Measurement and Unit Conversion
The Measurement and Unit Conversion domain tests whether you can move confidently between metric and household units, choose the right conversion factor, and keep units consistent. Items are usually short but unforgiving: one inverted factor and the answer is wrong even though the arithmetic looks fine.
Metric prefixes
The base units are the meter (length), liter (volume), and gram (mass). Kilo- means 1000 base units; centi- means one-hundredth (0.01); milli- means one-thousandth (0.001); micro- means one-millionth (0.000001). Going from a larger unit to a smaller unit multiplies (2 L = 2000 mL); going from a smaller unit to a larger unit divides (2500 mL = 2.5 L). Moving between kilo, base, and milli shifts the decimal three places; centi shifts it two.
Equivalents worth memorizing
- 1 kg = 2.2 lb; 1 lb = 16 oz
- 1 in = 2.54 cm
- 1 L = 1000 mL; 1 mL = 1 cc
- 1 tsp = 5 mL; 1 tbsp = 15 mL = 3 tsp
- 1 oz (liquid) = 30 mL; 1 cup = 8 oz = 240 mL
- 1 quart = 4 cups = 960 mL
Dimensional analysis
Write the given quantity as a fraction over 1 and multiply by conversion factors arranged so the unwanted unit cancels. Chain as many factors as needed; the remaining unit should be the one requested.
Worked example
A patient weighs 154 lb. What is the weight in kilograms, and what is a 0.5 mg/kg dose?
- Convert: 154 lb ÷ 2.2 lb/kg = 70 kg. Dividing is correct because kilograms are larger than pounds.
- Dose: 70 kg × 0.5 mg/kg = 35 mg.
- Check: 70 × 2.2 = 154, so both steps are consistent.
Traps that cost marks
Multiplying when converting pounds to kilograms. Using 2.5 or 2.4 instead of 2.2 lb/kg. Confusing milligrams and micrograms - 1 mg = 1000 mcg, so 0.25 mg = 250 mcg. Mixing weight ounces with fluid ounces. Losing a zero when writing 1.5 L as 150 mL. Rounding a converted weight before using it in a dose calculation.