Pediatric dosing: why weight matters more than age
Children aren't small adults. Their kidneys clear drugs more slowly in infancy, their liver enzymes mature at different rates, and their body water composition changes dramatically from birth through adolescence. A dosing approach that works for a 5-year-old won't work for a neonate. And an adult dose scaled down by age alone is rarely safe.
mg/kg/dose vs mg/kg/day: know which one you're using
This is the most common source of pediatric dosing errors. mg/kg/dose gives you the amount for each individual administration. mg/kg/day gives you the total daily amount, which you then divide by the number of doses per day.
Amoxicillin for otitis media is often prescribed at 40mg/kg/day divided 3 times daily. A 20kg child needs 800mg/day total, so roughly 267mg per dose. If you accidentally treat 40mg/kg/dose as the single dose, you've just tripled the intended amount. Always clarify with the prescribing reference which convention is being used.
The dose cap rule
Always cap the calculated dose at the adult maximum. A heavy 12-year-old at 60kg prescribed amoxicillin at 40mg/kg/day would calculate out at 2400mg/day. But the adult maximum is 1500mg/day. You use 1500mg, not 2400mg. This rule applies across the board.
Age-based rules: Young's and Clark's
These formulas (Young's uses age, Clark's uses weight in pounds) give rough estimates when mg/kg references aren't available. They're fine for community settings with OTC medications. For hospital settings or narrow therapeutic index drugs, use a validated pediatric dosing reference and weight-based calculation every time.
Liquid formulations and measuring accurately
Most pediatric doses come as suspensions. Enter the available concentration below to get the volume to give. A 125mg/5mL suspension delivers 25mg per mL. If your calculated dose is 150mg, that's 6mL. Give it with an oral syringe, not a kitchen spoon. Teaspoon measurements vary by up to 40% between spoons. For patients who also need a drug interaction check before administering, use our Drug Interaction Checker. Renal impairment in children also affects dosing - see the Renal Dose Adjustment Calculator for guidance.