How is the National Merit Selection Index calculated?

The formula, with an example

Let $R$ be Reading and Writing and $M$ be Math. Then $SI=(2R+M)/10$. For $R=720$ and $M=710$, compute $2(720)=1440$, then $1440+710=2150$, then $2150/10=215$.

Use section scores, not your total score or a percentile. The formula here is for PSAT/NMSQT screening; do not enter SAT section scores and call the result your qualifying PSAT index.

Why Reading and Writing matters twice

A ten-point Reading and Writing increase adds two index points. A ten-point Math increase adds one. If two students have the same total, the one with the higher Reading and Writing score has the higher index.

That weighting does not change the SAT total-score formula. SAT section scores still add once each. It also does not remove Math from National Merit preparation; your next practice priority depends on what you can actually learn.

Compare against the right cutoff

Use the index shown on your score report as the official reference. Then check the cutoffs by graduation class and state and your program eligibility. Reaching a listed number is not the same as receiving a scholarship.