Hi All, I got a dumb question (not exactly a math person). I know I can get the in-the-money call and put implied volatility on the IVolatility.com website. My question is how do I calculate it by hand to get the same numbers? Thank you for all your help. Regards, William

Thanks Moreagr and gummy. Hi Peter, I found your site while researching on Bollinger Band... a great great site. Thank you for the spreadsheet! Too bad my math brain took the worse turn after high school or college... I plan to reverse the process as my son grows up... hope it's not too late Regards, William

At the money, you can approximate it as Implied Vol = P / (0.4 F Sqrt(t) ) where P is price of option, F is forward price (for short dated stuff it's ok to use spot price), t is time to expiry in years.

It's the forward price, which is spot price times (1 + risk free rate). Now, you should remember that this approximation would be a bit off because of discounting and the fact that this is a first order approximation. In general, it' would be off by less then half of BS vol, though, so it's a reasonable way to check.