PURPOSE: To know the forming allowance from a distance up to a limit line by setting up the elongation and plate thickness strain of a blank on one and the other axes of a plane coordinate system, entering forming limit lines, drawing a chart, plotting the thickness strain of a press forming product, and converting the thickness strain into elongation.

CONSTITUTION: Limit lines such as a surface roughening limit line and a deformation limit line are entered into the plane coordinate system drawn by setting up the elongation Δl/l of the blank on one axis and the plate thickness strain Δt/t on the other axis to draw the chart. The feature of extension in a press forming product deformed state graphic is separated into parts (a) to (e) corresponding to forming portions. The plate thickness strain of the portion (e) is found out and plotted on the range (e) of the graphic, the reading of the vertical line is set up as A and a deformation route forecasting quadratic curve passing the plotted point and the origin is found out. The obtained quadratic curve is extended, the reading B of the vertical axis on an intersecting point between the extended curve and the forming limit line is found out and a difference (a distance between the reading values AB) between both the reading values A, B is calculated. The difference indicates the forming allowance on the measuring portion (e) of the forming product. Thus, said constitution makes it possible to simplify the deciding work of the forming allowance.