I have two questions:

One: I know that you need better pot odds than hand odds to be profitable in the long run. Let's say I flop a flush draw and getting the flush is my only realistic chance of winning. The odds of completing on the next card is around 4.5 to 1 so you need pot odds of better than 4.5 to be profitable. What about when the pot is small? Let's say you faced a 2.5bb raise preflop and a called so 2.5 + 2.5 + 1.5 blinds so a pot of 6.5bb post-flop (everyone else folded). An opponents raise of 2.5bb (therefore 9bb in the pot) would give you pot odds of 3.6 to 1 and therefore would not be profitable. Is this still how it works as it seems very nitty to me to fold a flush draw like this, or is it different when the pot is small and even a small raise makes up a big percentage of the pot.

Two: How do you figure out the % of times you need to be right facing a river bet in order to be profitable? Let's say I'm facing a river bet of 20bb and the pot is now 46bb, so I have odds of around 2.25 to 1. Does this mean I only need to be right once every 2.25 times as this seems very small given it's such a big bet?

Thanks so much.

1) this is the magic of implied odds
2) well, you need to be right about one in three times. it is not a big bet given the pot size.

sizfour is spot on about implied odds. To clarify the concept: if you risk $1 to chase your flush draw you must get paid $4.50 when you hit to break even. However, it doesn’t matter if that $4.50 already is in the pot when you hit or if it goes in AFTER you hit. If you have good reason to think your opponent will pay off your flush, you can profitably call even if the current pot odds aren’t favorable.

Be careful with this idea, though. Don’t just say “implied odds” and call any old crappy draws in any situation. If effective stacks are short, for example, you may not even theoretically be able to get enough payoff to make the call profitable. If you have an opponent opening light, cbet bluffing often and folding often to turn action, you may not get paid off often enough to count on implied odds. If you have a really crappy draw, like a gutshot, even implied odds may not be sufficient to justify a call.

They're betting 2.5 into a flop pot of 6.5bb which means you need 21.39% equity to call. You should have direct odds to call a flush draw even without considering implied odds.

You're either calculating equity wrong, or you're converting your odds to percentages incorrectly.

a) required equity = (call)/(pot after you call) = 21%
b) odds to percentage: 1:3.6 => 1 / (3.6 + 1) ~= 21%

---+

The more complicated answer is that the raw equity of a hand is not the value of a hand.

EV considers the times you call and fold next street, the times you hit that draw and stack them, the times you hit your draw and get stacked anyway, the times you hit and get called on the turn but don't extract river, the times you call flop and turn then fold river, and everything in between.

When you read "I need 21% equity" what you should be thinking is "I need to win 21% of the value of the pot to breakeven".