The ISO 26262 is a functional safety standard for road vehicles. The standard requires manufacturers to conduct quantitative assessment of the diagnostic coverage (DC) of products. The DC is defined as the percentage of failure probability covered by safety mechanisms. However, DC evaluation methods for drift faults, in which the change in element values is not constant, have not been discussed. In this paper, we propose a DC evaluation method for analog circuits with drift faults. With this method, we first parameterize the effect of drift faults onto a bounded region then split the region into safe fault, hazardous detectable fault, and hazardous undetectable fault regions. We evaluate the classification rate distribution by the area ratios of these regions. Afterward, we evaluate the hazardous failure probability by multiplication of the failure probability of a hardware element, the fault rate distribution of fault models, and classification rate distribution of the hazardous undetectable fault. The DC of a functional block is given as the sum of the failure probabilities of hardware elements and that of the hazardous failure probabilities.We developed a DC evaluation toolkit to reduce the workload of DC evaluation. This toolkit takes into account a circuit design and fault models for each hardware element and automatically determines the DC under specified operating conditions. The overall productivity gain from using this toolkit is more than 10 times that of manual evaluation, based on a comparison of the evaluation time. This toolkit removes overhead workload and aids in quantitative DC evaluation.