In this paper, a new method is suggested for calculating the absorption amount of drug. The method is based on the general linear compartment model for pharmacokinetics, hence applicable to any proper linear compartment model for pharmacokinetics. For the linear one compartment model, it is in agreement with the Wagner-Nelson method which is often used nowadays. For the linear two compartment model, it has advantages over the Loo-Riegelman method, which is also in common use in, for example, simplicity in mathematical derivation, less computation, etc. And it can be easily extended to some particular nonlinear compartment model.