You receive a prescription from the doctor, it’s for 500mg of Levetiracetam solution taken B.I.D.
The stock bottle lists 100 mg/mL and the Rx is for 30 days, but your pharmacy’s dispensing bottles list ounces instead of mL
You need to be able first to find how much the patient will need to last 30 days, then convert mL to oz
Why is it important
There are hundreds of applications for unit conversions in pharmacies, all of which are important for you to know as the technician
For instance, we calculate Amoxicillin dosing by first finding the patient’s weight and converting pounds (lbs) to kilograms (kg) to see how many milligrams (mg) their dose should be
Miscalculating a dose could have harmful side effects on the patient, so we must calculate each conversion correctly
Understanding Unit Prefixes
Consider measurements like milligrams and kilograms. The metric prefixes milli- and kilo- denote the factor of measurement
Note the bolded measurements you will see most often in pharmacy, but others have been added to show the trend in factors
Take note of each symbol as well. Is it capitalized? Lowercase? How does letter casing change the measurement?
| Prefix | Symbol | Base 10 | Value | Example |
| Giga | G | 109 | 1 000 000 000 | gigagram (Gg) |
| Mega | M | 106 | 1 000 000 | megameter (Mm) |
| Kilo | k | 103 | 1 000 | kilogram (kg) |
| Hecto | h | 102 | 100 | hectogram (hg) |
| Deca | da | 101 | 10 | decameter (dam) |
| – | – | 100 | 1 | – |
| Deci | d | 10-1 | 0 . 1 | decigram (dg) |
| Centi | c | 10-2 | 0 . 01 | centimeter (cm) |
| Milli | m | 10-3 | 0 . 001 | milliliter (mL) |
| Micro | μ | 10-6 | 0 . 000 1 | microgram (μg) |
| Nano | n | 10-9 | 0 . 000 000 1 | nanometer (nm) |
| Pico | p | 10-12 | 0 . 000 000 000 1 | picogram (pg) |
We use these prefixes to improve notation. For example, it’s much easier to write 72 kg than it is to write 72 000 g
If we measured weight (grams) or volume (liters) in the base measurement at all times, we would end up with tiny decimals or massive numbers which are much harder to calculate with
Knowing these terms makes conversions much easier. If you know kilo- is 1000, it doesn’t matter what the unit is
Your patient is 1.76 meters tall? That’s 176 centimeters (move the decimal right 2 times for 10-2)
You need 230 mL for a patient and the pharmacist tells you the 1 liter stock bottle has about 1/3 left in it
Well, you know 1 liter = 1000 mL, and 1/3 of 1000 is about 333 mL so you should have plenty!
These are ways we can apply our prefixes without needing to memorize each measurement
Unit Conversions
Unfortunately, our prefixes only get us so far. And not all units are the same
Here are the unit conversions you’ll need to memorize come exam day
1 kg = 2.2 lbs
1 lb = 454 g
1 lb = 16 oz
1 oz = 28.35 g
1 oz = 29.57 ml (about 30 mL)
1 tsp = 5 mL
1 tbsp = 15 mL
1 grain = 64.8 mg
You may have noticed the “grain” above, which historically is based on the actual measurement of a grain
A grain is a unit of weight, equal to 0.0648 g or 64.8 mg
Apothecaries used this measurement centuries ago; it is the weight of a single grain. This was adopted into medicine due to the small weight of tablets, pills, and capsules and is still seen in some calculations today
Practice Questions
Grab a piece of paper and a pencil and give these a go. You will have a calculator on the exam, so feel free to use one here
1. How many mL are in 5 tsp?
2. How many ounces are in a 1-liter bottle (approximately)?
3. How many kg is a patient that weighs 184 lbs?
4. I have 2 kg and 4 ounces of medication, how many grams do I have?
5. You have a 464-gram tub of Triamcinolone. How many kg is this tub?
6. A 324 mg tablet of Aspirin is the same as how many grains?
7. How many liters are in 5 tsp and 10 tbsp?
Challenge Questions
8. The prescription reads “Take 3 tsp PO q6h for 7d.” How much should you fill for the patient?
9. The pharmacist needs you to fix an incorrectly filled medication. The order is a 30-day prescription of liquid Levetiracetam, 5 mL PO BID. The technician before you filled 5 oz in the bottle. How much more needs to be added?
10. You have two bottles on the shelf with the same NDC and lot number. You need 1 liter to fill this prescription. You have 30 tbsp in one bottle and 80 tsp in the second bottle, do you have enough to fill this prescription? If so, how much is left over? If not, how much more do you need?
Practice Answers
1. There are 25 mL in 5 tsp
1 tsp = 5 mL
5 * 5 = 25 mL
2. In a 1-liter bottle there are 33.3 oz
1 L = 1000 mL and 1 oz = 30 mL (approximate)
1000 mL / 30 mL = 33.33333 which is about 33.3 oz
3. An 184 lb patient weighs 83.6 kg
1 kg = 2.2 lbs
184 lbs / 2.2 lbs = 83.63636 kg
4. In 2 kg and 4 oz there are 2113.4 grams
1 kg = 1000 g, so 2 kg is 2000 g
1 oz = 28.35 g, so 4 oz is 113.4 g
2000 g + 113.4 g = 2113.4 grams
5. The 464 g tub is equal to 0.464 kg
1 kg = 1000 g
464 g / 1000 g = 0.464 kg
6. In a 324 mg tablet there are 5 grains
1 grain = 64.8 mg
324 mg / 64.8 mg = 5 grains
7. In 5 tsp and 10 tbsp there are 0.175 liters
5 tsp = 25 mL and 10 tbsp = 150 mL
1000 mL = 1 liter
25 mL + 150 mL = 175 mL
175 mL / 1000 mL = 0.175 liters
8. The Rx reading “Take 3 tsp PO q6h for 7d” needs 420 mL
3 tsp = 15 mL
q6h = every 6 hours
7d = 7 days
So the dose is 15 mL every 6 hours (4x/day) for 7 days
15 * 4 = 60 mL/day * 7 days = 420 mL
9. The prescription needs 5 oz added
5 mL PO (by mouth) BID (twice a day) for 30 days
5 mL * 2 = 10 mL/day * 30 days = 300 mL needed
1 oz = 30 mL (approximate)
300 mL / 30 mL = 10 oz to fill this RX
If 5 oz have already been filled…
10 oz – 5 oz = 5 oz need to be added to fulfill the RX
10. You do not have enough, you need 150 mL more
30 tbsp = 450 mL and 80 tsp = 400 mL
1 liter = 1000 mL
450 mL + 400 mL = 850 mL
1000 mL – 850 mL = 150 mL needed