The question
A call costs you chips now and pays you the pot later. Break-even is the point where those two cancel out: win often enough and calling makes money, win less often and it burns money. The number you want is that threshold.
It has one form and it never changes:
Everything difficult about this is in the denominator, and everything people get wrong is in the denominator too.
A worked example
The pot is 800. Your opponent bets 400. You have to put in 400 to continue.
Three things are going into that pot, so add all three:
Then the price is your call against that total:
You need to win at least a quarter of the time. Above that, calling makes money. Below it, folding does.
The shortcut worth having
At a table you will not be doing long division. What you actually learn is the handful of bet sizes people use, because each one has a fixed answer no matter how big the pot is.
Notice how little the answer moves. A bet eight times larger than another only shifts the threshold from 17% to 40%, which is why big bets are less punishing than they feel and small bets are harder to fold to than they look.
The three ways to get it wrong
The game's wrong answers are not random numbers. Each one is a specific error, so when you miss, the results screen can tell you which of these you did.
Leaving your own call out of the pot
The most common one by far. Using 800 + 400 = 1,200 instead of 1,600 gives 33% rather than 25%, and makes every call look worse than it is. The chips you put in are part of the pot you win.
Using the raw bet-to-pot ratio
400 ÷ 800 = 50% is a description of the bet size, not a probability. It is a useful number for a different purpose and it is never the break-even point.
Counting your call twice
Adding the call to a total that already contains it gives 2,000 and an answer of 20%. Your money goes in once.
What this does not tell you
Pot odds give you a threshold, not a decision. To act on it you also need some sense of how often you actually win, which is what counting outs is for. The two together are the whole calculation: this is the price, that is whether you can pay it.