Pediatric dosage calculation: mg/kg/day divided
Pediatric orders are weight-based and often written per day, so the calculation adds one step: dividing the daily dose by the number of doses. This page shows the sequence, the safe dose check, and two worked examples.
Formula
Doses per day come from the frequency: q12h is 2, q8h is 3, q6h is 4. Round mg to the tenth and mL to the tenth, once, at the end of each.
Example 1 · divided daily dose
Order: amoxicillin 40 mg/kg/day PO divided q8h. Child 22 lb. Available: 250 mg / 5 mL. mL per dose?
- 22 lb ÷ 2.2 = 10 kg
- 10 × 40 = 400 mg/day
- q8h = 3 doses → 400 ÷ 3 = 133.33 mg/dose
- 133.33 ÷ 250 × 5 = 2.67 → 2.7 mL
Answer: 2.7 mL
Example 2 · once daily
Order: azithromycin 10 mg/kg/day PO once daily. Child 33 lb. Available: 200 mg / 5 mL. mL per dose?
- 33 lb ÷ 2.2 = 15 kg
- 15 × 10 = 150 mg (one dose per day)
- 150 ÷ 200 × 5 = 3.75 → 3.8 mL
Answer: 3.8 mL
Mistakes that cost points
- Not dividing the daily dose. 400 mg is the day's total. Each q8h dose is 133.3 mg.
- Pounds as kilograms. 22 × 40 = 880 mg/day, more than double. Convert to 10 kg first.
- Comparing per-dose with per-day. A range in mg/kg/day is checked against the order's daily total.
Practice for this topic opens soon
Generated pediatric problems join the drills shortly. Until then, check your own working with the mg per kg calculator, and practise the steps it builds on: Conversions.
Why pediatric doses are per kilogram
Children vary too much in size for fixed doses, so most pediatric orders are mg/kg, and a dosing error is proportionally larger. Weigh in kilograms; if the weight is given in pounds, divide by 2.2. A 22 lb toddler weighs 10 kg.
References give pediatric doses two ways: mg/kg/dose, and mg/kg/day divided into a number of doses. Read which one the order and the reference use before you start.
Dividing the daily dose
When the order is mg/kg/day, multiply by the weight to get the daily total, then divide by the doses per day from the frequency. Amoxicillin 40 mg/kg/day divided q8h for 10 kg is 400 mg/day, and q8h is three doses, so each dose is 133.3 mg.
Giving the whole daily amount as one dose triples or quadruples the dose. It is the error exam writers test most in pediatric questions, and it is why the divide step deserves its own line in your work.
Checking the safe dose range
A safe dose question gives a reference range, for example amoxicillin 20 to 40 mg/kg/day, and an order. Multiply both ends by the weight: for 10 kg, 200 to 400 mg/day. Then express the order the same way: 150 mg q8h is 450 mg/day. That is above the range, so the order is not safe and you would hold it and contact the prescriber.
Compare like with like. If the range is per day, convert the order to per day; if the range is per dose, compare each dose.
Measuring small volumes
Oral pediatric doses are measured in an oral syringe marked in tenths of a mL, so round to the tenth. Under 1 mL, round to the hundredth when the syringe allows it. 2.67 mL becomes 2.7 mL; 3.75 mL becomes 3.8 mL.
A sense check: most oral pediatric doses are between about 1 and 15 mL. A dose of 40 mL for a toddler usually means the daily dose was not divided.
Frequencies and doses per day
Convert the frequency to doses per day before dividing: q24h or daily is 1, q12h or BID is 2, q8h or TID is 3, q6h or QID is 4, q4h is 6. Use the interval form when the order gives it, because BID and q12h are not always interchangeable in practice, but for the division they give the same count.
Body surface area
A few pediatric drugs, mostly chemotherapy, are dosed per square metre of body surface area rather than per kilogram. The Mosteller formula is BSA (m²) = √(height in cm × weight in kg ÷ 3,600). For a child 100 cm tall weighing 16 kg, BSA is √(1,600 ÷ 3,600) = 0.67 m². Most dosage exams for students include BSA only as an occasional item; when they do, they give the formula or a nomogram.
How exams test it
Pediatric items combine three skills: pounds to kilograms, the daily dose divided by the frequency, and a safe dose check. Expect a reference range in the stem and a question that asks both whether the order is safe and how many mL to give. Work the safe dose check first; if the order is unsafe, the volume question becomes a question about holding the dose.
Measuring and giving oral doses to children
Use an oral syringe, never an injection syringe, for oral liquids, so an oral dose cannot be injected by mistake. For infants, give small amounts into the side of the cheek and let the child swallow between each. Teach caregivers with the same syringe they will use at home, and write the dose in mL on the instructions, not in teaspoons, because household spoons vary widely in volume.
Check before you submit
Is the weight in kilograms? Is the order per day, and did you divide by the doses per day from the frequency? Did you compare the order with the safe range in the same terms, per dose or per day? Is the volume rounded to the tenth of a mL, or the hundredth under 1 mL? Is the volume plausible for a child of this size, usually 1 to 15 mL for an oral dose?
If the order is unsafe, the answer to give is that you would hold the dose and clarify it, not a volume. Read what the question asks after the calculation.
Questions people ask
How do you calculate a pediatric dose?
Convert weight to kg, multiply by the mg/kg ordered, divide by doses per day if the order is per day, then divide by the concentration for the volume.
How do I know if a pediatric dose is safe?
Multiply the reference range by the weight, express the order in the same terms, and check it falls inside the range.
How many doses per day is q6h?
Four. q8h is three and q12h is two.
Why are pediatric doses checked against a safe range?
Children vary widely in size and a small error is proportionally larger. Checking every order against the weight-based reference range catches prescribing and calculation errors before the dose is given.
Do I round pediatric doses to the tenth?
Round mg to the tenth and mL to the tenth, or the hundredth under 1 mL, unless the question says otherwise.
What is body surface area dosing?
Dosing per square metre of body surface area, used for some drugs such as chemotherapy. BSA = √(height cm × weight kg ÷ 3,600).