An eccentric reducer calculator device is provided. The device is comprised of a planar sheet body having a plurality of concentric circle markings of progressively increasing diameter, corresponding to standard and non-standard pipe outer diameters. Extending from the central axis are horizontal centerline markings that reach opposite edges of the body, optionally labeled with numerical indicators, and in some embodiments vertical centerline markings that are orthogonally aligned for complete alignment referencing. The device functions by placing the relevant piping component in alignment with the concentric circle markings and centerlines, thereby enabling direct marking of the correct centerline for cutting, welding, or installation operations.