Weight
These are the solid measures in the metric system and their abbreviations:
Gram: g
Milligram: mg
Microgram: mcg (μg, which uses the Greek letter mu [μ], is no longer accepted by the Joint Commission for its approved abbreviation list)
Kilogram: kg
Weight Equivalents
These are the basic weight equivalents in the metric system:
1 kg = 1000 g
1 g = 1000 mg
1 mg = 1000 mcg
As you can see, the kilogram is the largest of these.
To equal the weight of a single kilogram, you need 1000 grams.
To equal the weight of a single gram, you need 1000 mg.
To equal the weight of a single milligram, you need 1000 mcg.
The symbol >, which means “greater than,” indicates these relationships:
kg > g > mg > mcg
Read this notation as “A kilogram is greater than a gram, which a gram is greater than a milligram, which is greater than a microgram.”
Converting Solid Equivalents
If the available supply is not in the same weight measure as the medication order, you will have to calculate how much of a drug to give.
EXAMPLE
Order: 0.25 g
Supply: tablets labeled 125 mg
Since 1 g = 1000 mg, you change 0.25 g to milligrams by multiplying the number of grams by 1000:
To convert the order, you use 0.25 g = 250 mg. (drop the “trailing zeroes” to the right of the decimal place)
Here’s an easy rule to help you remember this type of conversion:
Large to small—multiply by 1000
Small to large—divide by 1000
Following this rule, if you are converting grams to milligrams (a larger measurement to a smaller one), multiply the original number by 1000. If you are converting from micro grams to milligrams (a smaller measurement to a larger one), divide the original number by 1000.
There’s another method of conversion as well. In decimals, the thousandth place is three numbers after the decimal point. You can change grams to milligrams by moving the decimal point three places to the right, which produces the same answer as multiplying by 1000. You can also change milligrams to grams by moving the decimal point three places to the left, which is the same as dividing by 1000. You’ll be using this method in some of the calculations to come.
RULE 
CHANGING GRAMS TO MILLIGRAMS
To multiply by 1000, move the decimal point three places to the right.
EXAMPLE
EXAMPLE 1
0.25 g = _____ mg
0.25 g = 250 mg
EXAMPLE 2
0.1 g = _____ mg
0.1 g = 100 mg
GRAMS TO MILLIGRAMS QUICK METHOD. Should you move the decimal point to the left or to the right? This Quick Method can help you decide.
1. First, write the order.
2. Write the supply.
3. Show which way the decimal point should move by drawing a “greater than” sign (>).
4. Remember that in the equivalent 1 g = 1000 mg, the gram is the larger measure, with 1000 mg equaling the weight of 1 g.
5. Move the decimal point three places to the right.
EXAMPLE
EXAMPLE 1
Order: 0.25 g
Supply: 125 mg
You want to convert grams to milligrams.
0.25 g > _____ mg
The “greater than” sign tells you to move the decimal point three places to the right.
Therefore, 0.25 g = 250 mg.
EXAMPLE 2
Order: 1.5 g
Supply: 500 mg
You want to convert grams to milligrams.
1.5 g > _____ mg
The “greater than” sign tells you to move the decimal point three places to the right.
Therefore, 1.5 g = 1500 mg.
SELF-TEST 1
Grams to Milligrams
Convert from grams to milligrams. For correct answers, see the end of the chapter.
1. 0.3 g = _________________________ mg
2. 0.001 g = _______________________ mg
3. 0.02 g = ________________________ mg
4. 1.2 g = _________________________ mg
5. 5 g = ___________________________ mg
6. 0.4 g = _________________________ mg
7. 0.08 g = _______________________ mg
8. 0.275 g = ______________________ mg
9. 0.04 g = _______________________ mg
10. 0.325 g = ______________________ mg
11. 2 g = __________________________ mg
12. 0.0004 g = _____________________ mg
RULE 
CHANGING MILLIGRAMS TO GRAMS
To divide by 1000, move the decimal point three places to the left.
EXAMPLE
EXAMPLE 1
100 mg = _____ g
100 mg = 0.1 g
EXAMPLE 2
8 mg = _____ g
8 mg = 0.008 g
MILLIGRAMS TO GRAMS QUICK METHOD. This method also works to convert milli grams to grams.
