This document discusses various methods for calculating drug doses, including definitions of dose-related terms and factors considered in dose determination such as age, weight, body surface area, organ function, and condition being treated. Equations are provided for calculating the size of a dose, number of doses, or total quantity based on one given value. Examples demonstrate dose calculations for specific patients based on weight, body surface area, and dosing tables. Nomograms are also described as a tool for determining body surface area.

1. Calculation of Doses
Chapter -3
1/23/2015
By: Negese Sewagegn
(Bpharm, MSc in Clinical Pharmacy)

2. • Dose Definitions: The dose of a drug is the
quantitative amount administered or taken by a
patient for the intended medicinal effect.
• The dose may be expressed as a single dose, the
amount taken at one time; a daily dose; or a total
dose, the amount taken during the course of therapy.
• A daily dose may be subdivided and taken in divided
doses, two or more times per day depending on the
characteristics of the drug and the illness.
• The schedule of dosing (e.g., four times per day for
10 days) is referred to as the dosage regimen.
• Quantitatively, drug doses vary greatly among drug
substances; some drugs have small doses, other
drugs have relatively large doses. 2

3. • The dose of a drug is based on its biochemical and
pharmacologic activity, its physical and chemical
properties, the dosage form used, the route of
administration, and various patient factors.
• The dose of a drug for a particular patient may be
determined in part on the basis of the patient’s age,
weight, body surface area, general physical health,
liver and kidney function (for drug metabolism and
elimination), and the severity of the illness being
treated.
• Pharmacokinetic dosing takes into account a
patient’s ability to metabolize and eliminate drugs
from the body due to impaired liver or renal
function, which often necessitates a reduction in
dosage.
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4. Doses
• Given two factors in the following equation, by
rearrangement, the third may be calculated :
• Number of doses = Total quantity
Size of dose
• In using the equation, the total quantity and the size
of dose must be in the same unit of measure
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5. General Dose Calculation
• A pharmacist often needs to calculate the size of a dose,
the number of doses, or the total quantity of medication to
dispense.
• For these calculations the following equation is useful with
the terms rearranged depending on the answer required.
• In using the equation, the units of weight or volume must
be the same for the total quantity and size of the dose.
• Number of doses = Total quantity
Size of dose
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6. Exercise
If 1 tablespoon is prescribed as the dose,
approximately how many doses will be
contained in 1 pint of the medicine?
– 1 tablespoon = 15 mL
– 1 pint = 473mL
• Number of doses = 473 mL = 31.5 or 31 doses
15 mL
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7. Example Calculations of the Size of a Dose
 Size of dose = Total quantity
Number of doses
 The size of the dose is expressed in whatever denomination
is chosen for measuring the given total quantity.
 How many drops would be prescribed in each dose of a
liquid medicine if 15 mL contained 60 doses? The
dispensing dropper calibrates 32 drops/mL.
• 15 mL = 15 x 32 drops = 480 drops
• Size of dose = 480 (drops) = 8 drops, answer
60
Or, solving by dimensional analysis:
32 drops x 1 x 15 mL = 8 drops/dose, answer.
1ml 60 doses
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8. Example Calculations of the Total Quantity of
Product
• Total quantity = number of doses x size of
dose
• It is convenient first to convert the given dose
to the denomination in which the total
quantity is to be expressed.
• How many milliliters of a liquid medicine
would provide a patient with 2 tablespoonfuls
twice a day for 8 days?
– Number of doses =16
– Size of dose = 2 tablespoonfuls or 30 mL
– Total quantity =16 x 30 mL= 480 mL, answer.
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9. Drug Dosage Based on Age
• The age of the patient is a consideration in the
determination of drug dosage.
• Neonates have immature hepatic and renal functions
that affect drug response.
• The elderly, in addition to diminished organ function,
frequently have issues of concomitant pathologies and
increased sensitivities to drugs.
• Before the physiologic differences between adult and
pediatric patients were clarified, the latter were treated
with drugs as if they were merely miniature adults.
• Various rules of dosage in which the pediatric dose was a
fraction of the adult dose, based on relative age, were
created for youngsters (e.g., Young’s rule).
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10. • Today these rules are not in general use because age
alone is no longer considered a singularly valid
criterion in the determination of accurate dosage for
a child, especially when calculated from the usual
adult dose, which itself provides wide clinical
variations in response.
• Currently, when age is considered in determining
dosage of a potent therapeutic agent, it is used
generally in conjunction with another factor, such as
weight.
• The dose of the drug digoxin is determined by a
combination of the patient’s age and weight.
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11. Note: The value of 150 in Fried’s rule was an estimate of the age (12.5
years or 150 months) of an individual who would normally receive an
adult dose, and the number 150 in Clark’s rule was an estimate of the
weight of an individual who likewise would receive an adult dose. 11

12. Special Considerations in Dose Determinations
for Elderly Patients
 Dose determinations for elderly patients frequently require
consideration of some or all of the following:
 Therapy is often initiated with a lower-than-usual adult dose.
 Dose adjustment may be required based on the therapeutic
response.
 The patient’s physical condition may determine the drug dose and
the route of administration employed.
 The dose may be determined, in part, on the patient’s weight, body
surface area, health and disease status, and pharmacokinetic factors.
 Concomitant drug therapy may affect drug/dose effectiveness.
 A drug’s dose may produce undesired adverse effects and may affect
patient compliance.
 Complex dosage regimens of multiple drug therapy may affect
patient compliance. 12

