formula of probability of option being expired

Hi,

I heard we can use the delta to find the probability of option being expired. Anyone knows the exact formula? Thanks.

 

the lower the delta the less likely it will finish in the money...

does this help...

 

I knew that. I want the exact formula.

 

http://www.elitetrader.com/vb/showthread.php?s=&threadid=44490&pagenumber=2

Regards

 

nothing is exact in options.

 

Do not need to waste your brain. You can check out each option"s probability in thinkorswim's software for free.

 

I know some free software. I'm just curious why they are different.

In some other software, you just need to enter S, X, IV, T. Other software requires you to have expected return u as well.

Which one should I trust?