1. First, write the order.
2. Write the supply.
3. Show which way the decimal point should move by drawing a “less than” sign (<).
4. Remember that in the equivalent 1 g = 1000 mg, the gram is the larger measure.
5. Move the decimal point three places to the left.
EXAMPLE
EXAMPLE 1
Order: 15 mg
Supply: 0.03 g
You want to convert milligrams to grams.
15 mg < _____ g
The “less than” sign tells you to move the decimal point three places to the left.
Therefore, 15 mg = 0.015 g.
EXAMPLE 2
Order: 500 mg
Supply: 1 g
You want to convert milligrams to grams.
500 mg < _____ g
The “less than” sign tells you to move the decimal point three places to the left.
Therefore, 500 mg = 0.5 g.
Milligrams to Grams
Convert from milligrams to grams. For correct answers, see the end of the chapter.
1. 4 mg = _____________________ g
2. 120 mg = _____________________ g
3. 40 mg = _____________________ g
4. 75 mg = _____________________g
5. 250 mg = _____________________ g
6. 1 mg = _____________________ g
7. 50 mg = _____________________ g
8. 600 mg = _____________________ g
9. 5 mg = _____________________ g
10. 360 mg = _____________________ g
11. 10 mg = _____________________ g
12. 0.1 mg = _____________________ g
RULE 
CHANGING MILLIGRAMS TO MICROGRAMS
The second major weight equivalent in the metric system is 1 mg = 1000 mcg.
Some medications are so powerful that smaller microgram doses are sufficient to produce a therapeutic effect. Rather than using milligrams written as decimals, it’s easier to write orders in micrograms as whole numbers.
To multiply by 1000, move the decimal point three places to the right.
EXAMPLE
EXAMPLE 1
0.1 mg = _____ mcg
0.1 mg = 100 mcg
EXAMPLE 2
0.25 mg = _____ mcg
0.25 mg = 250 mcg
MILLIGRAMS TO MICROGRAMS QUICK METHOD. Should you move the decimal point to the left or to the right? Here are the steps:
1. First, write the order.
2. Write the supply.
3. Show which way the decimal point should move by drawing a “greater than” sign (>).
4. Remember that in the equivalent 1 mg = 1000 mcg, the milligram is the larger measure, with 1000 mcg equaling the weight of 1 mg.
5. Move the decimal point three places to the right.
EXAMPLE 1
Order: 0.1 mg
Supply: 200 mcg
You want to convert milligrams to micrograms.
0.1 mg > _____ mcg
The “greater than” sign tells you to move the decimal point three places to the right.
Therefore, 0.1 mg = 100 mcg.
EXAMPLE 2
Order: 0.3 mg
Supply: 600 mcg
You want to convert milligrams to micrograms.
0.3 mg > _____ mcg
The “greater than” sign tells you to move the decimal point three places to the right.
Therefore, 0.3 mg = 300 mcg.
SELF-TEST 3
Milligrams to Micrograms
Convert from milligrams to micrograms. For correct answers, see the end of the chapter.
1. 0.3 mg = _____________________ mcg
2. 0.001 mg = _____________________ mcg
3. 0.02 mg = _____________________ mcg
4. 0.08 mg = _____________________ mcg
5. 1.2 mg = _____________________ mcg
6. 0.4 mg = _____________________ mcg
7. 5 mg = _____________________ mcg
8. 0.7 mg = _____________________ mcg
9. 0.04 mg = _____________________ mcg
10. 10 mg = _____________________ mcg
11. 0.9 mg = _____________________ mcg
12. 0.01 mg = _____________________ mcg
RULE 
CHANGING MICROGRAMS TO MILLIGRAMS
To divide by 1000, move the decimal point three places to the left.
EXAMPLE
EXAMPLE 1
300 mcg = _____ mg
300 mcg = 0.3 mg
EXAMPLE 2
50 mcg = _____ mg
50 mcg = 0.05 mg
MICROGRAMS TO MILLIGRAMS QUICK METHOD. This method also converts micrograms to milligrams.
1. First, write the order.
2. Write the supply.
3. Show which way the decimal point should move by drawing a “less than” sign (<).
4. Remember that in the equivalent 1 mg = 1000 mcg, the milligram is the larger measure.
5. Move the decimal point three places to the left.
EXAMPLE
EXAMPLE 1
Order: 100 mcg
Supply: 0.1 mg
You want to convert micrograms to milligrams.
100 mcg < _____ mg
The “less than” sign tells you to move the decimal point three places to the left.
Therefore, 100 mcg = 0.1 mg.
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