13. Drug Dosage Based on Body Weight
• The usual doses of drugs are considered generally
suitable for the majority of individuals likely to take the
medication.
• The usual dose is expressed as a specific quantity of
drug per unit of patient weight, such as milligrams of
drug per kilogram of body weight(abbreviated mg/kg).
• Dosing in this manner makes the quantity of drug
administered specific to the weight of the patient being
treated.
• The patient’s weight is an important factor in dosing
since the size of the body influences the drug’s
concentration in the body fluids and at its site of
action.
• Dose calculations based on body weight have become
standard for certain drugs in dosing both adult and
pediatric patients. 13

14. Example Calculations of Dose Based on Body Weight:
• The doses of the majority of drugs based on body weight are
conveniently expressed in terms of mg/kg, since the doses of
most drugs are administered in milligram amounts.
• Depending on the drug, dosage form, and/or route of
administration, the doses of some drugs are expressed in other
units of measure, such as micrograms or milliliters per pound
or kilogram of body weight.
• A useful equation for the calculation of dose based on body
weight is:
-Patient’s dose (mg) = Patient’s weight (kg) x Drug dose (mg)
1 (kg)
This equation is based on a drug dose in mg/kg and the
patient’s weight in kilograms. When different units are given or
desired, other units may be substituted in the equation as long
as the terms used are consistently applied. 14

15. Example:
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16. TABLE1. DOSING BY BODY WEIGHT FOR A HYPOTHETICAL DRUG
Dosing Tables
•For some drugs dosed according to body weight or body surface
area, dosing tables appear in product literature to assist the
physician and pharmacist. An example is presented in Table 1.
•Using Table 1. and a daily dose of 0.5 mg/kg, how many 20-mg
capsules of the drug product should be dispensed to a patient
weighing 176 lb. if the dosage regimen calls for 15 weeks of
therapy?
2 capsules/day x 7 days/week x 15 weeks = 210 capsules, answer.16

17. Drug Dosage Based on Body Surface Area(BSA)
• The BSA method of calculating drug doses is widely used for two types
of patient groups:
– cancer patients receiving chemotherapy and
– pediatric patients, with the general exception of neonates, who are
usually dosed on a weight basis with consideration of age and a variety of
biochemical, physiologic, functional, pathologic, and immunologic
factors.
• The approximate relation between body weight and BSA, in square
meters (m2), based on average body dimensions (Table 2).
• The average adult is considered to have a BSA of 1.73 m2.
• A person with a BSA of 1.30 (or about 75% of that of the average
adult) would receive about 75% of the adult dose (Table 2).
Dosing Tables
• For certain drugs, dosing tables may be provided to determine the
approximate dose based on a patient’s body surface area. Table 2.
presents an example for a hypothetical drug.
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18. Table 2. approximate relation of surface area and
weights of individuals of average body dimension
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19. Example calculations of dose based on BSA
• A useful equation for the calculation of dose based on
BSA is:
• Patient’s dose = Patient’s BSA (m2) x Drug dose (mg)
1.73 m2
• If the adult dose of a drug is 100 mg, calculate the
approximate dose for a child with a BSA of 0.83 m2,
using (a) the equation and (b) Table 2.
(a) Child’s dose = 0.83m2x100 mg=47.97 or 48mg, answer.
1.73m2
(b) According to Table 2., a BSA of 0.83 m2 represents 48%
of the average adult BSA of 1.73 m2; thus, the child dose
would be 48% of the usual adult dose:
100 mg x 0.48 = 48-mg dose for child, answer.
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20. Table 3. pediatric dosing guideline for a hypothetical
drug based on BSA
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• Using Table 3., find the dose of the hypothetical drug at a dose level
of 300 mg/m2 for a child determined to have a BSA of 1.25 m2.
Calculate to verify.
• From Table 3., the dose = 375 mg, answer.
• From calculations: 300 mg/m2 x 1.25 m2 = 375 mg dose, answer.

21. Nomograms
• Most BSA calculations use a standard nomogram, which
includes both weight and height. Nomograms for children
and adults .
• The BSA of an individual is determined by drawing a
straight line connecting the person’s height and weight.
• The point at which the line intersects the center column
indicates the person’s BSA in square meters.
• In the example shown in Figure 1, a child weighing 15 kg
and measuring 100 cm in height has a BSA of 0.64 m2.
• If the adult dose of a drug is 75 mg, what would be the
dose for a child weighing 40 lb. and measuring 32 in. in
height using the BSA nomogram?
• From the nomogram, the BSA = 0.60 m2
0.60 m2 x 75 mg = 26 mg, answer.
1.73 m2
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24. BSA Equation
• In addition to the use of the nomogram, BSA
may be determined through use of the
following formula:
• Calculate the BSA for a patient measuring 165 cm in
height and weighing 65 kg.
Note: For the sake of comparison, check Figure 1. to derive the BSA for the same
patient using the nomogram.
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