Underbet

Also known as: underbet, under-bet, small bet, blocker bet

A small bet well under half the pot, used to extract thin value, set a cheap price, or deny equity on the right ranges.

An underbet is a bet far below half-pot — often 20–40%, sometimes tiny. It's the opposite end of bet sizing from the overbet and serves several precise jobs: extracting thin value from a wide calling range, denying equity cheaply on dry boards, setting a small price as a blocker bet to discourage a larger bet from the opponent, and protecting a capped but ahead range.

The math: a small bet gives the opponent excellent pot odds, so their MDF is high — they should fold rarely. For a bet of fraction \(s\), \(\text{MDF}=\tfrac{1}{1+s}\); at \(s=0.25\) that's 80%. Correspondingly your bluffs need few folds to profit (\(\tfrac{s}{1+s}=20\%\)), but each successful bluff wins little. Underbets are a merged/value-leaning size: they thrive when you are capped-but-good and want to realise equity without bloating the pot, or when a wide opponent range pays a small value-bet far more often than a big one.

Example

River pot 100, you bet 25 (25% pot, \(s=0.25\)). Villain's MDF is \(\tfrac{1}{1.25}=80\%\) — they continue with almost everything that beats a bluff. With a thin value hand against a wide range, a 25 bet that gets called 70% earns \(0.70\times 25=17.5\) on average; a pot-sized 100 might only get called 30%, earning \(0.30\times100=30\) but risking far more and folding out the hands you beat.