1.6 Math and Measurement for Medical Assistants
Key Takeaways
- The metric system is the standard in healthcare: length (meters/centimeters), mass (grams/kilograms), volume (liters/milliliters), and temperature (Celsius)
- Key conversions: 1 kg = 2.2 lbs, 1 inch = 2.54 cm, 1 tsp = 5 mL, 1 tbsp = 15 mL, 1 oz = 30 mL, 1 cup = 240 mL
- Temperature conversions: °F to °C = (°F - 32) × 5/9; °C to °F = (°C × 9/5) + 32
- Medication math uses the formula D/A × Q = Amount (Desired dose ÷ Available dose × Quantity)
- Intake and output (I&O) measures all fluids consumed and all fluids excreted to assess fluid balance
- Military time (24-hour clock) is used in healthcare documentation: 1:00 PM = 1300, midnight = 0000, noon = 1200
Accurate math and measurement skills are essential for safe clinical practice. Medical assistants must be comfortable with metric conversions, medication calculations, and documentation standards.
Metric System Basics
| Measurement | Base Unit | Common Clinical Units |
|---|---|---|
| Length | Meter (m) | Centimeters (cm), millimeters (mm) |
| Mass/Weight | Gram (g) | Kilograms (kg), milligrams (mg), micrograms (mcg) |
| Volume | Liter (L) | Milliliters (mL), cubic centimeters (cc) — Note: 1 mL = 1 cc |
| Temperature | Celsius (°C) | Also Fahrenheit (°F) in some clinical settings |
Metric Prefixes:
- kilo- (k) = 1,000 (1 kg = 1,000 g)
- centi- (c) = 0.01 (1 cm = 0.01 m)
- milli- (m) = 0.001 (1 mg = 0.001 g; 1 mL = 0.001 L)
- micro- (mc/μ) = 0.000001 (1 mcg = 0.001 mg)
Essential Conversions
| Conversion | Value |
|---|---|
| 1 kg | 2.2 pounds (lbs) |
| 1 inch | 2.54 centimeters |
| 1 teaspoon (tsp) | 5 mL |
| 1 tablespoon (tbsp) | 15 mL |
| 1 ounce (oz) | 30 mL |
| 1 cup | 240 mL (8 oz) |
| 1 pint | 480 mL (16 oz) |
| 1 quart | 960 mL (32 oz) |
| 1 liter | 1,000 mL |
Weight Conversion Examples:
- Patient weighs 154 lbs → 154 ÷ 2.2 = 70 kg
- Patient weighs 80 kg → 80 × 2.2 = 176 lbs
Temperature Conversions
- °F to °C: (°F - 32) × 5/9 = °C
- Example: 98.6°F → (98.6 - 32) × 5/9 = 37°C
- °C to °F: (°C × 9/5) + 32 = °F
- Example: 39°C → (39 × 9/5) + 32 = 102.2°F
Military Time (24-Hour Clock)
Healthcare documentation uses 24-hour (military) time to prevent AM/PM confusion:
| Standard Time | Military Time |
|---|---|
| 12:00 AM (midnight) | 0000 |
| 1:00 AM | 0100 |
| 6:00 AM | 0600 |
| 12:00 PM (noon) | 1200 |
| 1:00 PM | 1300 |
| 3:00 PM | 1500 |
| 6:00 PM | 1800 |
| 9:00 PM | 2100 |
| 11:59 PM | 2359 |
Conversion rules:
- For times 1:00 AM to 12:59 PM: Same number (add leading zero if needed)
- For times 1:00 PM to 11:59 PM: Add 12 to the hour
- To convert from military time back: If ≥1300, subtract 12
Intake and Output (I&O)
I&O measures all fluids consumed (intake) and all fluids excreted (output) over a defined period:
Intake includes: Oral fluids, IV fluids, tube feedings, ice chips (counted at half volume) Output includes: Urine, vomit, diarrhea, wound drainage, blood loss
Normal adult daily fluid balance: Intake ≈ Output (approximately 2,000-2,500 mL/day)
A patient weighs 176 pounds. What is their weight in kilograms?
What is 3:30 PM in military (24-hour) time?
How many milliliters (mL) are in 2 tablespoons of liquid medication?