After reading 101 Options Trading Secrets and The Option Trader's Guide to Probability, Volatility and Timing, I've come to learn about the use of options probability calculation. So I went in search of free options probability calculator available online. Most of the trading tools that I use are free of charge, as I believe that fixed cost for trading should be kept to minimal. So here's a few places where you can find a free options probability calculator:
Tradersoft :This one has to be downloaded and can only be used for limited period after which you have to register. The good thing is since it is installed in PC, can be used offline.
OptionVeResearch: This is the one I'm currently using for its ease of use and simplicity.
Here's how you can use it. For example you are bearish and want to do a Feb 07 bear call spread on SPX, say sell 1470, buy 1480. By keying in the necessary info, you can see that the probability of your sold leg being safe at expiration is 85.5%. We can also tell that for SPX to stay within one standard deviation boundary, it has to be contained between 1408-1468.