This free Dosage Calculations Practice Test #11 covers pediatric body surface area (BSA) and weight-based dosing: the Mosteller BSA formula in metric and household units, mg/m² doses, BSA doses converted to mL, estimating a child’s dose from an adult dose, BSA-based safe-dose checks, daily BSA doses divided by frequency, mL/kg fluid boluses, maintenance fluid rates by weight, and mg/kg/hr continuous infusions. Work through 10 timed questions, then check every answer with the full rationales below. No sign-up required.
All 10 Questions With Answers & Rationales
Prefer to work at your own pace? Every question from the timed quiz above is listed here in full. Choose your answer, then tap Show Answer & Rationale to check it.
Question 1: BSA with the Mosteller formula (metric)
A child is 120 cm tall and weighs 25 kg. Using the Mosteller formula, what is the child's body surface area (BSA)? (Round to the nearest hundredth.)
- A. 0.83 m²
- B. 0.98 m²
- C. 0.91 m²
- D. 1.10 m²
Show Answer & Rationale
Answer: C. 0.91 m²
Rationale: Mosteller formula with metric units: BSA (m²) = √[height (cm) × weight (kg) ÷ 3,600]. 120 × 25 = 3,000. 3,000 ÷ 3,600 = 0.833. √0.833 = 0.913, rounded to 0.91 m². 0.83 m² forgets to take the square root. 0.98 m² uses 3,131, which is the constant for inches and pounds, not centimeters and kilograms. 1.10 m² is the reciprocal of the correct answer. Other BSA formulas and nomograms give slightly different values, so use the method your facility or drug reference specifies.
Question 2: BSA with the Mosteller formula (household units)
A child is 40 inches tall and weighs 35 lb. Using the Mosteller formula for inches and pounds, what is the child's BSA? (Round to the nearest hundredth.)
- A. 0.67 m²
- B. 0.45 m²
- C. 0.62 m²
- D. 1.50 m²
Show Answer & Rationale
Answer: A. 0.67 m²
Rationale: Mosteller formula with household units: BSA (m²) = √[height (in) × weight (lb) ÷ 3,131]. 40 × 35 = 1,400. 1,400 ÷ 3,131 = 0.447. √0.447 = 0.669, rounded to 0.67 m². 0.45 m² forgets the square root. 0.62 m² uses 3,600, the constant for centimeters and kilograms. 1.50 m² is the reciprocal. Match the constant to the units: 3,600 for cm and kg, 3,131 for inches and pounds. You can also convert first (40 in × 2.54 = 101.6 cm; 35 lb ÷ 2.2 = 15.9 kg) and use 3,600; the answer is the same.
Question 3: Dose from mg/m²
A child with a BSA of 0.8 m² is prescribed a medication at 250 mg/m² per dose. How many mg should the child receive per dose?
- A. 312.5 mg
- B. 20 mg
- C. 250 mg
- D. 200 mg
Show Answer & Rationale
Answer: D. 200 mg
Rationale: A BSA-based dose is calculated by multiplying: dose = prescribed mg/m² × BSA. 250 mg/m² × 0.8 m² = 200 mg. 312.5 mg divides by the BSA instead of multiplying. 250 mg gives the dose for 1 m², ignoring the child's actual size. 20 mg is a decimal-place error. BSA dosing is used most often for chemotherapy and other high-alert drugs, so many facilities require an independent double check of the BSA and the calculated dose.
Question 4: BSA dose converted to mL
A child with a BSA of 0.6 m² is prescribed a medication at 75 mg/m² IV. The vial is labeled 10 mg/mL. How many mL should the nurse prepare?
- A. 45 mL
- B. 4.5 mL
- C. 0.45 mL
- D. 12.5 mL
Show Answer & Rationale
Answer: B. 4.5 mL
Rationale: Step 1, find the dose: 75 mg/m² × 0.6 m² = 45 mg. Step 2, convert to volume: 45 mg ÷ 10 mg/mL = 4.5 mL. 45 mL treats the mg dose as mL and skips step 2. 0.45 mL is a decimal-place error. 12.5 mL divides by the BSA (75 ÷ 0.6 = 125 mg) and then by the concentration. Ask: does the answer make sense? A 45 mg dose from a 10 mg/mL vial must be a little under 5 mL.
Question 5: Estimating a child's dose from an adult dose
A drug reference gives an adult dose of 500 mg. A child has a BSA of 0.86 m². Using the BSA ratio method with an average adult BSA of 1.73 m², what is the estimated child's dose? (Round to the nearest whole mg.)
- A. 430 mg
- B. 289 mg
- C. 249 mg
- D. 865 mg
Show Answer & Rationale
Answer: C. 249 mg
Rationale: BSA ratio method: child's dose = (child's BSA ÷ 1.73 m²) × adult dose. 0.86 ÷ 1.73 = 0.497. 0.497 × 500 mg = 248.6 mg, rounded to 249 mg. 430 mg multiplies the adult dose by the child's BSA without dividing by 1.73. 289 mg divides the adult dose by 1.73 but never applies the child's BSA. 865 mg multiplies by 1.73. This method only gives an estimate. When a drug has its own pediatric dosing (mg/kg or mg/m²), use that, and check the result against the drug reference.
Question 6: BSA-based safe-dose check
A child with a BSA of 0.7 m² is prescribed a medication 60 mg IV every 12 hours. The drug reference lists a safe range of 100–150 mg/m²/day divided every 12 hours. What should the nurse conclude?
