Although these are arguably the most important odds (as the flop is commonly seen as the most important street in Hold'em), playing profitable poker means making the correct decisions on all streets. A brilliant call on the flop is completely negated by misplaying the turn.
For these players who have merely memorized the flop numbers, they common system they use for the turn is simply cutting the flop numbers in half. Although this will give you an idea of where to start from, it's not exactly a perfect solution.
Pot odds can be a little bit tricky. Almost everyone knows that a flopped flush draw is about 2-1 to hit, meaning they need better than 2-1 odds to play the hand.
I commonly see players making the mistake of using the same equation for the turn. Let's run a hypothetical scenario:
I hear Liv Boeree is impressed by strong odds comprehension.
You're in a $2/$5 game on the button with $500 in your stack, holding A♦ K♦. Middle position makes a standard raise of $25. You call after it's folded to you; the blinds muck their hands, leading you heads-up to the flop.
The flop comes 2♦ T♦ 4♣ - you've flopped the nut-flush draw. For some reason, poker players seem to ignore odd dead money when calculating the pot size, meaning your opponent ignores the blinds, and views the pot as $50. They bet $50.
You need to call $50 - you're getting just barely over 2-1 on your money. On straight pot odds you're pretty much breaking even here, but if you hit on the turn you feel you can make a few more bets, plus you might be good if you can hit an ace. You call.
Turn: [2♦ T♦ 4♣] [8♥]
The pot is now starting to get large. You have $425 in your stack, making the pot over one-third of your stack. This is turning into a sizable pot for the table, with two streets of betting left to go.
Your opponent thinks you might be on the draw, or you have a set and are trapping. In case you're on the draw, they don't want to let you draw for free: they bet $100.
Now from your experience playing with this player, you now know that they have AA or KK. But since you don't know which one, you can only count on hitting your flush for the win. You have to call $100 to see a river. What should you do?
Most beginner players here make an instant call. The bet is less than the pot, and the pot is going to be nice and large for them if you hit their nut draw.
You're getting 2.57-1 on your money, even better odds than you got on the flop, and it was an easy call on the flop. Seems like it should be an easy call on the turn as well.
In reality, you're 20% to win the pot here (assuming you're up against AA). You're a 4-1 dog, getting 2.5-1 on your money.
Your pot odds here are horrific. Your opponent would have had to bet around $50 for you to make a profitable call here.
An Interesting Brain Fart
The concept of drawing odds on the flop used to mess with my brain when I was a beginner. I used to wonder "How can it be correct to call with +2-1 odds on the flop when you know they will bet the turn again?"
When you think about it, in reality you're only seeing one card, not two, before you encounter another bet.
Your odds for hitting the flush on the turn specifically are about 20%. I figured it made more sense to need 4-1 odds to make a call even on a flop.
My beginner brainwaves are actually logically sound - in reality you are only seeing one card. The reason you can safely call getting better than 2-1 odds on the flop has to do with a whole bunch of factors beyond simple pot odds.
You have to consider implied odds, setups and live-pair draws. Most importantly, you have to consider how often the flop bet is a bluff, semi-bluff or c-bet.
In the majority of these situations, you are going to see a river without a bet on the turn, allowing you to draw two cards for your call. On top of that, you'll actually be ahead of many of these bluffs.
The concept behind this reasoning is this: You do not know what will happen on later streets, you must make the correct choice for the current action with the information you have.
It is simply not possible to play profitable poker on assumptions and hopes.
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