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Top Pharmacy Calculation Tips for the PTCB Exam!

Jan 30th, 2020
ptcb pharmacy calculation tips


Here we review some of the top pharmacy calculation tips for the PTCB exam you need to know.

We cover the following 5 topics:

  • 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 set to appear on the 2020 PTCB test.

Let’s get started.

Tip 1 – Conversions and Formulae

During the PTCB exam, candidates are 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 (this division eliminates the ratio)

Then: 0.0002 x 100 = 0.02%

It’s that easy.

You also need to know how to do the reverse:

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

Consult our earlier article to learn the rules by which we make these conversions.

Tip 4 – Know How to Calculate BMI

BMI is used to calculate the body mass index of a patient. It allows clinicians to evaluate whether a patient is overweight, obese, or within the normal weight for their height.

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

One of the most common pharmacy calculation questions on the PTCB exam asks you to find the BMI of a patient.

When reading the question, always 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 PTCB candidates simply divide kilograms by meters, forgetting to square the value.

Here is an example of how to do it:

  • 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 2.2lbs in 1kg, the patient weighs 86.4 kilograms.

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

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

BMI = 86.4 kilograms / 2.89m2 = 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 that the patient is prescribed 7-days supply.

Second, you must understand the concept of concentration.

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

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

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

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

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

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

Final Thoughts

That concludes our top 5 pharmacy calculation tips for the PTCB exam.

We’ve covered quite a lot in a short period. But that’s always 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.

Candidates with solid coverage of the above 5 topics can expect to score well on the day of the PTCB pharmacy calculation section. Of course, there is much more to know, too, and that requires long-term preparation and diligence.

Registered members already have access to our pharmacy calculation database and complete theory explanations. If you have not yet registered to PTCB, take a few moments to complete registration now. At PTCB Test Prep, we get you ready.