The essays are graded by two readers on a holistic scale ranging from 0-6, with 6 being the highest possible score. In holistic grading, the essay is evaluated as a whole while considering the major elements of content, structure, and stances. If the grades awarded by the two readers differ by more than one point, the essay is then graded by a third reader.
The final score is calculated by taking the average of the four grades and rounding it off to the nearest half-point. You must have a look at the scoring guide before taking the GRE as it will give you an insight on how the essays are graded. Go through the ‘Reader’s Commentary’ thoroughly to understand what the readers expect in these two essay