Set 4 of 10 · Weight-based
Weight-Based Dosage Calculation Practice
Many drugs are prescribed by body weight, as mg/kg or mcg/kg, so the dose fits the client. A weight-based calculation has three steps: convert the weight to kilograms, multiply by the dose per kg, then work out the volume to give. The practice questions on this page are free, with no sign-up.
Nurses use it for many IV drugs and for most pediatric doses. An accurate, current weight matters, because every later step depends on it.
Weight-based dosage calculation formula (mg/kg)
- Weight in kg
- kg = lb ÷ 2.2, rounded to the nearest tenth
- Dose
- dose = mg/kg × weight in kg
- Volume to give
- mL = dose (mg) ÷ concentration (mg/mL)
- Divide pounds by 2.2; don't multiply. The weight in kg is always the smaller number.
- Check whether the reference dose is per dose or per day. A daily amount split into several doses needs one more step.
- If the prescription or reference states a maximum dose, compare your result with it.
Worked example: weight-based dosage calculation
A client who weighs 147 lb is prescribed Drug W 1.5 mg/kg IV. The vial is labeled 40 mg/mL. What is the dose in mg, and how many mL will the nurse draw up? Round the weight and each answer to the nearest tenth.
- Weight: 147 lb ÷ 2.2 = 66.818 kg. Body weight is rounded to the nearest tenth before it is used: 66.8 kg.
- Dose: 1.5 mg/kg × 66.8 kg = 100.2 mg. A drug amount in mg is kept to the nearest tenth.
- Volume: 100.2 mg ÷ 40 mg/mL = 2.505 mL.
- Round: a volume of 1 mL or more goes to the nearest tenth, so 2.5 mL.
Answer: 100.2 mg, given as 2.5 mL
Practice this set on the exam screen
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Answer key
Weight-based practice questions: answers and rationales
Try the questions first. Open one to read its answer and rationale.
Question 1A 40-year-old client in the emergency department is in status epilepticus and weighs 68 kg. Phenytoin 15 mg/kg IV is prescribed as a loading dose. How many mg will the nurse give? Round to the nearest tenth. Record your answer using 1 decimal place.
The weight is already in kg, so one step is needed: dose = mg/kg × weight in kg. 15 mg/kg × 68 kg = 1,020 mg. kg cancels, leaving mg. The answer is exact and is written as 1,020 mg. Sense check: 10 mg/kg would be 680 mg, and half as much again is 340 mg, so 15 mg/kg is 680 + 340 = 1,020 mg. IV phenytoin is given no faster than 50 mg/min in adults, so this dose takes at least 1,020 ÷ 50 = 20.4 minutes, with cardiac monitoring. Takeaway: when the weight is given in kg, multiply the dose per kg by the weight and check that kg cancels.
Sources: Open RN Nursing Skills (ACC adaptation), 5.11 Weight-Based Calculations; DailyMed: Phenytoin sodium injection label (adult loading dose of 10 to 15 mg/kg; no faster than 50 mg/min in adults; cardiac monitoring)
Question 2A 55-year-old client on a medical unit has a deep vein thrombosis and weighs 187 lb. Enoxaparin 1 mg/kg subcutaneously every 12 hours is prescribed. How many mg will the nurse give for each dose? Round the weight to the nearest tenth of a kilogram and the dose to the nearest tenth. Record your answer using 1 decimal place.
Step 1: convert the weight. 187 lb ÷ 2.2 = 85 kg. The result is exact, so no rounding is needed. Step 2: dose = mg/kg × weight in kg. 1 mg/kg × 85 kg = 85 mg for each dose. The common error is using the weight in pounds, which gives 187 mg, more than twice the correct dose of an anticoagulant. A second error is doubling or halving the answer because the drug is given twice a day; the prescription is 1 mg/kg for each dose, not for the whole day. Takeaway: weight-based doses use kilograms, so divide pounds by 2.2 before multiplying.
Sources: Open RN Nursing Skills (ACC adaptation), 5.11 Weight-Based Calculations; Open RN Nursing Skills (ACC adaptation), 5.4 Equivalencies; DailyMed: Enoxaparin sodium injection label, section 2 (1 mg/kg subcutaneously every 12 hours for deep vein thrombosis)
Question 3A 26-year-old client in the emergency department has a fractured femur and weighs 54 kg. Morphine sulfate 0.1 mg/kg IV is prescribed. The vial is labeled 10 mg/mL. How many mL will the nurse give? Round to the nearest hundredth. Record your answer using 2 decimal places.
