The basic formula
Pot Odds = your call / (pot + your call)
Example: pot is $100. Opponent bets $50. To stay in you must call $50. Pot is now $150, and you invest another $50 — total $200. Your Pot Odds: 50/200 = 25%.
Meaning: as long as you have more than 25% chance to win the hand — calling is +EV. Less than 25% — fold.