Poker Strategy: Choosing our bet sizes preflop - open raise and isolation

Poker Strategy | estimated reading time
2024. June 10.
Today, we'll start a mini-series discussing a fundamental part of the game: preflop bet and raise sizes.
bet-sizes

We'll enumerate all common spots, starting with the open raise and isolation sizes and the reasoning behind them. Later, we'll see 3-bet and 4-bet spots (part 2) and adjustments to unconventional spots (part 3), like different stack depths and exploits.

Please bear in mind that our basic setup for these spots is 100bb effective stacks and a GTO approach. You'll be notified when the setup changes.

Open size by position

Why do regulars open specific sizings? Or rather, the question is, why do solvers suggest those common sizings we see daily? The answer is mathematical and somewhat logical at the same time.

As you already know, the action is provided by introducing blind bets. Without blinds, typically, you wouldn't be incentivized to put money into the pot unless you have aces. This statement probably oversimplifies the question, but you get the point. These blind bets incentivize the players to fight for the pot, depending on their position, cards, the rake, etc. It's straightforward to see a correlation between the bet size we use and the frequency (in this case, the width of the range) we are mathematically "allowed" to use. Solvers introduce bet sizes based on risk-reward ratio, meaning to risk the least possible but forcing certain parts of the others' range out of the pot.

Therefore, the most profitable or theoretically approved opening sizes will be the following:

PositionSize
UTG2bb
MP2bb
CO2,3bb
BTN2,5bb
SB3bb

Can you or should you differ from these sizes? The answer is you could, and it all depends on you. What you should always take into account when you raise your preflop sizes is that you need to decrease the opening range, and the contrary is also true. When you use small sizes, you can bet a bit wider.

Should you adjust your preflop sizes? As a beginner, probably not. It's more advisable to follow a solver-based preflop chart and differ from it when you already know what you are doing. Because the best answer is to play always exploitatively if the blinds are indifferent to your sizings, you can get away with min opens; if the BB is not 3-betting enough and calling the same frequency, you could go smaller, etc. You might simplify your strategy to use 2,2bb from UTG-CO and 2,5bb from BB or something like that. It all depends on you. Just remember the consequences sizes imply.

Isolation size

How about spots when someone posted a blind out of their turn just to get dealt immediately or a recreational limps? Or what if a regular uses a mixed strategy (limping and opening) from SB?

We should distinguish the spots by position and also consider who the limper (regular or recreational) is. Usually, when we are in position to the limper, a 3,5bb is advised as an isolation size. When out of position, we tend to mitigate our positional disadvantage by lowering the EV of calling. Therefore, we use a 5bb iso size or even bigger. You may add at least 1bb for each overlimper to your original sizing. For example, if someone limped from UTG and the CO overlimped, you should isolate at least 4,5bb from the BTN.

When facing an SB limp as the BB, we are in position, so usually, we should use a 3,5bb size as an isolation. Some simulations suggest multiple sizings. However, we recommend sticking to one for simplification. Since it's a theoretical mistake to limp from the SB in a high-rake environment (almost on any site, except high stakes) and due to its complexity, only a few regulars imply this strategy. When they do, they probably commit far bigger mistakes than the EV gain of using multiple iso sizes.

Last, we need to talk about limps from weak, recreational players. Normally, you want to attack them pretty aggressively and size-wise, so you should go as big as possible. There is a common agreement that the bigger their VPIP, the bigger you want to go because it's more likely they are sticky with their holdings preflop. This means that if you play against someone who plays 35/10/1, you might iso 3,5bb in position, but if a recreational plays 60/10/6, you might go as big as 6bb in position. Another strategy you might apply is increasing your iso size by 1bb each time you have the chance. This way, you can test out Villain's limits and how much she is willing to invest preflop. Sometimes, you'll be amazed that some players are ready to put 15-25bb preflop as limp/call, dramatically boosting your winrate compared to your conservative 3,5bb iso in position.

Summary

We've discussed that the main incentivizer of opening is the blind bets. As solvers and common sense suggest, we would like to win (steal) the most by risking the least possible in the process. Therefore, our open sizes will vary between 2-3bb, respectively. Another topic we've examined is isolation when another player limps or posts blinds out of their turn. We must distinguish SB limping strategy and open limps from recreationals since they imply very different ranges. We can generally establish that our isolation size will start from 3,5bb to any size we wish. Positions and Villain's player type are the most essential factors in deciding the sizing.

The following article will discuss 3-bet and 4-bet pots' preflop sizings.

Stay tuned!

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