The answer is not completly straight forward. Normally you need to be 100 pounds overweight, with a BMI of 40 or above. But........if you have comorbitities (it's too early in the am, I think that's how you spell it.) Such as diabetes, sleep apnea, gerd, anything that can be weight related, if you have things like that your BMI can be lower.
Here is a web site that has miles and miles of information about wls. www.obesityhelp.com
you can spend hours on this web site going through the information that is compiled there. Lots of before and after pictures too.