Dosage Calculations

DRUG
ARITHMETIC
S
LEARNING OBJECTIVES
✔ Describe four measuring
systems that can be used in drug
therapy.
✔ Convert between different
measuring systems when given drug
orders and available forms of the
drugs.
NOTE:
✔ To determine the correct dose of a
particular drug for a patient, the
nurse must consider the patient’s
sex, weight, age, and physical
condition, as well as the other
drugs that the patient is taking.
MEASURING SYSTEMS
1.) Metric System- It is based on
the decimal system, so all units are
determined as multiples of 10.
✔ Uses the gram as the basic unit of
solid measure and the liter as the
basic unit of liquid measure.
2.) Apothecary System- Uses the
minim as the basic unit of liquid
measure and the grain as the basic unit
of solid measure.
✔ It uses Roman numerals placed after
the unit of measure to denote amount.
✔For example, 15 grains would be
written “gr xv.”
3.) Household System-
found in recipe books and uses
the teaspoon as the basic unit
of fluid measure and the
pound as the basic unit of solid
measure.
4.) Avoirdupois System- another
older system that was very popular when
pharmacists routinely had to compound
medications.
✔ It is seldom used by prescribers but
may be used for bulk medications that
come directly from the manufacturer.
5.) Other System- Some drugs are
measured in “units”that reflect
chemical activity or biological
equivalence.
✔ A “unit” usually reflects the
biological activity of the drug in 1
mL of solution.
✔ The unit is unique for the drug it measures;
a unit of heparin is not comparable to a unit of
insulin.
✔ Milliequivalents (mEq) are used to measure
electrolytes (e.g., potassium, sodium, etc).
✔ The milliequivalent refers to the ionic
activity of the drug in question; the order is
usually written for a number of
milliequivalents instead of a volume of drug.
✔ International units are
sometimes used to measure
certain vitamins or enzymes.
✔ These are also unique to each
drug and cannot be converted to
another measuring form.
CONVERSION
BETWEEN
SYSTEMS
EXAMPLE # 1
✔ Convert 6 fl oz (apothecary
system) to the metric system.
✔ Given:
1 fl oz = 30 mL
SOLUTION:
1 fl oz 6 fl oz
30 mL X
1 fl oz (X) 6 fl oz (30 mL)
1 fl oz X 180 fl oz (mL)
1 fl oz 1 fl oz
180 mL
EXAMPLE # 2
✔ Convert 32 gr (apothecary) to its
equivalent in the metric system,
expressing the answer in milligrams.
✔ Given:
1 gr = 60 mg
SOLUTION:
1 gr 32 gr
60 mg X
1gr (X) = 60 mg (32 gr)
1 gr X = 1920 (mg) (gr)
1 gr 1 gr
x= 1920 mg
DOSAGE
CALCULATIO
NS
LEARNING OBJECTIVES
✔ Calculate the correct dose
of a drug when given
examples of drug orders
and available forms of the
drugs ordered.
ORAL DRUGS
✔ Frequently, tablets or capsules for
oral administration are not available in
the exact dose that has been ordered.
✔ The easiest way to determine dose is
to set up a ratio and proportion
equation.
GENERAL
FORMULA:
Example # 1
✔ An order is written for 10 grains
of aspirin (gr x, aspirin). The
tablets that are available each
contain 5 grains. How many
tablets should the nurse give?
Solution:
Example # 2
✔ An order is written for 0.05 g
Aldactone (spironolactone) to be
given orally (PO). The Aldactone is
available in 25-mg tablets. How
many tablets would you have to
give?
Solution:
1.) Convert gram to milligram
0.05 gram x1000 mg

1 gram
= 50 mg
Solution:
Example # 3
✔ An order has been written for
250 mg of sulfisoxazole. The
bottle states that the solution
contains 125 mg/5 mL. How much
of the liquid should you give?
Solution:
PARENTERAL DRUGS
✔ All drugs administered parenterally
must be administered in liquid form.
✔ The person administering the drug
needs to calculate the volume of the
liquid that must be given to administer
the prescribed dose.
GENERAL
FORMULA:
Example # 1
✔ An order has been written for
75 mg of meperidine to be given
intramuscularly.
✔ The vial states that it contains
meperidine, 1.0 mL = 50.0 mg
Solution:
INTRAVENOUS
SOLUTIONS
✔ Used to deliver a fluids, electrolytes,
vitamins, nutrients, or drugs directly into the
bloodstream.
✔ Microdrip delivery system delivers 60 drops
per milliliter.
✔ Macrodrip system delivers 15 drops per
mililiter and is used when a large volume must
be delivered quickly.
DRIP RATES
✔ It is when the infusion volume is
calculated into drops.
✔ General formula is:
Example # 1
✔ A patient is ordered to
receive 1000 mL of intravenous
fluids to run for 8 hours.
Calculate the drip rate.
Solution:
DRIP RATES IN DROPS PER
MINUTE
✔ There are two standard giving sets of drip
rates.
❖ Macro drop factor = 15 drops (divide by 4)
❖ Micro drop factor = 60 drops (divide by 1)
❖ The formula in determining DR in DPM:
Example # 2
✔ A patient is to receive 1 liter
of PNSS over the next 12 hours.
What is the rate of infusion in
drops per minute, if the drop
factor is 60 drops per milliliter.
Solution:
The formula in determining the
number of hours to complete
before it runs out is:
Example # 3
✔ The volume of the fluid is
1000 ml and the IV pump is set
at 62 ml / hr. How long will it
take for the fluid to run?
Solution:

Dosage Calculations

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DRUG

0.05 gram x1000 mg

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