In this paper, we propose a simple and analytical method to design a multi-band tunable graphene-based absorber using circuit model. The presented paper provides a flowchart and equations to design a multi-band absorber which tremendously simplifies the design procedure of a multi-band absorber. This paper only investigates the disk-shaped graphene-based absorber, but we believe the proposed method is extendable to all presented shapes of graphene which previously designers provided with a circuit model. To verify the accuracy of the presented method; we have designed several multi-band absorbers and used full-wave numerical modeling of the structure to approve our method. Also, this paper studies the reliability of our proposed model by investigating the fabrication sensitivity of the structure. Lastly, this paper investigates to see how a multi-band absorption spectra behave, in case there are any errors during the fabrication phase. The designed absorbers have potential applications in various fields of active THz metamaterials, tunable filters, and sensors. The graphene-based disk-shaped absorber has the advantage of polarization insensitivity, wide-angle absorption and ease of implementation due to its simple shape. Another advantage of using graphene in the absorber structure is that with the availability of changing the fermi energy level, it is possible to tune the absorption frequency of the multi-band absorbers.