The conventional area-balancing method uses a single regional slope, commonly parallel to basal detachment and generally horizontal, to restore a deformed geological section to an initial rectangular shape. However, most fold-thrust belts exhibit a wedge or trapezium (i.e., a quadrilateral with at least one pair of parallel sides) shape in cross section with stratigraphic sequence thinning into the basin. Two new, simple, quantitative solutions are developed to restore a thrust belt with more realistic geometries, which assume that the initial shape is a trapezium. This allows exploration of the function of initial wedge taper (represented by surface slope and basal dip) in controlling shortening estimate using area balancing. The new methods are tested against sandbox models and natural examples of fold-thrust belts using a range of input parameters to evaluate the uncertainties in shortening calculation. We find the results are very sensitive to both initial surface slope and initial basal slope. This study highlights the significance of initial wedge taper in determining shortening across a fold-thrust belt, with accurate shortening estimates requiring independent constraint of this important geometry. Previous studies that used area balancing to predict depth to detachment and/or to estimate shortening without considering the effects of initial wedge taper may need reevaluation.