Pharmaceutical Calculations:
The International System of Units
Danielle DelVillano, Pharm.D.
1
Objectives
• Demonstrate an understanding of the
International System of Units
• Convert measures within the International
System of Units
• State equivalent measures and complete
conversions between the International System of
Units and other systems of measure used in
pharmacy
• Apply the International System of Units correctly
in calculations
2
Introduction
• International System of Units (SI)
– Formerly called metric system
– Base units
• Meter
• Kilogram
• Liter
http://kaffee.50webs.com/Science/activities/Activity.SI.Unit.Conversions.2.htm
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Guidelines for Correct Use
• Two symbols for
microgram
– mcg
– μg
• Decimals are used, not
fractions
– 5.25 mg
– NOT 5 ¼ mg
• Leading zero should be
used before all decimals
– 0.5 mg
– NOT .5 mg
• Trailing zeros should not
be placed after whole
numbers
– 5 mg
– NOT 5.0 mg
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Common Conversions
• Length
– 1 m = 100 cm
– 100 cm = 1000 mm
• Volume
• Weight
– 1 g = 1000 mg
– 1 g = 0.001 kg
– 1 mg = 1000 mcg
– 1 L = 1000 mL
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Useful Equivalents
Ansel, H. C. (2009) Phamaceutical Calculations (13th Ed.). Philadelphia:Lippincott Williams &
6
Wilkins, and Wolters Kluwer Publishers
Practice Problem 1: Reducing SI Units
• By decimal shift
– Reduce 9876 milligrams to grams
•
•
To convert from smaller to larger number shift the
decimal RIGHT
9876 mg = 9.876 g
– Reduce 2.525 liters to microliters
•
•
To convert from larger to smaller number shift the
decimal LEFT
2.525 L = 2525 mL = 2,525,000 μL
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Practice Problem 2: Reducing SI Units
• Reduce 85 micrometers to centimeters
– By ratio and proportion: (KNOW 1 cm = 10000
μm)
1 cm
10000 mcg
=
x cm
85 mcg
(85 mcg)(1 cm) = x cm
(10000 mcg)
x = 0.0085 cm
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Practice Problem 2: Reducing SI Units
• Reduce 85 micrometers to centimeters
– By dimensional analysis: (KNOW 1 cm = 10000 μm)
85 μm X
1 cm__ = 0.0085 cm
10000 μm
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Practice Problem 3: Addition
• Reduce to common denomination then add or
subtract
• Add 1 kg, 250 mg, and 7.5 g
– 1 kg = 1000 g
– 250 mg = 0.25 g
1000 g
0.25 g
+
7.5 g
1007.75 g
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Practice Problem 4: Subtraction
• Reduce to common denomination then add or
subtract
• Subtract 2.5 mg from 4.85 g
– 2.5 mg = 0.0025 mg
-
4.85
g
0.0025 g
4.8475 g
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Practice Problem 5: Multiplication
• Multiply 820 mL by 12.5 and express the
result in liters
820 mL
x 12.5 _
10250 mL
10250 mL = 10.25 L
12
Practice Problem 6: Division
• Divide 0.465 g by 15 and express the result in
milligrams
0.465 g
÷
15__
0.031 g
0.031 g = 31 mg
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Questions
14
Reference
• Ansel, H. C. (2009) Phamaceutical Calculations
(13th Ed.). Philadelphia:Lippincott Williams &
Wilkins, and Wolters Kluwer Publishers
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Additional Problems
16
Chapter 2 Problem 2
• Add 7.25 L and 875 cL. Reduce the result to
mL.
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Chapter 2 Problem 8
• A liquid contains 0.25 mg of a substance per
milliliter. How many grams of the substance
will 3.5 L contain?
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Chapter 2 Problem 13
• Aspirin tablets generally contain 325 mg of
aspirin. How many such tablets may be
prepared from the 5 kg of aspirin?
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Chapter 2 Problem 15
• Norgestrel and ethinyl estradiol tablets are
available containing 0.5 mg of norgestrel and
50 mcg of ethinyl estradiol. How many grams
of each ingredient would be used in making
10,000 tablets?
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Chapter 2 Problem 20
• A patient is instructed to take three 50 mcg
tablets of pergolide mesylate (PERMAX) daily.
How many mg of the drug would the patient
receive weekly?
21
Chapter 2 Problem 31
• A prefilled syringe contains 20 mg of a drug in
2 mL of solution. How many micrograms of
drug would be administered by an injection of
0.5 mL of the solution?
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Chapter 2 Problem 38
• If an albuterol inhaler contains 18 mg of
albuterol, how many inhalation doses can be
delivered if each inhalation dose contains 90
mcg?
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Chapter 2 Problem 49
• The product of biotechnology, filgrastim
(NEUPOGEN), is available in vials containing
0.3 mg of drug in each milliliter. Which choice
is equivalent in concentration?
(a) 0.03 mg/0.1 dL
(b) 300 mcg/0.01 dL
(c) 3 mcg/0.01 cL
(d) 300 mcg/10 cL
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