Converting Between Units¶
Overview¶
Unit conversion is the process of expressing a measurement in a different unit while keeping the same value. In nursing, you will constantly convert between units — metric to metric, household to metric, and back again.
The Conversion Factor Method¶
A conversion factor is a fraction that equals 1, where the numerator and denominator represent the same quantity in different units.
Because it equals 1, multiplying by it changes the unit without changing the value.
Step-by-Step Method¶
- Write down what you have
- Write down what you want
- Choose a conversion factor that cancels the unwanted unit
- Multiply and simplify
- Check that your answer makes sense
Example: Convert 2.5 g to mg.
| Step | Work |
|---|---|
| Have | 2.5 g |
| Want | mg |
| Conversion factor | \(\frac{1000 \text{ mg}}{1 \text{ g}}\) |
| Calculate | \(2.5 \text{ g} \times \frac{1000 \text{ mg}}{1 \text{ g}} = 2500 \text{ mg}\) |
| Check | mg is smaller than g, so number should be larger ✅ |
Common Conversions Reference¶
Weight¶
[1 \text{ kg} = 2.2 \text{ lb}] [1 \text{ kg} = 1000 \text{ g}] [1 \text{ g} = 1000 \text{ mg}] [1 \text{ mg} = 1000 \text{ mcg}]
Volume¶
[1 \text{ L} = 1000 \text{ mL}] [1 \text{ tbsp} = 15 \text{ mL}] [1 \text{ tsp} = 5 \text{ mL}] [1 \text{ fl oz} = 30 \text{ mL}]
Multi-Step Conversions¶
Sometimes you need more than one conversion factor.
Example: Convert 3 tbsp to mL, then to L.
Step 1 — tbsp to mL: [3 \text{ tbsp} \times \frac{15 \text{ mL}}{1 \text{ tbsp}} = 45 \text{ mL}]
Step 2 — mL to L: [45 \text{ mL} \times \frac{1 \text{ L}}{1000 \text{ mL}} = 0.045 \text{ L}]
Or in one step: [3 \text{ tbsp} \times \frac{15 \text{ mL}}{1 \text{ tbsp}} \times \frac{1 \text{ L}}{1000 \text{ mL}} = 0.045 \text{ L}]
Sanity Checks¶
Always Ask Yourself
- Am I converting to a smaller unit? → answer should be a larger number
- Am I converting to a larger unit? → answer should be a smaller number
If your answer goes the wrong direction, flip your conversion factor.
Clinical Application¶
Example 1: An order reads 0.25 g of amoxicillin. Stock is labeled 250 mg per tablet. Are these the same dose?
Yes — give 1 tablet.
Example 2: A patient weighs 176 lb. A drug is dosed at 5 mg/kg. What is the patient's weight in kg?
Example 3: An IV bag contains 1.5 L. How many mL is that?
Practice Problems¶
Problem 1
Convert 0.75 g to mg.
Answer
Problem 2
Convert 1500 mcg to mg.
Answer
Problem 3
A patient weighs 132 lb. Convert to kg.
Answer
Problem 4
An order is for 500 mcg. Stock is labeled 0.25 mg per tablet. Are these equivalent?
Answer
Convert order to mg: [500 \text{ mcg} \div 1000 = 0.5 \text{ mg}]
0.5 mg ≠ 0.25 mg — not equivalent. You would need 2 tablets.
Problem 5
Convert 2.5 L to mL.
Answer
Problem 6
A patient takes 4 tsp of liquid medication at home. Convert to mL and then to tablespoons for documentation.
Answer
Step 1 — tsp to mL: [4 \text{ tsp} \times 5 \text{ mL/tsp} = 20 \text{ mL}]
Step 2 — mL to tbsp: [20 \text{ mL} \div 15 \text{ mL/tbsp} = 1.33 \text{ tbsp}]
Clinical Tip
When in doubt, convert everything to the same unit before comparing or calculating. Never try to mentally jump between units mid-calculation — write every step out until conversions become second nature.