Dosage Calculation Practice 👑
Med math is not hard, it is unforgiving. One formula, one unit check, one decimal point. Work these and the drip rate stops being scary.
The three formulas that cover almost everything
- Desired over Have: (Desired dose ÷ Dose on hand) × Quantity = amount to give. Use this for tablets and simple liquids.
- IV drip rate in mL/hr: total volume ÷ total hours. For gtt/min, multiply mL/hr by the drop factor and divide by 60.
- Weight-based infusions: mcg/kg/min × kg × 60 ÷ concentration in mcg/mL = mL/hr. Convert mg to mcg before you start.
Worked practice problems
Cover the answer, work it, then check. Show your units at every step — the units are what catch the error.
- 1. Order: 0.25 mg digoxin PO. Available: 0.125 mg tablets. → (0.25 ÷ 0.125) × 1 = 2 tablets.
- 2. Order: 1,000 mL NS over 8 hours. → 1,000 ÷ 8 = 125 mL/hr.
- 3. Order: 500 mL over 4 hours with a 15 gtt/mL set. → 125 mL/hr × 15 ÷ 60 = 31 gtt/min.
- 4. Order: heparin 1,200 units/hr. Bag: 25,000 units in 250 mL. → 25,000 ÷ 250 = 100 units/mL; 1,200 ÷ 100 = 12 mL/hr.
- 5. Order: dopamine 5 mcg/kg/min for an 80 kg patient. Bag: 400 mg in 250 mL. → 400 mg = 400,000 mcg ÷ 250 = 1,600 mcg/mL. 5 × 80 × 60 ÷ 1,600 = 15 mL/hr.
- 6. Order: amoxicillin 40 mg/kg/day divided every 8 hours for a 22 lb child. → 22 ÷ 2.2 = 10 kg; 400 mg/day ÷ 3 doses = 133 mg per dose.
- 7. Order: 750 mg cefazolin. Available: 1 g in 10 mL. → 1 g = 1,000 mg; (750 ÷ 1,000) × 10 = 7.5 mL.
Conversions you must know cold
| From | To | Factor |
|---|---|---|
| 1 kg | pounds | 2.2 lb |
| 1 g | milligrams | 1,000 mg |
| 1 mg | micrograms | 1,000 mcg |
| 1 L | milliliters | 1,000 mL |
| 1 tsp | milliliters | 5 mL |
| 1 tbsp | milliliters | 15 mL |
| 1 oz | milliliters | 30 mL |
| 1 grain | milligrams | 60 mg |
The four errors that cause almost every wrong answer
- Skipping the unit conversion — mg and mcg are a factor of 1,000 apart and that is a fatal overdose, not a rounding issue.
- Using pounds instead of kilograms in a weight-based dose. Divide by 2.2 first, every time.
- Forgetting the drop factor when the question asks for gtt/min instead of mL/hr.
- Not sanity-checking the answer. Forty tablets is never the right answer. If it looks wild, it is wrong.
Frequently asked
What is the formula for dosage calculation?
The core formula is Desired ÷ Have × Quantity. For IV rates use total volume ÷ total hours for mL/hr, and multiply by the drop factor divided by 60 for gtt/min.
How do you calculate mcg/kg/min?
Multiply the ordered mcg/kg/min by the patient's weight in kilograms and by 60, then divide by the bag concentration in mcg/mL. That gives you mL/hr for the pump.
How do I convert pounds to kilograms for a weight-based dose?
Divide the weight in pounds by 2.2. A 154 lb patient is 70 kg. Always convert before you calculate, never after.
Are these dosage calculation practice problems free?
Yes, every problem and worked solution on this page is free. Timed Med Math Sprint drills with scoring and streak tracking are part of the Baddie RN membership.
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