A nurse is preparing to administer amoxicillin to a 4-year-old child who weighs 18 kg. The prescribed dose is 40 mg/kg/day divided into three equal doses. The available medication is amoxicillin oral suspension 250 mg/5 mL. How many milliliters should the nurse administer for each dose?

The correct dose is 4.8 mL, calculated as (18 kg × 40 mg/kg/day ÷ 3 doses) × (5 mL / 250 mg). Other options result from errors in weight-based calculation, dose division, or concentration conversion.

Understanding the Prescription
The first step in any pediatric dosage calculation is to clearly identify the prescribed daily dose. The order states amoxicillin 40 mg/kg/day. For a child weighing 18 kg, the total amount of medication needed per day is calculated by multiplying the weight by the dose per kilogram.

Calculating the Total Daily Dose
Total daily dose = 40 mg × 18 kg = 720 mg. This means the child must receive a total of 720 mg of amoxicillin over the course of the day.

Determining the Single Dose
The prescription specifies that this total daily amount is to be divided into three equal doses. To find the amount for one single dose, divide the total daily dose by 3.
Single dose = 720 mg / 3 = 240 mg. The nurse must administer 240 mg of amoxicillin for each dose.

Converting the Dose to Volume
The medication available is an oral suspension with a concentration of 250 mg per 5 mL. This ratio is used to convert the required mass (240 mg) into the volume to be drawn up and administered. Using the formula (Desired dose / Available dose) × Available volume:
Volume per dose = (240 mg / 250 mg) × 5 mL = 0.96 × 5 mL = 4.8 mL.

Clinical Significance of Accurate Calculation
The result of 4.8 mL per dose is the precise volume the nurse should administer. Mastery of medication dosage calculation (MDC) is a non-negotiable clinical skill, as highlighted in research on healthcare education. A randomized quasi-experimental study by Baran and Öztürk (2025) emphasizes that MDC is a vital component of clinical competence, where miscalculations can lead to life-threatening outcomes ^[1]. This principle is especially critical in pediatric populations, where doses are weight-based and errors can have amplified consequences. The prescription of antibiotics like amoxicillin must be precise not only for therapeutic efficacy but also to support antimicrobial stewardship. Inappropriate dosing, whether subtherapeutic or excessive, can contribute to the development of antimicrobial resistance (AMR), a significant global health challenge addressed in clinical practice guidance for children . Ensuring the calculated volume of 4.8 mL is accurately drawn and administered is a direct application of safe prescribing quality, a key factor in combating AMR and ensuring positive patient outcomes . The process of verifying the total daily dose (720 mg) and the single dose (240 mg) before the final volume conversion is a critical safety checkpoint that prevents potentially harmful tenfold errors.