What is the Drug Dosage Calculator?
This calculator works out a weight-based medication dose. Many drugs, especially in pediatrics, are prescribed in milligrams per kilogram of body weight (mg/kg). Given the patient weight and the prescribed dose per kilogram, it returns the dose per administration, the total daily dose, and — if you enter a liquid concentration — the volume in millilitres to measure for each dose. It is an educational tool and does not replace clinical judgement or a prescriber.
How to use it
Enter the patient weight in kilograms, the dose in mg per kg, the number of doses per day, and the concentration of the liquid preparation in mg per mL (leave at 0 if using tablets). The calculator multiplies weight by the dose to get a single dose, multiplies by frequency for the daily total, and divides by concentration to convert milligrams into millilitres.
The formula explained
The core relationship is \(\text{Dose} = \text{Weight} \times \text{Dose per kg}\). To find the liquid volume, divide the dose in milligrams by the concentration in mg/mL: \(\text{Volume} = \text{Dose} \div \text{Concentration}\). The daily figures simply multiply the single dose by the number of doses given each day.
$$\begin{gathered} \text{Dose}_{\text{admin}} = \text{Weight (kg)} \times \text{Dose (mg/kg)} \\[1.5em] \text{where}\quad \left\{ \begin{aligned} \text{Daily Dose} &= \text{Dose}_{\text{admin}} \times \text{Doses/day} \\ \text{Vol}_{\text{admin}} &= \frac{\text{Dose}_{\text{admin}}}{\text{Conc. (mg/mL)}} \\ \text{Daily Vol} &= \frac{\text{Daily Dose}}{\text{Conc. (mg/mL)}} \end{aligned} \right. \end{gathered}$$
Worked example
A child weighs 20 kg and is prescribed 10 mg/kg three times a day, using a syrup at 100 mg/mL. Dose per administration = $$20 \times 10 = 200 \text{ mg}$$ Daily dose = $$200 \times 3 = 600 \text{ mg/day}$$ Volume per dose = $$200 \div 100 = 2 \text{ mL}$$ and daily volume = $$600 \div 100 = 6 \text{ mL/day}$$
FAQ
Can I use it for tablets? Yes — set concentration to 0 and read only the milligram results, then choose tablet strengths accordingly.
Is the result the maximum safe dose? No. It only applies the mg/kg figure you enter; always check the maximum dose and contraindications in an approved reference.
What units does it expect? Weight in kilograms, dose in mg/kg, concentration in mg/mL. Volume results are in millilitres.
