This is literally the most important concept I have learned from BTP - implied odds/stack sizes when playing small-mid pairs.

I believe the rule is that the opponenet needs to have 10x the original bet behind him for the call to be warranted.

The rule of thumb that I use (and I'm pretty sure I stole it from Nash) is if the bet is 5% or less than my stack and the raiser's stack then it is an automatic call. If it is more than 10% of either stack then it is an automatic fold. In between is a typical "it depends" poker decision. I like to see more than 10X in the raiser's stack.

I think I remember Iceman saying he has raised that to 15X in the raiser's stack but I'll let him speak for himself.

What I have learned is to not call when the raiser has a short stack. There just isn't enough to win.

Hope this helps some.

I know this is pretty obvious but it's worth reiterating, the opponents stack size is moot unless you have him covered, if not, then it's in relation to the size of your stack (the maximum amount more that you could win)

...one other thing too, it depends on your position relative to the raiser too. If a loose raiser pops it from UTG and you are immediately to his right then you should probably fold with so many people yet to act behind. There is a decent chance that someone will reraise and then you will have to fold. If you are last to act then you can be a little more liberal with the call. I think Ciaffone/Reuben talks about this concept. My 2 cents anyway.