Drug dosage is the amount of a drug or medicinal substance, which needs to be administered to a patient in the form of a single dose or in the form of multiple doses, in order to achieve desirable therapeutic effect. Determining the adequate dose for a child represents a challenge, since the drug pharmacokinetics are different in the bodies of children, compared to those in adults, therefore children usually require lower doses of medication compared to adults. There are numerous formulas to calculate the appropriate drug dose for children, based on the child's age, weight, etc. The aim of this project is to design a computer program that will calculate the drug dosage for children quicker, and requiring less effort. Using the programming language C#, a user interface was developed. The application enables calculating the dose for children based on the adult dosage and the age or weight of a child requiring the medication, and it also regards children under the age of one as a special group and enables the calculation of the dose for those children specifically. The application simplifies the work of a pharmacist in a drugstore in terms of dispensing medication for children and it can be also used by paediatricians while prescribing drugs for children.