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The offset position and the expanded width, with which the lightning equivalent area is calculated, are analyzed by establishing the related model and mathematical formula, according to the logical relationship of equivalent area calculation, taking the slopeshaped building and domeshaped building as examples. For the single eaves, the double eaves of ancient buildings or ancient towers, the offset positions should be judged from the relationship of the horizontal distance（〖WTBX〗L〖WTBZ〗）and the vertical distance （〖WTBX〗H〖WTBZ〗） between the eaves and roof (top). For the domeshaped or ellipticalshaped roof buildings，the mathematical formulas from the outside to the central axis are established by using the formula of the roof model. Then the derivatives of the formulas are evaluated. The coordinates with the derivatives being zero (i.e., offset point positions) and the corresponding expanded widths are obtained. The equivalent areas are gained by the means of offsetting and area calculation with AutoCAD software, which provides reference for the accurate calculation of equivalent areas in lightning protection design and disaster risk assessment.