Background: The Psoriasis Area and Severity Index (PASI) is the score of choice to grade psoriasis severity and detect clinical changes. Due to low resolution based on the calculation of the score by fixed area classes, PASI scores <10 have little value. Methods: At 756 patient examinations, psoriasis activity was measured with both PASI and PrecisePASI. Results: PrecisePASI has a linear increase while PASI has a staircase pattern. Both scores meet at the endpoint-relevant values of body surface area (BSA) 10, 30, 50, 70 and 90%. PASI and PrecisePASI correlate significantly over the whole range of BSA. In the region of BSA <5%, PrecisePASI shows a significantly higher resolution (p < 0.0001). Conclusion: The calculation of PrecisePASI corrects the undesired inaccuracies of PASI in the lower BSA ranges and is a tool to use as an endpoint in trials aiming to detect differences in the lower ranges of BSA.