Medical Math for Medical Assistants: Dosage Calculations and Conversions

Why Math Shows Up on a Clinical Exam

Dosage calculation errors kill people. This is not an exaggeration — medication errors are consistently among the top preventable causes of patient harm in healthcare. Medical assistants measure, prepare, and in many settings administer medications. The CMA and RMA exams test math because math mistakes have real consequences. Work through every problem systematically, and always double-check your answer before preparing or administering anything.

The Metric System

Healthcare uses the metric system almost exclusively for medications. You need to convert fluently within the metric system and between metric and other units.

Weight conversions:

• 1 gram (g) = 1,000 milligrams (mg)
• 1 milligram (mg) = 1,000 micrograms (mcg or µg)
• 1 kilogram (kg) = 1,000 grams (g)
• 1 kilogram (kg) = 2.2 pounds (lb)

Volume conversions:

• 1 liter (L) = 1,000 milliliters (mL)
• 1 mL = 1 cubic centimeter (cc) — these terms are interchangeable in clinical practice

Common conversions to memorize:

• 1 teaspoon (tsp) = 5 mL
• 1 tablespoon (tbsp) = 15 mL = 3 tsp
• 1 fluid ounce = 30 mL
• 1 cup = 240 mL
• 1 pound (lb) = 0.454 kg (or divide lb by 2.2 to get kg)

Temperature Conversion

Two formulas. You need both.

Fahrenheit to Celsius: C = (F - 32) × 5/9

Celsius to Fahrenheit: F = (C × 9/5) + 32

Key reference temperatures:

• Normal body temperature: 98.6°F = 37.0°C
• Freezing: 32°F = 0°C
• Boiling: 212°F = 100°C
• Low-grade fever: begins at 99.5°F (37.5°C)
• High fever: 103°F (39.4°C) and above

Worked example: Convert 101.3°F to Celsius.

C = (101.3 - 32) × 5/9 = 69.3 × 5/9 = 346.5 ÷ 9 = 38.5°C

Dosage Calculation Formula

The standard formula for calculating how much of a medication to give:

Dose = (Desired dose ÷ Have) × Quantity

Or written as a fraction: D/H × Q

• D (Desired): The dose ordered by the physician
• H (Have): The dose strength available (on hand)
• Q (Quantity): The amount the available dose comes in (1 tablet, 5 mL, etc.)

Worked example 1: The physician orders 500 mg of amoxicillin. You have amoxicillin 250 mg/5 mL suspension. How many mL do you give?

D = 500 mg, H = 250 mg, Q = 5 mL

Dose = (500 ÷ 250) × 5 = 2 × 5 = 10 mL

Worked example 2: The physician orders 0.5 mg of a drug. You have 0.25 mg tablets. How many tablets?

D = 0.5 mg, H = 0.25 mg, Q = 1 tablet

Dose = (0.5 ÷ 0.25) × 1 = 2 × 1 = 2 tablets

Worked example 3: Order: 75 mg IM. Available: 100 mg/2 mL. How many mL?

D = 75 mg, H = 100 mg, Q = 2 mL

Dose = (75 ÷ 100) × 2 = 0.75 × 2 = 1.5 mL

Pediatric Dosage Calculations

Pediatric dosing is weight-based because children cannot receive the same doses as adults. There are two methods you need to know for the exam.

Clark's Rule (body weight method):

Child's dose = (Child's weight in lb ÷ 150) × Adult dose

The 150 represents the average adult weight in pounds.

Worked example: Adult dose is 500 mg. Child weighs 50 lb. What is the child's dose?

Child's dose = (50 ÷ 150) × 500 = 0.333 × 500 = 167 mg

Body weight method (mg/kg):

This is the standard method used clinically. The physician orders a dose in mg per kg of body weight.

Child's dose = Ordered dose (mg/kg) × Child's weight (kg)

Worked example: Physician orders 10 mg/kg of ibuprofen. Child weighs 22 lb. What is the dose?

Step 1: Convert pounds to kg: 22 ÷ 2.2 = 10 kg

Step 2: 10 mg/kg × 10 kg = 100 mg

Always convert pounds to kilograms first when using mg/kg dosing. A common error is forgetting this step and using pounds directly — that gives a dose 2.2 times too high.

Household to Metric Conversions

Patients often use household measurements at home. You need to translate between them:

• 1 teaspoon = 5 mL
• 1 tablespoon = 15 mL
• 2 tablespoons = 1 fluid ounce = 30 mL
• 1 cup = 8 fluid ounces = 240 mL
• 1 pint = 16 fluid ounces = 480 mL
• 1 quart = 2 pints = 32 fluid ounces = 960 mL ≈ 1 L

When a patient asks "how many teaspoons is 10 mL?" you should be able to answer immediately: 2 teaspoons. This comes up often with liquid medications for children.

IV Drip Rate Basics

In some MA roles and states, IV therapy is within scope under direct physician supervision. Even where it is not, the formula appears on exams.

Drops per minute (gtts/min) = (Volume in mL × Drop factor) ÷ Time in minutes

Drop factor (gtts/mL) is printed on the IV tubing package. Common drop factors: 10 gtts/mL (macrodrip), 15 gtts/mL (macrodrip), 20 gtts/mL (macrodrip), 60 gtts/mL (microdrip).

Worked example: Physician orders 500 mL normal saline over 4 hours. Tubing is 20 gtts/mL. What is the drip rate?

Time in minutes = 4 × 60 = 240 minutes

Drip rate = (500 × 20) ÷ 240 = 10,000 ÷ 240 = 41.7 gtts/min ≈ 42 gtts/min

Practice Questions