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Chapter 9
Calculating Drug Dosages
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Chapter 9
Lesson 9.1
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Learning Objectives
• Use formulas to determine the dosages of
tablets, capsules, or liquids
• Use formulas to determine the total number
of tablets or capsules or the amount of liquid
to be ordered for a specified time
• Use information about the apothecaries',
metric, and household measurements
systems to accurately calculate drug dosages
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Learning Objectives (cont.)
• Calculate dosages for parenteral
injections, including those for special
preparations such as insulin
• Calculate flow rates for infusions
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Calculating Medication
Dosages
Three Steps
1. Verify the drug available is the same
measurement system as the drug
dosage desired (convert if needed)
2. Reduce to lowest terms
3. Calculate dosage quantity to be
administered
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Drug Calculation Methods
• Fraction Method
– 600 mg = 200 mg
x tablets
1 tablet
Solve for x
• Ratios or Proportion Method
– 600 mg : x tablets :: 200 mg : 1 tablet
– Solve for x
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Drug Calculation Methods
(cont.)
• Desired over Available Method
Desired units (conversion factor) x
Quantity of drug form = Quantity to give
Quantity available (x conversion factor)
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Forms of Oral Medications
• Capsules
– Cannot be broken or divided
– If amount to be given is more than 0.5, round to next
whole number
• Tablets
– Only divide if scored
– Coated tablets are not to be broken
• Liquids
– May be measured in a medication cup, syringe, or
calibrated dropper
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Scored and Unscored Tablets
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Parenteral Medications
• Medication available in three forms:
– Prefilled syringe labeled with specific dosage
• For example: meperidine (Demerol) 100 mg in 1 mL
– Single-dose ampule or multiple-dose vial labeled
with a specific dosage per volume
• For example: epinephrine (Adrenalin) 1:1000 in 0.1 mL
– A vial with powder that requires a specific fluid be
added to it to obtain a specific dosage
(Reconstitution)
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Insulin
• A critical medication that replaces the
insulin not being produced by the
patient’s pancreas
• Insulin comes in a standardized
measure called a “Unit”
• Smallest amounts may be given; errors
are critical
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U-100 Vial
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Insulin (cont.)
• Strengths
– U-100 (100 Units of insulin per 1 mL)
– U-500 (500 Units of insulin per 1 mL)
• Preparation 5 times stronger, rarely used
• Syringe
• Calibrated in Units also
• Tuberculin syringe used in emergency
– Minims used; 16 minims = 1 mL
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U-100 Syringe
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Tuberculin Syringe
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Intravenous Medications
• Medications administered into the vein
– IV push
– IV hanging by gravity (flow rate formula)
– IV pump (mL/min or hr)
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Flow Rate Formula
• Gtts/min =
Volume to be administered × gtt factor
Time in minutes
• Drop factor of tubing:
Macrodrip = 10, 15, or 20 gtt/mL
Microdrip = 60 gtt/mL
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Chapter 9
Lesson 9.2
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Learning Objectives
• List the rule used to calculate
medication dosages for children
• Calculate flow rates for infusions for
children
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Clark’s Rule
• Formula
Weight of the child
________________ x Adult dose = Child’s dose
Weight of the adult
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Body Surface Area
• Body surface area (BSA) = the total tissue
area
• A nomogram is used to easily calculate the
BSA in square meters
• BSA formula
Surface area of the child (M2) × Usual adult dose
Surface area of an adult (1.73 M2) = Child’s dose
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Dimensional Analysis
• Steps
– Numbers in the dosage calculation
problem are placed on a grid along with
their labels
– The labels are cross-canceled to assure
only one label is left (one for answer)
– Numbers in calculation are placed along
grid next to their labels
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Dimensional Analysis (cont.)
– Numbers are cross-canceled
– Numbers are multiplied across the top and
bottom of the grid to yield a fraction
– The fraction is divided, and the remaining
label is applied to the answer
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