Pharmacy Tech Dosage Calculations

This guide will aim to give you a comprehensive understanding of some of the most common pharmacy calculations and drug dosage calculations that pharmacy technicians will use most frequently.

Pharmacy Calculations

When it comes to your PTCB exam, there are five main topics you will need to fully grasp. These include:

Conversions and formulae
Converting between ratios and percentage strengths
Roman numerals
Body mass index
Dosage calculations

Let’s start looking at each of these separately and in more detail.

Conversions and formulae

Within the PTCB exam, pharmacy technicians are often asked the answer to questions based on conversions.

There are quite a few conversions a pharmacy technician will be required to know and understand by heart.

Here are some of the must-know conversions and formulae you need to understand:

1 ounce = 28.3 grams
1 fluid ounce = 30ml
1 teaspoon = 5 m
1 tablespoon = 15 m
Celsius to Fahrenheit = multiply by 1.8 and add 32
Fahrenheit to Celsius = deduct 32, multiply by 5, then divide by 9
BMI = kg/m2
BSA = (height cm x weight kg) / 3600, then take the square root of the final total
Kg to lbs = there are 2.2lbs in 1 kilogram
1 US liquid quart = 0.95 liters
1 grain = 65 milligrams

Converting ratios to percentage strength

Routine questions are also asked on the exam paper regarding converting ratios to a percentage strength.

For example, you may be asked to convert 1:2,000 to a percentage strength.

While it might initially appear difficult, getting the correct answer is relatively straightforward.

All you are required to do is divide 1 by 2,000.

The answer you should get is 0.0005.

Then, multiply this figure by 100 i.e., 0.0005 x 100.

The correct answer is 0.05.

Simply add a percentage sign and the correct answer to the question is 0.05%.

Apart from this, you will often be required to perform the below calculation in reverse. For example:

Convert 0.50% into a ratio strength.

0.50% is also the same as 0.5 out of 100.

So, 100/0.75 = 200.

The answer is 1:200.

Roman numerals

One of the most important aspects of pharmacy technicians’ jobs is to understand the difference between Roman numerals and the Arabic numbers we are used to.

You must be able to convert these easily, and the exam will most definitely ask you to demonstrate your knowledge and understanding by switching between both sets of numerals.

Below is a list of some of the common numerals you’ll come across:

1 = I
2 = II
3 = III
4 = IV
5 = V
6 = VI
7 = VII
8 = VIII
9 = IX
10 = X
11 = XI
12 = XII
13 = XIII
14 = XIV
15 = XV
16 = XVI
17 = XVII
18 = XVIII
19 = XIX
20 = XX
21 = XXI
22 = XXII
23 = XXIII
24 = XXIV
25 = XXV
26 = XXVI
27 = XXVII
28 = XXVIII
29 = XXIX
30 = XXX
31 = XXXI
32 = XXXII
33 = XXXIII
34 = XXXIV
35 = XXXV
36 = XXXVI
37 = XXXVII
38 = XXXVIII
39 = XXXIX
40 = XL
41 = XLI
42 = XLII
43 = XLIII
44 = XLIV
45 = XLV
46 = XLVI
47 = XLVII
48 = XLVIII
49 = XLIX
50 = L
51 = LI
52 = LII
53 = LIII
54 = LIV
55 = LV
56 = LVI
57 = LVII
58 = LVIII
59 = LIX
60 = LX
61 = LXI
62 = LXII
63 = LXIII
64 = LXIV
65 = LXV
66 = LXVI
67 = LXVII
68 = LXVIII
69 = LXIX
70 = LXX
71 = LXXI
72 = LXXII
73 = LXXIII
74 = LXXIV
75 = LXXV
76 = LXXVI
77 = LXXVII
78 = LXXVIII
79 = LXXIX
80 = LXXX
81 = LXXXI
82 = LXXXII
83 = LXXXIII
84 = LXXXIV
85 = LXXXV
86 = LXXXV
I87 = LXXXVII
88 = LXXXVIII
89 = LXXXIX
90 = XC
91 = XCI
92 = XCII
93 = XCIII
94 = XCIV
95 = XCV
96 = XCVI
97 = XCVII
98 = XCVIII
99 = XCIX
100 = C
C = 100
D = 500
M = 1,000

Know how to calculate BMI

Another important aspect of the exam is how to accurately calculate a patient’s BMI or body mass index. BMI will help to indicate to a clinician whether a person is underweight, in the ideal weight range, or overweight.

For example, a score:

Under 18.5 – underweight range
Between 18.5 and 24.9 – ideal body weight range
Between 25 and 29.9 – overweight range
30+ – obese range

In order to calculate BMI, the following formula is used: kg/m2

When reading the question, ensure you fully consider whether the patient’s weight is in kilograms or pounds. As well as this, check (and double-check) to make sure whether the patient’s height is given as meters or centimeters.

You will have to convert pounds to kilograms and centimeters into meters.

For example, you might be presented with a question like this:

What is the BMI of a patient who weighs 180 pounds and who is 1.8m in height?

180 pounds will need to be converted from pounds to kilograms.

To work this out, you need to know that there are 2.2 lbs in every 1 kg.

The answer, therefore, is 81.6kg.

The height is in the correct format, so it will not require changing: 1.8m.

1.8m is now squared: 1.8m multiplied by 1.8m = 3.6m2

The question gives us the correct format for the patient’s height: 1.7m

We must now square that value: 1.8m multiplied by 1.8m = 3.6m2

Therefore:

BMI = 81.6kg/3.6 = 22.7

This means the patient is in the healthy weight range.

Dosage Calculations

Dosage calculations are the last of the five calculations that a pharmacy technician should be able to carry out. There are a lot of acronyms and abbreviations to learn in order to understand the correct dose required.

For example: Seroquel, 25mg i cap PO tid

This example means that the patient is required to take 1 25mg of Seroquel orally three times a day.

If you are asked to dispense 28, it means that the patient will have a little over 9 days’ supply of Seroquel.

Final Thoughts

Being a pharmacy technician is hard work, and it requires a high degree of pharmaceutical calculations. You’ll be required to undertake dosage calculations, help patients under their ideal body weight, know how many tablets to give, as well as a host of conversion factors, too.

While it’s not always easy or straightforward, getting yourself accustomed to these drug dosage calculations will make your role as a pharmacy technician much more straightforward when it comes to exams and real-life working experiences.