Conversions and Rounding
The numbers you need memorized, and three ways to set up any problem
Metric
| 1 kg | = 1,000 g |
| 1 g | = 1,000 mg |
| 1 mg | = 1,000 mcg |
| 1 L | = 1,000 mL |
| 1 mL | = 1 cc (write "mL", not "cc") |
Moving down a step (bigger unit → smaller unit): × 1,000. Moving up: ÷ 1,000.
Household and weight
| 1 kg | = 2.2 lb |
| 1 lb | = 16 oz |
| 1 in | = 2.54 cm |
| 1 tsp | = 5 mL |
| 1 tbsp | = 15 mL (3 tsp) |
| 1 fl oz | = 30 mL |
| 1 cup | = 240 mL (8 oz) |
Temperature
°F = (°C × 9/5) + 32
°C = (°F − 32) × 5/9
Check yourself: 37°C = 98.6°F; 38°C = 100.4°F.
Rounding rules (common exam rules)
- Round only the final answer, not each step.
- Volumes >1 mL: nearest tenth (2.46 → 2.5 mL).
- Volumes <1 mL: nearest hundredth (0.756 → 0.76 mL): use a 1 mL syringe.
- Tablets: whole, or half only if scored.
- Drops per minute: whole number. Weight in kg: nearest tenth.
- Always a leading zero (0.5 mg); never a trailing zero (5 mg, not 5.0 mg).
- Follow any rounding instruction in the question.
Three ways to set up a problem
Example: order amoxicillin 500 mg; available 250 mg per 5 mL.
Formula
Desired ÷ Have × Quantity
500 ÷ 250 × 5 mL = 10 mL
Ratio–proportion
250 mg : 5 mL = 500 mg : x mL
250x = 2,500 → x = 10 mL
Dimensional analysis
500 mg × (5 mL ÷ 250 mg) = 10 mL
Units you don't want cancel out.
Pick one method and use it every time. Check the units first: convert the order and the supply to the same unit before calculating.
Practice
- 154 lb = ___ kg
- 0.25 mg = ___ mcg
- 38.5°C = ___ °F
- 102.2°F = ___ °C
- 2 tbsp = ___ mL
Answers
- 154 ÷ 2.2 = 70 kg
- 0.25 × 1,000 = 250 mcg
- 38.5 × 9/5 + 32 = 101.3°F
- (102.2 − 32) × 5/9 = 39°C
- 2 × 15 = 30 mL