A supporting shell for a seat is provided by carrying out in-seat measurements with a plurality of test subjects, wherein the test subjects sit down on a measuring mat placed on a neutral seat frame. The measuring mat has a readily displaceable filling and adapts to the contour of the body of the respective test subject. After the test subject has sat down on the measuring mat and before the test subject leaves the same, the filling inside the measuring mat is fixed in place by applying a negative pressure. The geometric data of the surface contour of the measuring mat are determined and stored for each test subject. The measured surface contours are then superimposed onto one another. This produces an average surface contour which can subsequently be further altered slightly for adaptation to large seat users, and therefore a modified surface contour is created as a basis for the production of the supporting shell of the seat.