The present invention discloses a computer program and a method including an algorithm for computing the uncertainty of drug concentration in pharmaceutical dosage forms. The invention also provides a computer system, a website and an IC chip incorporating the program product. The invention determines the allowable sensitivity of the instruments measuring each of the components and the least allowable weight (LAW) within a maximum allowable error of each ingredient at the predetermined assay standard. The method and products allow the accurate error evaluation of drug dose in pharmaceutical mixtures and unit dosage forms, which may be composed of an infinite number of ingredients that are measured on multiple equipments of variable sensitivities. Thus this invention provides instant guidance on the pharmaceutical ingredient preparation.