What Is a Weight-Based Dosage Calculator?
Many medications—especially in pediatrics, oncology, and critical care—are prescribed according to a patient's body weight rather than a fixed amount. This calculator multiplies a dose per kilogram (mg/kg) by the patient's weight in kilograms to determine the total daily dose, then optionally divides it across the number of administrations per day. This is an educational tool only and is not a substitute for professional medical judgment. Always confirm dosing against the prescribing information and your clinician or pharmacist.
How to Use It
Enter three values: the patient's weight in kilograms, the prescribed dose in milligrams per kilogram (mg/kg), and how many doses are given per day. The calculator returns the total daily dose and the amount for each individual administration.
The Formula Explained
The core relationship is simple: \(\text{Dose (mg)} = \text{Dose per kg (mg/kg)} \times \text{Weight (kg)}\). The dose per kg is the prescriber's chosen intensity of therapy. Multiplying by weight personalizes the amount to the individual. If a drug is split into several daily administrations, the per-dose amount equals the total daily dose divided by the number of doses.
$$\begin{gathered} \text{Dose per Administration} = \frac{D_{\text{daily}}}{\text{Doses/Day}} \\[1.5em] \text{where}\quad \left\{ \begin{aligned} D_{\text{daily}} &= \text{Dose (mg/kg)} \times \text{Weight (kg)} \end{aligned} \right. \end{gathered}$$
Worked Example
A child weighs 20 kg and is prescribed amoxicillin at 25 mg/kg/day divided into 3 doses. Total daily dose = \(25 \times 20 = \textbf{500 mg/day}\). Per dose = \(500 \div 3 \approx \textbf{166.67 mg}\) given three times daily.
FAQ
Do I use total body weight or ideal body weight? It depends on the drug. Some medications use ideal or adjusted body weight, especially in obesity. Follow the specific drug reference.
What if weight is in pounds? Convert to kilograms first by dividing pounds by 2.205, then enter the kilogram value.
Why divide into multiple doses? Splitting maintains steadier drug levels and can reduce side effects, but the prescriber sets the frequency.
