[uuDevil]

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I was always told that for an easy odds estimate on the flop, you take each out and add 4% for each one.

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That's a rough approximation that breaks down for large numbers of outs. A better approximation is:

1-P(missing on turn)*P(missing on river)

1-((47-21)/47)*((46-21)/46)=0.699

This is still just an approximation though. Somethimes you will hit one of your outs and still lose.

Edit: Since you know your opponents cards, that should be:

1-(45-21)/45*(44-21)/44=0.721