How the Weight-Based Dosage Formula Works
Weight-based medication dosing scales the amount of drug given to a patient's body weight, expressed in milligrams of drug per kilogram of body weight (mg/kg). This is the standard convention used across pediatrics and adult medicine for drugs where the effective and safe dose depends on body size. The core formula is Dose (mg) = Weight (kg) × prescribed dose rate (mg/kg). This calculator is an educational tool for checking that arithmetic — it does not know the correct mg/kg rate for any specific drug or patient, so the dose rate, frequency, and maximum dose must always come from a prescriber, pharmacist, or the medication's official labeling, never from a guess.
From weight and mg/kg to a real dose
First, weight is converted to kilograms if entered in pounds, using 1 lb = 0.453592 kg (or kg = lb ÷ 2.2046). The per-administration dose is then weight (kg) multiplied by the prescribed mg/kg rate. If a maximum single dose is entered, the calculator caps the result at that ceiling, because many drugs have a flat maximum dose that applies regardless of how large the weight-based calculation is for a bigger patient. The total daily dose is the (possibly capped) per-dose amount multiplied by the number of doses per day, and the suggested dosing interval is simply 24 hours divided by the number of doses per day.
Common mistakes and safety notes
- Using pounds instead of kilograms: mg/kg rates are defined in kilograms. Forgetting to convert from pounds inflates the calculated dose by roughly 2.2 times.
- Confusing mg/kg/dose with mg/kg/day: a rate meant for the total daily amount should not be given at every single dose — divide it by the number of doses per day first, or the patient will receive several times the intended daily total.
- Ignoring the maximum dose: weight-based math can exceed a drug's labeled adult maximum for larger patients; always apply the lower of the calculated dose and the labeled maximum.
- This is not a prescription: results are an educational arithmetic check on a mg/kg convention you provide. Always verify the dose rate, frequency, and maximum against the prescriber's order, a pharmacist, or the drug's official prescribing information before administering any medication, and never use this tool to dose a patient without professional guidance.