Chapter 3 –
MEDICATIONS &
CALCULATIONS
SYSTEM OF
MEASUREMENT WITH
CONVERSION
Metric System
• METRIC SYSTEM is a decimal system
based on the power of 10 (degree of
magnitude is always multiple of 10)
• Developed in the late eighteenth century;
• Is the internationally accepted system of
measurement
• The basic units of measure are:
Gram (gm, G) for weight
Liter (L) for volume
Meter (m) for linear measurement or
length
Conversion within the Metric System
•Metric units most frequently used:
1g= 1000 mg
1L= 1000 ml
1mg= 1000 mcg
Metric Conversion
• When converting LARGER units to
SMALLER units in the metric system,
move the decimal point one space to the
RIGHT for each degree of magnitude
change
• Example: Change 1 gram to milligrams
1g=1.000mg or 1g=1000mg
Metric Conversion
• When converting SMALLER units to
LARGER units, move the decimal point
one space to the LEFT for each degree
in magnitude
• Example: Change 1000 mg to 1gram
1000mg=1000.g or 1000mg=1g
NOTE:
When changing LARGER units to smaller units, move
the decimal point to the RIGHT
When changing smaller units to larger units, move the
decimal point to the LEFT
Practice Problem
Larger to Smaller units
1. Change 2 g to mg ____
2. Change 0.5 g to mg ____
3. Change 2.5 L to ml ____
Smaller to Larger Units ____
4. Change 1500 mg to g ____
5. Change 3 g to kg ____
6. Change 500 ml to L ____
Household System
• Is not as accurate as the metric
system because of the lack of
standardization of spoons, cups and
glasses
• Likely to be used by the patient at
home where hospital measuring
devices are not available
• Measurements are approximate
Household System
• Example:
a teaspoon (t) is considered to
be equivalent to 5ml
Three teaspoons equal 1
tablespoon (T)
Milliliters (ml) is the same as
cubic centimeters (cc) in value
Household Equivalent in Fluid Volume
1 coffee cup (c) 6 ounces (oz)
1 medium size glass 8 ounces (oz)
1 ounce (oz) 2 tablespoons (T)
1 tablespoon (T) 3 teaspoons (t)
1 teaspoon (t) 60 drops (gtts)
Household Conversion
• To change LARGER units to SMALLER
units, MULTIPLY the requested number
of units by the basic equivalent value
• To change SMALLER units to LARGER
units, DIVIDE the requested number of
units by the basic equivalent value
Practice Problem
• Larger to Smaller Units
1. 3 oz to T ____
2. 5 T to t ____
3. Change 3 coffee cups to oz ____
• Smaller to Larger units ____
4. 3 T to oz ____
5. 16 oz to measuring cup ____
6. Change 12 t to T ____
METHODS FOR
CALCULATION
Methods for Calculation
Four general methods for the
calculation of drug doses are:
Basic formula
Ratio and Proportion
Fractional equation
Dimensional analysis
Methods for Calculation
These methods are used to calculate
oral and injectable drug doses
Keep in mind that the goal is to
prepare and administer medications in
safe and correct manner
Methods for Calculation
Before calculating drug doses, all units
of measure must be converted to a
single system
If the drug ordered in grams (g,G) and
the drug label gives the dose in
milligrams (mg), the convert grams to
milligrams
Interpreting Oral and
Injectable Drug Labels
Pharmaceutical companies usually
label their drugs with the brand name
of the drug in large letters and generic
name in small letters
The dose per tablet, capsule or liquid
is printed on the drug label
Example no. 1
Example no. 2
Methods for Calculation
The following recommendations are
offered:
Think
Read accurately
Picture the problem
Identify an expected range for the answer
Seek help when unsure of the calculation
Method 1: BASIC FORMULA
• The basic formula is easy to recall and is most
frequently used in calculating drug dosages
•Formula:
• D xV = A
H
• Where:
D is the desired dose (desired dose
ordered by the physician)
H is the on-hand dose (drug dose on label
of container)
V is the vehicle (drug form in which the
drug comes)
A is the amount calculated to be given to
the client
EXAMPLE no. 1
