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Top Pharmacy Calculation Tips for the PTCB Exam!

Jan 30th, 2020
ptcb pharmacy calculation tips

Pharmacy Math and the Pharmacy Technician Exam

In this rapid revision guide, we review the the following five subject areas:

  • Conversions and Formulae
  • Converting between Ratios and Percentage Strengths
  • Roman Numerals
  • Body Mass Index
  • Dosage Calculations

Take note – as all five of these topics are likely to appear on your PTCB test.

Let’s get started.

Tip 1 – Conversions and Formulae

During the PTCB exam, candidates are often asked questions that are based on knowledge of conversions.

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

  • 1 ounce = 28.3 grams
  • 1 fluid ounce = 29.57 mL (or 30mL rounded-up)
  • 1 teaspoon = 5 mL
  • 1 tablespoon = 15 mL
  • Convert Celsius to Fahrenheit = multiply by 1.8 and add 32
  • Convert Fahrenheit to Celsius = deduct 32, multiply by 5, then divide by 9
  • BMI = kg/m2
  • BSA = (height cm x weight kg) / 3600, then take square root of this value
  • Kg to lbs = there are 2.2lbs in 1 kilogram
  • 1 US liquid quart = 0.95 liters
  • 1 grain = 65 milligrams

The PTCB exam routinely asks questions based on these conversions and formulae.

Tip 2 – Converting Ratios to Percentage Strength

You may be asked questions such as:

Convert 1 : 5,000 to a percentage strength.

Whilst it may at first appear confusing, the answer method is rather easy. Simply divide 1 by 5,000 and multiply x 100.

In this case: 1 / 5,000 = 0.0002 (the division eliminates the ratio)

Then, 0.0002 x 100 = 0.02%

Not too difficult, right? You also need to know how to do the reverse, namely:

Convert 0.75% to a ratio strength.

In this case, we know that 0.75% is the same as 0.75 out of 100.

Therefore: 100 / 0.75 = 133.33 (we are finding out how many “0.75”s are in 100).

Answer – 0.75% is the same as a ratio of 1 : 133.33

Tip 3 – Roman Numerals

It’s vital that you understand how to convert between Arabic and Roman numerals.

We’ve dealt with this topic before but, for the purposes of revision, here are the letters you need to know. As well as these, you must understand the rules for converting between Roman numerals and Arabic numbers.

  • I, II, III – 1, 2, 3
  • V = 5
  • X = 10
  • L = 50
  • C = 100
  • D = 500
  • M = 1,000

Tip 4 – Know How to Calculate BMI

BMI is used to calculate the body mass index of a patient. It lets clinicians evaluate whether a patient is overweight, obese, or within the normal weight for their height (of course there are limitations and drawbacks to BMI, but it still largely applies to the majority of the general population. Bodybuilders and the very elderly, for example, would not be suited to a BMI evaluation).

To calculate BMI, we use this formula: kg/m2  

You may be asked on the PTCB to calculate the BMI of a patient.

When reading the question, identify the following:

  • Is the patient’s weight in pounds or kilograms?
  • Is the patient’s height given in cm or meters?

If not, you need to convert pounds to kilograms and cm to meters.

One of the most common mistakes is to forget to square the meter value. Many technicians make the mistake of dividing kilograms by meters, but forgetting to square the value.

  • Calculate the BMI of a patient who weights 190 pounds and who is 1.7m tall.

In this case, we need to convert 190 pounds to kilograms. As there are 2.2 lbs in 1 kg, the patient weighs 86.4 kilograms.

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

We must now square that value: 1.7 m multiplied by 1.7m = 2.89 m2

BMI = 86.4 kilograms / 2.89 m2 = 29.9

In this case, the patient would be classified as overweight, bordering on obese.

Tip 5 – Dosage Calculations

Dosage calculations are routinely examined on the PTCB test.

For this, candidates are expected to have a thorough knowledge of pharmacy abbreviations and acronyms.

For example: amoxicillin 500mg, ii cap po tid

Translation: 2 capsules to be taken orally three times daily

If you are asked to dispense 42, it means the patient is prescribed 7-days supply. Second, technicians need to apply their understanding of concentrations.

For example: 100 mg / 5 mL – means there is 100 mg of drug in each 5 mL of solution.

  • How many milligrams of active ingredient is in a 120 mL solution which has a concentration of 60 mg / 5 mL?

The total solution is 120 mL – each 5 mL of which contains 60 mg.

If 5 mL contains 60 mg, then 10 mL contains 120 mg.

If 10 mL contains 120 mg, then 120 mL contains 1,440 mg.

Conclusion – there is 1,440 mg of drug in the 120 mL solution.

Final Thoughts

That about wraps up our rapid revision of some of the top tips for pharmacy math technicians must know.

We’ve covered quite a lot, but that’s what always makes the best way to study. Collect what you need to know, put that knowledge into practice by taking MCQs – and learn and grow from there. Learn from mistakes, and always take note of where you could improve next time around.

Technicians with solid coverage of the above five topics can expect to score well on the day of the PTCB pharmacy calculation section. Of course, there is much more to know, and that requires long-term preparation and diligence.

But with time, there is no reason why you cannot succeed at math on the PTCB exam.

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PTCB Test Prep Author


Elaine Walker

Elaine joined PTCB Test Prep in 2017, currently serving as the lead product development manager overseeing both course development and quality improvement. Mrs. Walker is a graduate of California State University and has worked as a pharmacy technician for over twenty years – with particular interests in pediatric pharmacy, extemporaneous compounding, and hospital pharmacy. Over the past 8-years, she has helped prepare thousands of students for the PTCB examination.