Math basics for medications
Medical math and dosage calculations
All medical assistants should be able to convert between units of measure. It is also important to be able to accurately calculate medication dosages. This section discusses different types of math problems that a medical assistant should be able to solve. Please note that in some states, calculating medical dosages may be outside the legal scope of practice for medical assistants. State laws vary.
The following sections present guidelines for writing and calculating dosages. The medical assistant should always verify answers for dose calculations with a co-worker or the provider.
Healthcare rules when writing numbers
With medications, it is important to be accurate in calculating and documenting amounts. There are three rules to follow when writing medication dosages in healthcare:
Rule 1: Use the correct unit abbreviation
Follow the number with the correct abbreviation for the unit of measure. Leave a space between the number and the abbreviation. Do not use a period with the abbreviations.
Correct examples
- 2 mg
- 10 mL
Incorrect examples
- 2 mg.
- 10mL
Rule 2: Write fractions as decimals
Write a fraction of a dose as a decimal.
Correct examples
- 0.2 mg
- 0.5 mg
Rule 3: Use a leading zero, never a trailing zero
If the dose is less than 1, place a zero to the left of the decimal point so the dose isn’t misread as a whole number. Never place a decimal point and a zero after a whole number - it can be misread as a much larger dose and cause an overdose.
Correct examples
- 0.75 mcg
- 0.2 mL
- 2 mL
- 5 mL
Incorrect examples
- .75 mcg
- .2 mL
- 2.0 mL
- 5.0 mL
Rounding numbers
When you calculate answers, sometimes the number needs to be rounded.
Steps for rounding
- Find the place value where you want to end up.
- Look at the number to the right of the place value.
- 4 or below → drop the remaining digits.
- 5 or above → add 1 to the rounded digit and drop the remaining digits.
Examples
Example 1
The 3 is in the tenths place. The next digit is 6, so round up.
Answer: 569.4
Example 2
The 9 is in the tenths place. The next digit is 2, so drop the remaining digits.
Answer: 339.9
Measurement systems and conversions
Three systems of measurement turn up in medication orders, and a medical assistant has to move between them. The metric system is the standard for healthcare - it is what drug labels, syringes, and dosage calculations use. The household system is what patients use at home: teaspoons, tablespoons, cups, ounces, and pounds. It is the system a medical assistant coaches a patient in, which is why an order written in metric so often has to be translated at the end of a visit. The apothecary system is an older system that has largely been retired, but grains (gr) still appear on a few labels.
Metric equivalents
| Unit | Equivalent |
| 1 kilogram (kg) | 1000 grams (g) |
| 1 gram (g) | 1000 milligrams (mg) |
| 1 milligram (mg) | 1000 micrograms (mcg) |
| 1 liter (L) | 1000 milliliters (mL) |
| 1 milliliter (mL) | 1 cubic centimeter (cc) |
| 1 meter (m) | 100 centimeters (cm) |
| 1 centimeter (cm) | 10 millimeters (mm) |
Household and metric equivalents
| Household | Metric |
| 1 teaspoon (tsp) | 5 mL |
| 1 tablespoon (tbsp), or 3 teaspoons | 15 mL |
| 1 fluid ounce (fl oz), or 2 tablespoons | 30 mL |
| 1 cup, or 8 fluid ounces | 240 mL |
| 1 pint, or 2 cups | 480 mL |
| 1 quart, or 2 pints | 960 mL |
| 2.2 pounds (lb) | 1 kilogram (kg) |
| 1 inch | 2.54 cm |
Apothecary equivalents
| Apothecary | Metric or household |
| 1 grain (gr) | 60 mg in most medical assisting references, 65 mg in some (which is why aspirin gr V is labelled 325 mg). Use one or the other throughout a calculation, never both |
| 1 dram | 4 mL, or about 1 teaspoon |
| 1 ounce | 30 mL |
| 1 minim | About 0.06 mL, or roughly 1 drop (gtt) |
To convert pounds to kilograms, divide by 2.2; to convert kilograms to pounds, multiply by 2.2. Weight-based doses are always calculated from the weight in kilograms, so this is the conversion done most often.
Example: Weight-based dose
A pediatric patient weighs 44 lb, and the provider orders 10 mg/kg. What is the correct dose in milligrams?
- Convert the patient’s weight to kilograms: kg
- Multiply by the ordered dose per kilogram: mg
Answer: 200 mg
Example: Household-to-metric conversion
A provider orders 2 teaspoons of a liquid medication for a patient to take at home. How many milliliters is that?
- Each teaspoon equals 5 mL: mL
Answer: 10 mL
Temperature conversion
Medical assistants also convert between Fahrenheit (°F) and Celsius (°C) when documenting patient temperatures.
Here, is the temperature in degrees Fahrenheit and is the temperature in degrees Celsius.
Example: Converting Fahrenheit to Celsius
A patient’s temperature reads 102°F. What is this in Celsius?
Answer: 38.9°C (rounded to the nearest tenth)
Decimal-to-percentage conversion
To convert a decimal to a percentage, multiply by 100 and add a percent sign: .