A pure bluff were you have absolutely no chance to win if you get called are relatively simple to calculate the odds on. Just compare your bet/raise size to the pot size and estimate the chance of the opponent folding.

If you bet 5BB in a 5BB pot your opponent will have to fold more than 50% of the time for it to be +EV.

However, if you have some outs (semibluffing) or you think it's possible to get called by a worse hand it gets more complicated.

I think this is accurate:

EV = F * P - C * A

F = Estimated chance of opponent folding
P = Pot size
C = Estimated chance of opponent calling (or raising)
A = Avarage loss if you get called (If you have no chance of winning if you get called A = The size of your bet)

Obviously F = 1 - C

It's the avarage loss that's hard to estimate since it depends on so many things. If it's on the river the avarage loss equals to how much behind of your opponents calling range you are. But otherwise you have to consider the times you are forced to fold to a later bet. And if you are semibluffing you have to consider implied odds and redraws (if implied odds are big the variable A may even be avarage win). You also have to consider the times where you do hit your hand but villain had a better hand all along so you end up losing even more money.

I'm a little bit unsure of the math here but it seems like that's how it should be done, someone correct me if it's not.