https://www.compassprep.com/psat-national-merit-faq/
The Selection Index is double the sum of your Reading and Writing (RW) score, and Math score divided by 10. Alternatively, you can simply drop the last zero on your section scores, double the RW and add the Math. For example, a student with scores of 690 RW and 720 M would have a Selection Index of 69 x 2 + 72 = 210. You cannot directly calculate a Selection Index from a Total Score (320 – 1520). For students entering the competition with an SAT score through Alternate Entry, note that — when calculating a Selection Index — each SAT section is capped at 760. If, for example, you have a 700 RW and 800 Math, your Selection Index would be 70 x2 + 76 = 216.
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