1. Fundamentals of Pharmacology Review fractions, decimals, and percents
Before viewing this tutorial, you must have knowledge of fractions, decimals and percents. Click the link at the bottom of the screen for a review or click the arrow to advance to the next screen.
Pima Medical Institute Online
Review fractions, decimals, and percents

2. Using a Given Concentration
The concentration on a bottle of pills is
40 mg / 1 tablet. How many milligrams do you need to make 30 tablets?
Use the concentration
on the bottle
Unknown value
40 mg
1 tablet
n mg
30 tablets
(40 mg)(30 tablets)
(n mg) (1 tablet)
When filling a prescription, use the concentration on the bottle or vial to help you.
For example, the concentration on a bottle of pills is 40 mg / 1 tablet and you need to make a bottle containing 30 tablets.
How many milligrams do you need fill the prescription?
First, identify the ratios. You have the concentration and your ratio containing your unknown.
40 mg / 1 tablet
n mg / 30 tablets  Contains unknown value
To set up your proportion statement, put an equal sign between the ratios.
40 mg /1 tablet = n mg / 30 tablets
Now, when you cross multiply, you get the following:
(40 mg)(30 tablets) = (n mg)(1 tablet)
= n
So you’ll need 1200mg to make 30 tablets for the prescription.
(40 mg) (30 tablets)
1 tablet n
Identify the ratios.
Put an equals sign between.
Cross multiply.
1200 mg n

3. Examples of conversion factors
A conversion factor is a ratio describing the equivalence between different units of measurement
Use proportions to calculate a patient’s drug dose
Examples of conversion factors
Now that you have an understanding of ratio and proportion, let’s look at how you can use proportions to calculate a patient’s drug dose.
To convert between systems of measurement, you will need to use conversion factors. Conversion factors are ratios that describe the equivalence between different units of measurements. Here are some examples of conversion factors.
There are 1,000 g in 1 kg. There are 2.2 lbs in 1 kg: 2.2 lbs / 1 kg (Kg are heavier than lbs). There are 4.55 liters in 1 gallon. There are 50 mg in 1 tablet.
1,000 g
1 kg
2.2 lbs
1 kg
4.55 L
1 gallon
1 tablet
50 mg
kg are heavier than lbs

4. Calculating the Dose Be very careful when rounding
Volume to administer to patient
(Body weight)(Desired volume)
Concentration of drug
Dose
A dog weighs 100 lbs and is to be given an antibiotic dosage
of 10 mg/kg. The antibiotic comes in 50 mg tablets.
What is the dose?
Be very careful when rounding
your measurements up or down!
If you are unsure, don’t be afraid to ask!
100 lbs / 2.2 lbs = kg
( ) mg kg
(45.45 kg)
(100 lbs)
Dose
Now, to calculate the volume of drug you should give your patient (also known as the dose), use this formula.
Dose = Body weight x desired volume/ Concentration of the drug
For example, a dog weighs 100 lbs and is to be given an antibiotic dosage of 10mg/kg.   The antibiotic comes in 50 mg tablets. What is the dose? Let’s apply the formula for dose.
What is the dog’s body weight? 100 lbs
What is the desired volume? 10 mg/kg
What is the concentration of the drug? 50 mg per tablet
Notice that we need to convert the dog’s weight from pounds to kilograms because our other units of measurement are in kg and mg. Since there are 2.2 lbs in every kilogram, divide 100 by 2.2. This gives us kg.
Now, cancel out units that appear in both the numerator and denominator. Multiply then simplify your fraction. According to your calculation, the dose is .909 tablets. Typically, this would be rounded up to one tablet. However, you must be very careful when rounding your measurements up or down. If you are unsure, don’t be afraid to ask! Your decision will affect your patient’s progress. 50 mg
tablet
(45.45)(10) 50
tablets
Round up to 1 tablet
.909 tablets

5. Filling a Prescription
A prescription is how often a patient needs to take a certain dose as dictated by the veterinarian
If the dog is to be administered 1 tablet three times a day (t.i.d.) for 10 days, how many tablets will fill its prescription?
SUN
MON
TUE
WED
THUR
FRI
SAT
Keep in mind, that the dose is what the patient is administered. A prescription is how often and for how long the patient needs to take that dose. If you know the dose, you can then calculate the prescription.
Using the previous example, if the dog is to be admistered 1 tablet three times a day (t.i.d.) for 10 days, how many tablets will fill its prescription?
Since your starting value is tablets and your answer needs to be in tablets, no conversion is needed. Simply multiply.
1 tablet x 3 t.i.d. x 10 days = 30 tablets
It will take 30 tablets to fill the prescription.
1 tablet
3 t.i.d.
10 days 30
tablets

6. Self-Test: Conversions and Filling Prescriptions

7. Presented by PMI Online
Resources:
Essential Calculations for Veterinary Nurses and Technicians by Terry Lake and Nicola Green
Applied Pharmacology for Veterinary Technicians, 4th Edition by Boyce P. Wanabaker and Kathy Lockett Massey