- A. The dose is too high: 120 mg/day is above the 105 mg/day maximum, so hold it and contact the prescriber.
- B. The dose is safe: 120 mg/day is within the safe range.
- C. The dose is too low: 60 mg is below the 70 mg minimum, so give an extra dose.
- D. The dose is safe because 60 mg per dose is less than 105 mg.
Show Answer & Rationale
Answer: A. The dose is too high: 120 mg/day is above the 105 mg/day maximum, so hold it and contact the prescriber.
Rationale: Find the safe daily range for this child: 100 mg/m² × 0.7 m² = 70 mg/day and 150 mg/m² × 0.7 m² = 105 mg/day. The prescribed daily total is 60 mg × 2 doses = 120 mg/day, which is above the 105 mg/day maximum. The nurse holds the dose and clarifies with the prescriber. The safe per-dose range is 35–52.5 mg every 12 hours, so 60 mg is also too high per dose. Option B compares 120 with the per-m² numbers instead of the child's range. Option C compares one dose with the daily minimum. Option D compares a single dose with the daily maximum. A nurse never changes a dose without a new prescription.
Question 7: Daily BSA dose divided by frequency
A child with a BSA of 0.5 m² is prescribed a medication at 600 mg/m²/day, divided every 6 hours. How many mg should the child receive per dose?
- A. 300 mg
- B. 75 mg
- C. 150 mg
- D. 1,200 mg
Show Answer & Rationale
Answer: B. 75 mg
Rationale: Step 1, daily dose: 600 mg/m²/day × 0.5 m² = 300 mg/day. Step 2, number of doses: every 6 hours = 24 ÷ 6 = 4 doses per day. Step 3, per dose: 300 mg ÷ 4 = 75 mg. 300 mg gives the whole day's amount in a single dose, a fourfold overdose. 150 mg divides by 2 instead of 4. 1,200 mg divides by the BSA instead of multiplying. Read whether a mg/m² order is per dose or per day before you calculate.
Question 8: mL/kg fluid bolus and pump rate
A child who weighs 18 kg is prescribed a 0.9% sodium chloride bolus of 20 mL/kg to infuse over 30 minutes by pump. What volume will the child receive, and at what rate should the nurse set the pump?
- A. 180 mL at 360 mL/hr
- B. 360 mL at 360 mL/hr
- C. 792 mL at 1,584 mL/hr
- D. 360 mL at 720 mL/hr
Show Answer & Rationale
Answer: D. 360 mL at 720 mL/hr
Rationale: Volume: 20 mL/kg × 18 kg = 360 mL. Rate: pumps are set in mL/hr, so 360 mL ÷ 0.5 hour = 720 mL/hr (or 360 mL ÷ 30 min × 60). 180 mL uses 10 mL/kg. 360 mL at 360 mL/hr would take a full hour instead of 30 minutes. 792 mL treats the weight as pounds and multiplies by 2.2. Bolus volume, fluid type and speed are set by the prescriber and facility protocol; reassess heart rate, perfusion, lung sounds and work of breathing during and after every bolus.
Question 9: Maintenance IV fluid rate by weight
The prescriber orders maintenance IV fluids for a child who weighs 24 kg, calculated with the Holliday-Segar method (100 mL/kg/day for the first 10 kg, 50 mL/kg/day for the next 10 kg, and 20 mL/kg/day for each kg over 20). What hourly rate does this give? (Round to the nearest whole mL.)
- A. 100 mL/hr
- B. 66 mL/hr
- C. 1,580 mL/hr
- D. 74 mL/hr
Show Answer & Rationale
Answer: B. 66 mL/hr
Rationale: First 10 kg: 10 × 100 = 1,000 mL. Next 10 kg: 10 × 50 = 500 mL. Remaining 4 kg: 4 × 20 = 80 mL. Total = 1,580 mL/day. 1,580 ÷ 24 hours = 65.8, rounded to 66 mL/hr. 100 mL/hr applies 100 mL/kg to all 24 kg. 1,580 mL/hr sets the daily total as an hourly rate. 74 mL/hr counts 14 kg instead of 4 kg in the last step. The 4-2-1 hourly shortcut gives about 64 mL/hr; both are estimates. The prescriber sets the actual rate and adjusts it for the child's condition, losses and labs.
Question 10: mg/kg/hr continuous infusion
A child who weighs 33 lb is prescribed a continuous IV infusion of a medication at 0.1 mg/kg/hr. The bag contains 10 mg in 100 mL. At what rate in mL/hr should the nurse set the pump?
- A. 1.5 mL/hr
- B. 33 mL/hr
- C. 15 mL/hr
- D. 150 mL/hr
Show Answer & Rationale
Answer: C. 15 mL/hr
Rationale: Step 1, weight in kg: 33 lb ÷ 2.2 = 15 kg. Step 2, dose per hour: 0.1 mg/kg/hr × 15 kg = 1.5 mg/hr. Step 3, concentration: 10 mg ÷ 100 mL = 0.1 mg/mL. Step 4, rate: 1.5 mg/hr ÷ 0.1 mg/mL = 15 mL/hr. 1.5 mL/hr sets the mg/hr dose as mL/hr. 33 mL/hr uses the weight in pounds. 150 mL/hr is a tenfold error. Continuous pediatric infusions are often high-alert, so use a smart pump drug library and an independent double check where your facility requires it.
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