Pot means the size before the bet you are facing. They bet. You either call that bet or you don't. Your required equity is:
Worked example, because this is the acceptance test and also the spot you will actually see: pot 100, they bet 50. Final pot if you call is 200. You put in 50. 50 / 200 = 25%. You need to be good here more than one time in four.
That 25% is the break-even calling equity if the hand ended now. Real streets add implied odds (you get paid extra when you hit) and reverse implied odds (you hit the stupid end and pay extra). Draws with dirty outs are not the same as a clean flush draw. See pot odds, equity, SPR.
MDF is the other side of the same coin
If they bet 50 into 100, they are risking 50 to win 100. Minimum defense frequency is 1 − (bet / (pot + bet)) = 1 − 50/150 ≈ 67%. That is how often you need to continue, as a range, so they cannot auto-profit by betting air. MDF is a range idea. Pot odds is a hand idea. Mixing them up is how people hero-call 72o "because MDF."
What this tool will not do
It will not tell you your equity. That is a combinatorics problem: their range, the runout, the blockers in your hand. It will not tell you to call. Thin value and bluff-catchers live in the gap between "priced" and "good." If you are results-oriented about a single call, go reread the TPTK trap.
FAQ
Pot 100, bet 50 — what equity do I need?
25%. Call 50 to win a 200 pot (100 + 50 + 50). Formula: bet / (pot + 2 × bet). Pot is the size before the bet you are facing.
Is that the same as 33%?
Only if you defined "pot" as already including their bet. This tool treats pot as before the bet, so 100 / 50 is 25%, not 33%.