How Weight-Based Dosing Is Calculated
Weight-based dosing scales a dose to body mass instead of using one fixed amount for every patient. It is the standard approach in pediatrics, critical care, oncology and anesthesia, where the difference between a 6 kg infant and a 90 kg adult is far too large for a single dose to be appropriate.
Once the total dose is known, the volume to administer depends on the concentration of the preparation you are holding:
Those two steps are independent, and separating them is what prevents most arithmetic errors. Calculate the dose first, in milligrams. Only then convert to volume.
Worked Example
A 24 kg child, a drug ordered at 15 mg/kg, supplied as 250 mg per 5 mL:
- Total dose = 15 mg/kg × 24 kg = 360 mg
- Concentration = 250 mg ÷ 5 mL = 50 mg/mL
- Volume = 360 mg ÷ 50 mg/mL = 7.2 mL
Note that the concentration step is where the supplied preparation matters. The same 360 mg order becomes 3.6 mL if the drug is supplied at 100 mg/mL, and 14.4 mL at 25 mg/mL. The dose is identical; only the volume changes.
Pounds, Kilograms and the Most Common Error
Dosing references are written in mg/kg, but body weight is frequently recorded in pounds. Converting in the wrong direction produces a 2.2-fold error, which is the single most common weight-based dosing mistake.
A useful sanity check: the kilogram figure is always the smaller number. If your converted weight is larger than the pound figure, you multiplied when you should have divided.
Dose Caps
Many weight-based orders carry a maximum dose that applies regardless of body weight. Above a certain mass the linear mg/kg relationship stops holding, so the calculated figure is checked against the cap and the lower of the two is used. This calculator performs the arithmetic only and does not apply caps — those come from the prescribing reference for the specific drug.