Originally Posted by ypsieast

Some clarification:

Probability (p) = % chance you will get your card
Quick approx. calculation is 2x outs + 2 for 1 card; 4x outs for 2 cards
If you have 9 outs, your (p) of hitting an out on 1 card = 20% and 36% for 2 cards

Odds (o) = (1-p)/p
If p = 20% then o = (1-.20)/.20 = 4 to 1

Pot Odds = ($ of Pot)/($ to call)
If the pot is 500 and it costs you 100 to call then your pot odds = 5 to 1
Pot odds are favorable when greater than the odds to hit your out
If your odds to hit your hand are 4 to 1 and the pot odds are 5 to 1, then the situation offers favorable pot odds and you should usually call.
The problem is that if you calculate you (p) and in turn your (o) based on 2 cards to come, you are ignoring the fact that you may have to call another bet on the turn. If calling would put you all in, you would use the formula [outs x4] to calculate your (p).

Implied Odds = Pot Odds adjusted for the likelihood of winning additional bets, should you hit your outs.
There isn't a standard formula for implied odds. You just think e.g., "OK, if I complete my OESD, I can double up through this guy; even though my odds are 5.25 to one to complete my str8 and the pot odds are only 4 to 1, I'm going to call. Because if I hit my card, I will be paid 10 times this bet on the turn.

Breakeven Percentage = % times you need to win in that situation to break even win/loss over the long run.
Calculation: ($ to call)/($ in pot + $ to call)
Example: Pot is 800 and you have to call 200 to see the river. Breakeven %= 200/(800+200) = 20% This means that if you called a million times in this situation, you would break even in the long run if you had a 20% chance of winning the hand.
If your Breakeven % is less than your (p), then it is +EV to call.