# 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.

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.

### 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 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.