Apothecary, and Household Systems of Measurement


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:


image


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 image CHANGING GRAMS TO MILLIGRAMS


To multiply by 1000, move the decimal point three places to the right.


EXAMPLE



 


EXAMPLE 1


0.25 g = _____ mg


image


0.25 g = 250 mg


 



 


EXAMPLE 2


0.1 g = _____ mg


image


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.


image


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.


image


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 image CHANGING MILLIGRAMS TO GRAMS


To divide by 1000, move the decimal point three places to the left.


EXAMPLE



 


EXAMPLE 1


100 mg = _____ g


image


100 mg = 0.1 g


 



 


EXAMPLE 2


8 mg = _____ g


image


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.


image


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.


image


Therefore, 500 mg = 0.5 g.


 



SELF-TEST 2


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 image 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


image


0.1 mg = 100 mcg


 



 


EXAMPLE 2


0.25 mg = _____ mcg


image


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



 


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.


image


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.


image


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 image CHANGING MICROGRAMS TO MILLIGRAMS


To divide by 1000, move the decimal point three places to the left.


EXAMPLE



 


EXAMPLE 1


300 mcg = _____ mg


image


300 mcg = 0.3 mg


 



 


EXAMPLE 2


50 mcg = _____ mg


image


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.


image


Therefore, 100 mcg = 0.1 mg.

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Jul 12, 2017 | Posted by in PHARMACY | Comments Off on Apothecary, and Household Systems of Measurement

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