Stack depth forms a mathematical constraint on the decisions which can be made in a hand of poker. After all, you can’t bet $500 if you only have $100, and thus, our entire strategic framework changes if the stack sizes involved in the hand change. The shortest of the stacks remaining in a hand is referred to as the effective stack.

SPR stands for ‘Stack-to-Pot Ratio’, and refers to the relationship between the number of chips in the pot, and the number of chips in your stack. It is almost always used specifically in postflop scenarios. If there is $200 in the pot and $600 remaining in the effective stacks on the flop, the flop SPR is 3:1. This generally is considered a low SPR, while anything above 10:1 should be considered a high SPR. The majority of the most significant pots you’ll play, however, will come at moderate SPRs - somewhere between 3:1 and 10:1.

In developing a framework for ourselves to confidently adjust to changes in stack depth and postflop SPR, we need to identify some key principles that can help us in preflop hand selection.

Principle 1: the shallower the stacks, the closer we are to playing an all-in pot.

Principle 2: the closer we are to being all-in, the lower our hand requirements for being all-in.

Principle 3: our preflop hand selection should reflect our all-in requirements.

If our stack is five big blinds, we are going to be all-in any time we play a hand. As a result, our equity will be realized no matter what happens. Therefore, we don’t need to prioritize hands with suitedness or connectedness, only showdown value. As a consequence, a hand like King-Eight offsuit is stronger at this stack size than a hand like Six-Five suited.

If our stack is much deeper, for example 300 big blinds, we’re almost never going to be all-in before the river. As a consequence, we’ll need a very strong hand on the river (like a flush or a full house) to put our whole stack in. This means our preflop hand requirements should heavily favor hands that make these very strong river hands: strong pairs, strong suited connectors or suited Broadways, and suited Aces.

At stacks somewhere in between, we’ll find our requirements will vary. Therefore, our preflop hand selection should be adaptive according to the situation; but it should always prioritize hands which possess as many as possible of the three fundamental properties of a hand: again, showdown value, suitedness and connectedness. The deeper the stacks become, the more criteria a hand must fulfil in order for us to justify playing it, particularly when we are out of position.