Many universities take only the math&reading in consideration.
My question is: How do they calculate it? Do they convert it to the old SAT scores?
There is nothing to calculate. The reading and writing are combined into the Evidence-Based Reading and Writing score out of a possible 800 total. Together, EBRW and math can be combined for a composite score out of a possible 1600 total. If a college refers to the “reading” or “verbal” score, they would be referring to EBRW. No conversion to the old SAT is needed; almost all applicants will have New SAT scores and indeed, some colleges no longer even accept scores from the old version.
The SAT essay is on an entirely different scale out of 24 points and cannot be part of the composite. Do not confuse the essay with the multiple-choice writing section that is part of EBRW. Unfortunately, some college websites refer to “writing” when they really intend to refer to the essay.