Skin prick tests (SPT) represent the standard method for the diagnosis of type-1 allergies. The skin wheals provoked by the SPTs are considered positive above certain cut-off diameters, usually 3 mm. At present their size is mostly estimated by measuring the diameter with rulers. Since the shape of wheals usually is not circular, the measurement of diameters leads to imprecise results. Therefore, we developed an algorithm for precisely measuring the wheals' area in mm² automatically. The average deviation of the automatic measurement on a test set achieved with the developed algorithm was 6.9 %. Compared to the maximum standard deviation of 6.59 % when measuring the manually redrawn outlines, the automatic method works sufficiently well.