It seems to me that you would only need 33 % equity against villains calling range when making a pot sized value bet since let us say the pot is $10. You bet $10 and he calls. That is 10/30 = 33 %. Yet I've heard that you need 50 % equity to make this bet.

Am I missing something?

you need over 50% eq against calling range to make any valuebet, sizing here is meaningless. If you have <50% it's a bluff, not a valuebet.

When you are in position on the river, you need 50% equity against villain's calling range*. This is because you have to compare the EV of betting to the EV of checking, not to zero. The EV of checking back isn't going to be zero.


*This is an oversimplification if villain can raise and you should fold to a raise. That hurts your EV, and to make up for it, the minimum equity you'll need when called will end up being more than 50%.

This is only true in position on the river.

To the extent that 'valuebet' and 'bluff' are meaningful terms, this isn't really true. You could call hands in the 40-50%-ish region 'preemptive bets' or 'protection bets' or whatever. But from a practical standpoint those hands often are pretty much value bets anyway.