Order: cefaclor (Ceclor) 0.5 g PO bid
Example no. 2
Order: Codeine 60 mg PO STAT
Available:
Method 2:
RATIO & PROPORTION
• Is the oldest method currently used in the
calculation or drug dosages
• formula:
• H : V :: D : x
• Where:
H is the drug on hand
V is the vehicle or drug form
D is the desired dose
X is the unknown amount to give
• Note: Multiple the means and the extremes; solve
for x; x is the divisor
EXAMPLE no. 1
Order: amoxicillin (Amoxil) 100 mg PO q.i.d
Method 3:
FRACTIONAL EQUATION
• Similar to Ratio and Proportion except it is
written as a fraction
• Formula: H = D
V x
H is the dosage in hand
V for vehicle
D desired dose
X unknown
• Note: Cross multiply and solve for x
EXAMPLE
Order: ciprofloxacin (Cipro) 500 mgPOq12h
Method 4:
DIMENSIONAL ANALYSIS
• Is a calculation method known as
units and conversions
• Advantage: it decreases number of
steps required to calculate a drug
dosage
STEPS:
1. Identify the unit/form (tab., cap) of the drug to be
calculated then place equal sign
2. The known dose and unit/form from the drug label
follows the equal sign
e.g. Order: Amoxicillin 500mg, Available: 250/ capsule
capsule = 1 cap
250 mg
3. The mg (250 mg)is the denominator and it must
match the next numerator, which is 500mg (desired
dose)
capsule = 1 cap x 500 mg
250 mg x 1
4. Cancel out the mg, 250 and 500 mg. what remains is
the capsule and 2. answer 2 capsules
Example
Order: Amoxicillin 0.5g PO q8h
Body Weight
The body weight (BW) method of
calculation allows for the individualization
of the drug dose and involves the following
three steps
1. Convert to kilograms if necessary (lb /
2.2 = kg)
2. Determine the drug dose per BW by
multiplying as follows:
Drug dose x body weight = Client’s dose per day
3. Follow the basic formula
Example no. 1
1. Order: Fluorouracil (5 –FU), 12 mg/kg/day
IV, not to exceed 800 mg/day, the adult
weighs 132 lbs
a. Convert pounds to kilograms by directing
the number of pounds by 2.2 (1 kg = 2.2 lbs)
o132/ 2.2 = 60 kg
b. mg x Kg = client’s dose
o12 x 60 = 720 mg IV/day
Answer: Fluorouracil 12mg/kg/day = 720mg
Example no. 2
Order: cefaclor (Ceclor) 20 mg/kg/day in
three divided doses; the child weighs 31 lb
Available:
Answer
a. Convert lbs to kg
31/2.2 = 14kg
b. 20 mg x 14g = 280/day
280 mg/3 divided doses = 93mg/dose
c. D x V = 93mg x 5 = 3.7ml
H 125mg
Practice Problems
Oral and Injectable dosage calculation
Solve the problem and determine the
drug dose given the following:
1. Order: cimetidine (Tagamet) 0.4g PO, q6h
Available:
How many tablet (s) of Tagamet should the
patient receive? _____
Answer
a. Convert gm to mg by moving the decimal
point three spaces to the right.
0.4g = 400mg
b. D x V = 400mg x 1 tablet = 1 tablet
H 400mg
Practice Problems
Oral and Injectable dosage calculation
2. Order: doxycycline hyclate (Vibra Tab)
PO, initially 200 mg, then 50mg PO bid
Available:
a. How many tablets would you give as the initial
dose? _____
b. How many tablets would you give for each
dose after the initial dose? ____
Answer
a. Initially
D x V = 200mg x 1 = 2 tablets
H 100mg
b. Daily
D x V = 50mg x 1 = ½ tablet
H 100mg
Practice Problems
Oral and Injectable dosage calculation
3. Order: Azithromycin (Zithromax) 250
mg, PO, daily
Available:
How many ml should the patient receive
per day? ____
Answer
a. D x V = 250mg x 5ml = 6.25 ml
H 200mg
Practice Problems
Oral and Injectable dosage calculation
4. Order heparin 7500 units SQ
Available:
How many ml should the patient receive?
_____
Answer
a. D x V = 7500 units x 1ml = 0.75 ml
H 10,000 units
Practice Problems
Oral and Injectable dosage calculation
5. Order: atropine sulfate 0.5 mg subQ
Available:
How many ml should the patient receive?
_____
Answer
a. D x V = 0.5mg x 1ml = 1.25ml
H 0.4mg
Practice Problems
6. Order: oxacillin 250 mg IM q6h
Available: (Drug label states to add 2.7 ml
of sterile water = 3ml of drug solution)
How many ml should be given? ____
Answer
a. D x V = 250mg x 3ml = 1.5ml
H 500mg