Step 1: dose = mg/kg × weight in kg. 0.1 mg/kg × 54 kg = 5.4 mg. Step 2: volume = dose ÷ concentration. 5.4 mg ÷ 10 mg/mL = 0.54 mL. A volume under 1 mL is given to the nearest hundredth and written with a leading zero. Stopping after step 1 and recording 5.4 confuses the dose in mg with the volume in mL; drawing up 5.4 mL would be a tenfold overdose of an opioid. After giving the dose, the nurse reassesses pain, sedation level and breathing. Takeaway: a weight-based problem often has two parts, the dose in mg and then the volume in mL; finish both.
Sources: Open RN Nursing Skills (ACC adaptation), 5.11 Weight-Based Calculations; Open RN Nursing Skills (ACC adaptation), 5.5 Rounding; DailyMed: Morphine sulfate injection label, Hikma (0.1 to 0.2 mg/kg IV every 4 hours as needed; 10 mg/mL vials); AHRQ PSNet WebM&M: opioid dose stacking (reassess pain, sedation and breathing after an opioid)
Question 4A 60-year-old client in the intensive care unit has a life-threatening infection and weighs 72 kg. Tobramycin 1.5 mg/kg IV every 8 hours is prescribed. The drug reference states the maximum as an amount per day. How many mg of tobramycin will the client receive in 24 hours? Round to the nearest tenth. Record your answer using 1 decimal place.
Step 1: single dose = mg/kg × weight in kg. 1.5 mg/kg × 72 kg = 108 mg. Step 2: every 8 hours is 24 ÷ 8 = 3 doses, so the daily amount is 108 mg × 3 = 324 mg. The prescription is written per dose and the limit per day, so both must be on the same basis before they are compared. Here 324 mg ÷ 72 kg = 4.5 mg/kg/day, which is within the 5 mg/kg/day used for life-threatening infections. Stopping at 108 mg understates the daily amount of an aminoglycoside, a drug class that can damage the kidneys and hearing. Takeaway: check whether a weight-based dose is written per dose or per day, and convert one to the other before comparing it with a limit.
Sources: Open RN Nursing Skills (ACC adaptation), 5.11 Weight-Based Calculations; DailyMed: Tobramycin for injection label, Fresenius Kabi (boxed warning: kidney damage and hearing damage); DailyMed: Tobramycin injection label, Fresenius Kabi (adults with life-threatening infections: up to 5 mg/kg/day in 3 or 4 equal doses; not above 5 mg/kg/day unless serum levels are monitored)
Question 5A 51-year-old client at an outpatient infusion center weighs 262 lb and is prescribed ondansetron 0.15 mg/kg IV, not to exceed 16 mg per dose, 30 minutes before chemotherapy. The 20 mL multiple-dose vial is labeled 2 mg/mL. How many mL will the nurse draw up? Round the weight to the nearest tenth of a kilogram and the volume to the nearest tenth. Record your answer using 1 decimal place.
Step 1: weight. 262 lb ÷ 2.2 = 119.09 kg, rounded to 119.1 kg. Step 2: calculated dose. 0.15 mg/kg × 119.1 kg = 17.865 mg. Step 3: compare with the stated maximum. 17.865 mg is more than 16 mg, so the dose to give is 16 mg. Step 4: volume. 16 mg ÷ 2 mg/mL = 8 mL. The 30 minutes and the 20 mL vial size are not part of the calculation. Skipping the comparison gives 17.865 ÷ 2 = 8.9 mL, a dose above the stated maximum. Takeaway: when a prescription or reference states a maximum dose, compare the weight-based result with it before working out the volume.
Sources: Open RN Nursing Skills (ACC adaptation), 5.11 Weight-Based Calculations; Open RN Nursing Skills (ACC adaptation), 5.4 Equivalencies; DailyMed: Ondansetron injection label, Hospira, section 2.1 (0.15 mg/kg per dose, maximum 16 mg per dose; 2 mg/mL)
FAQ
Weight-based dosing: FAQ
What is weight-based dosing?
How to calculate drug per kg?
Can you give me some examples of weight-based medications?
Sources: Open RN Nursing Skills (ACC adaptation), 5.11 Weight-Based Calculations; DailyMed: Cephalexin for oral suspension label; DailyMed: Dopamine Hydrochloride in 5% Dextrose Injection label; DailyMed: Dobutamine in 5% Dextrose Injection label; Raschke RA et al. The weight-based heparin dosing nomogram. Ann Intern Med 1993