Why Unit Conversions Matter
Every dosage calculation starts with unit conversions. If you can't convert between grams, milligrams, and micrograms — or between pounds and kilograms — you can't safely calculate any medication dose.
The good news? There are only about a dozen conversions you actually need to memorize. Master these, and the rest of nursing math becomes dramatically easier.
Metric Weight Conversions
1 kg = 1,000 g 1 g = 1,000 mg 1 mg = 1,000 mcg 1 kg = 2.2 lb 1 lb = 16 oz
The most dangerous conversion error in nursing is confusing mg and mcg. A 1000-fold error (e.g., giving 1 mg instead of 1 mcg) can be lethal with drugs like digoxin, fentanyl, and levothyroxine.
The mg/mcg Danger Zone
Confusing milligrams (mg) and micrograms (mcg) is one of the deadliest medication errors. They differ by a factor of 1,000. Fentanyl is ordered in mcg — giving mg would be a 1000x overdose. Levothyroxine is ordered in mcg — giving mg would be catastrophic. Always write out "mcg" — never abbreviate to a symbol that could be confused with mg.
Metric Volume Conversions
1 L = 1,000 mL 1 mL = 1 cc (milliliter = cubic centimeter) 1 tsp = 5 mL 1 tbsp = 15 mL 1 oz = 30 mL 1 cup = 240 mL
You'll use the household conversions (tsp, tbsp, oz) primarily for patient education when teaching about liquid medications at home.
Temperature Conversions
Celsius to Fahrenheit: (C x 9/5) + 32 = F Fahrenheit to Celsius: (F - 32) x 5/9 = C Normal body temp: 37C = 98.6F
Quick mental check: 100.4F = 38C (fever threshold). This is worth memorizing as a clinical anchor point.
Special Medical Conversions
1 grain (gr) = 60 mg — still used for some drugs like aspirin and thyroid medications. You may see "aspirin gr V" which means 5 grains = 300 mg (close to the standard 325 mg tablet).
1 inch = 2.54 cm — needed for BSA calculations and height conversions.
1 dram = 4 mL — occasionally seen in pharmacy, rarely in clinical nursing.
The Decimal Movement Method
Within the metric system, converting between units is just moving the decimal point. kg to g: move 3 places RIGHT (multiply by 1000). g to mg: move 3 places RIGHT. mg to mcg: move 3 places RIGHT. Going the other direction, move LEFT. Example: 0.5 g = 500 mg = 500,000 mcg. Each jump is 3 places. Once you internalize this pattern, metric conversions become instant.
Practice with NurseTally Calculators
Convert any medical unit instantly and accurately.