The International System of Units
Yewande Dayo
Student Pharmacist
Objectives
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Demonstrate an understanding of SI units
Convert units within SI units
State equivalent measure between SI units and other
systems of measure used in pharmacy practice
Convert between SI units and other systems of measure
used in pharmacy practice
Apply SI units correctly in calculations
The International System of Units

Was formerly called the metric system and it is the
internationally recognized decimal system of weights and
measures
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The base units of the SI are the meter and kilogram
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For length, the primary unit is the meter
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For volume, the primary unit is the liter
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For weight, the primary unit is the gram
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Subdivisions and multiples of the primary units, their relative
values and corresponding prefixes are shown in the table
below
Guidelines for Using SI Units

Unit names and symbols are not capitalized except in
headings and beginning of a sentence. Exception for liter.
May be capitalized or in lower case.
 e.g. 2 L or 2 l, 3 mm and 4 g; not 3 Mm and 4 G

Periods are not used following SI symbols except at the
end of a sentence
 e.g. 4 mL and 4 g, not 4 mL. and 4 g.
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A compound unit that is a ratio or quotient of two units
is indicated by a solidus (/) or a negative exponent.
 e.g. 5 mL/h or 5 mLh-1 , not 5 mL per hour
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Symbols should not be combined with spelled-out terms
in the same expression
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e.g. 3 mg/mL not 3 mg/milliliter
Plurals of units when spelled out, have an added “s”.
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5 mLs is wrong!
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Two acceptable symbols of microgram – mcg & µg
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Symbol for square meter is m2
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Cubic centimeter cm3 = mL; CC not accepted in SI
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Decimal fractions are used not common fractions
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3.25 not 3 1/2
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To prevent medication errors, a zero should be placed in
front of a leading decimal point
 e.g. 0.5 g not .5 g
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Trailing zeros should not be placed following a whole
number on prescriptions
 e.g. 5 mg, not 5.0 mg
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when selecting units, the unit that will result in a numeric
value of between 1 and 1000 is selected
 e.g. 500g not 0.5kg; 1.96 kg not 1960 g; 750 mL not
0.75 L
Some Applications of SI units in Pharmacy
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Manufacture/labeling of pharmaceutical products
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Write, fill and compound Rx and medication orders
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Milligram or microgram amounts e.g. 325mg or 30
mg/5 mL
Express laboratory results
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Usually in gram, milligram and microgram quantities
Dose patients
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In kilogram and kiloliter quantities
mg/dL
Nanotechnology
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1 – 1000 nm
Measurements of Length
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The primary unit of length is the meter
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1 kilometer (km) = 1000.000 meters
1 hectometer (hm) = 100.000 meters
1 dekameter (dam) = 10.000 meters
1 meter (m) = 1.000 meter
1 decimeter (dm) = 0.100 meter
1 centimeter (cm) = 0.010 meter
1 millimeter (mm) = 0.001 meter
1 micrometer (μm) = 0.000,001 meter
1 nanometer (nm) = 0.000,000,001 meter
Conversion of the most common length denominations
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1000 millimeters (mm) = 100 centimeters (cm)
100 centimeters (cm) = 1 meter (m)
Measurements of Volume
 The
1
primary unit of volume is the liter
kiloliter (kL) = 1000.000 liters
 1 hectoliter (hL)= 100.000 liters
 1 decaliter (daL)= 10.000 liters
 1 liter (L) = 1.000 liter
 1 deciliter (dL)= 0.100 liter
 1 centiliter (cL) = 0.010 liter
 1 milliliter (mL) = 0.001 liter
 1 microliter (µL)= 0.000,001 liter
Measurements of Weight
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The primary unit of weight is the gram
1
kilogram (kg) = 1000.000 grams
 1 hectogram (hg)= 100.000 grams
 1 decagram (dag)= 10.000 grams
 1 gram (L) = 1.000 gram
 1 decigram (dL)= 0.100 gram
 1 centigram (cL) = 0.010 gram
 1 milligram (mL) = 0.001 gram
 1 microgram (µL)= 0.000,001gram
Prescriptions Written Using SI Units
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These use Arabic numerals before the abbreviations
for the denominations
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Weights are written as grams and decimals of a gram
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3 g or 3.4 g
Volumes are written as milliliters and decimals of a
milliliter
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10 g; 300 mL
5 mL or 20.5 mL
E.g. RX Dextromethorphan HBr 320 mg
Guiafenesin
3.2 g
Cherry Syrup, to make
240 mL
Conversion of SI Units
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To change a unit to the next smaller denomination, move the
decimal point one place to the right
To change a unit to the next larger denomination, move the
decimal point one place to the left
Other Systems of Measurement
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Two other systems of measurement exist
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The avoirdupois system (widely used in the US for measuring weight
and selling goods; predominant in commerce)
The apothecary system (used to be predominant in pharmacy for
volume and weight measure)
Example 1
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A patient receives 500 mcg of estradiol benzoate by
injection every day for 3 weeks. How many grams of
estradiol benzoate does the patient receive during the
course of therapy?
Example 2
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Two kilograms of a drug are used to make 40,000 tablets.
(a) How many milligrams are in each tablet
(b) How many grams would be required to make 60
tablets?
Example 3
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One pint of a cough syrup is dispensed to a patient who
takes 1 tablespoonful of the syrup four times a day for a
week. How many doses of the syrup will remain in the
original bottle after the full course of therapy?
Example 4
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If a vial contains 40 mg of tobramycin sulfate per milliliter,
how many micrograms of tobramycin sulfate would be in
0.1 mL?
Example 5
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A prescription calls for 0.060 g of one ingredient, 2.5 mg
of another and enough of a third to make 0.5 g. How
many milligrams of the third ingredient should be used?
Example 6
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Are the terms mcg/mL and mg/L equivalent or not
equivalent?
Example 7
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How many grams of digoxin (LANOXIN) would be
required to make 25,000 tablets each containing 250
mcg of digoxin?
Example 8
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A patient is instructed to take three 50-mcg tablets of
pergolide mesylate (PERMAX) daily. How many
milligrams of the drug would the patient receive
weekly?
Example 9
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A liquid oral concentrate of morphine sulfate
contains 2.4 g of morphine sulfate in a 120-mL bottle.
Calculate the concentration of morphine sulfate on a
mg/mL basis.
Example 10
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An intravenous solution contains 500 µg of a drug
substance in each milliliter. How many milligrams of
the drug would a patient receive from the
intravenous infusion of a liter of the solution?
Example 11
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A vial contains 80 mg of drug in 2 mL of injection.
How many milliliters of the injection should be
administered to obtain 0.02 g of drug?
Example 12
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A 125-mL container of amoxicillin contains 600 mg/5
mL. How many milliliters would be used to administer
400 mg of amoxicillin?
Case 1
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A nurse telephones a pharmacy regarding the proper
quantity of an injection to administer to a pediatric
patient from a 1-mL vial containing 0.1 mg of digoxin. The
attending physician had prescribed a dose of 25 mcg. How
many milliliters should be the pharmacist’s response?
Case 2
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A hospital pharmacist is asked to prepare an intravenous
infusion of dopamine. Based on the patient’s weight, the
pharmacist calculates a dose of 500 mcg/min for continuous
infusion. The concentration of a premixed dopamine infusion is
400 mg/250 mL. What is the concentration of the infusion on a
mcg/mL basis? How many milligrams of dopamine is the
patient to receive in the first hour of treatment? How long will
the infusion last?