I had dry humping. I ejaculated on her underwear. I was naked. She got postinor shortly after. How much is risk of pregnancy?

Unlikely. If you are positive that no semen passed through her underwear (and she was wearing it all times during the episode, and Postinor was taking within 120 hours, the chance is small, but not zero.
Extraordinarily low. Clothing (such as underwear) serves as a barrier to the sperm. The postinor is an emergency contraceptive. Take care.

Related Questions

I had dry humping and I ejaculated on her underwear. I was naked. She got postinor shortly after. How much is risk of pregnancy?

Absolutely. Anytime sperm have access to the vagina their is a possibility of pregnancy. It is less likely in this scenario your underwire is not an effective barrier to sperm. If you don't want to become pregnant some form of birth control must be used every time . Read more...