ConceptsBetting
SPR and commitment
Stack-to-pot ratio: how many pots are left behind, and so how committed a hand is.
What it is
SPR is the effective stack — the smaller of the two stacks still in — divided by the pot. Rail measures it at the start of each street, the way players quote it ("SPR 4 on the flop"), not after every bet.
Typical numbers at 100 bb: a single-raised pot starts the flop around SPR 15 to 18; a 3-bet pot around 4; a 4-bet pot around 1.
Why it matters
SPR tells you in advance which hands can go all in. At SPR 1 to 3, top pair is usually happy to get the money in; at SPR 15, one pair wants to keep the pot small and the stacks belong to sets, straights and better.
It lets you plan the hand: how big each bet must be to get the stacks in by the river, and whether a bet now leaves enough behind to fold to a raise.
Two of Rail's flags are SPR questions: a bet that leaves too little behind (all in, in all but name) and a fold after most of the stack is already in.
The maths
SPR equals the effective stack divided by the pot, at the start of the street.
The geometric bet, as a fraction of the pot, equals one plus two times SPR, to the power one over n, minus one, all divided by two.
- n
- the streets left to bet, each bet called
Worked example
A 3-bet pot
The cutoff opens to 2.5 bb, the button 3-bets to 10 bb, the blinds fold and the cutoff calls. Both started with 100 bb.
- The pot on the flop: 10 + 10 + 0.5 + 1 = 21.5 bb. Each player has 90 bb behind.
- SPR = 90 / 21.5 ≈ 4.2.
- To get the stacks in over three streets with equal bets, each bet should be about 55% of the pot.
- Over two streets it takes about pot-sized bets (103%).
- Compare a single-raised pot: 2.5 + 2.5 + 0.5 = 5.5 bb with 97.5 bb behind, SPR ≈ 17.7 — three bets of about 116% pot.
In the 3-bet pot, three ordinary bets put top pair all in; in the single-raised pot it takes three bigger-than-pot bets. Same hand, different plan.
Try it
Set the pot and the stacks, then the number of streets: the geometric bet is the size that gets it all in by the river.
Common mistakes
- Playing one pair for the whole stack at a high SPR.
- Folding top pair at SPR 1 after putting a third of the stack in.
- Betting so that a sliver is left behind: the opponent can call knowing you will never fold.
- Measuring SPR after a bet on the same street, which makes every spot look more committed than it is.
In your hands
Hands from your library where the analysis flagged this:
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Related concepts
- Bet sizing and polarisationSmall, big or overbet: every size has a job, and the job depends on the board and on which hands you bet.
- Pot oddsThe share of the final pot a call costs — and so the equity a call needs to break even.
- 3-bets and 4-betsThe first re-raise before the flop, and the re-raise after it: building pots with value and taking them with fold equity.