GradeDeflation.com is site where Princeton students can get a rough idea of what their GPA would have been if they were enrolled at Harvard.
The results are not meant to be factual. They are simply an estimate based on a limited array of known data points.
It is calculated based on aligning GPA's between the two universities by percentile. So the median GPA at Princeton will return the median GPA at Harvard, the 90th percentile GPA at Princeton will return the 90th percentile GPA at Harvard, etc.
GPA's below 2.75 and above 3.97 are difficult to find an equivalent for, so if you fall in one of these ranges your results might not be that accurate.