A put payoff is positive when price is below strike.
Long put payoff
A long put is the right, not an obligation, to sell at strike. At $80.00 payoff is $20.00; at $100.00 payoff is $0.00; at $120.00 payoff is $0.00.
payoff(S)=max(K−S,0)
Payoff convention
This is an expiry-only chart with no premium, carrying, or time path effects.
payoff(K=$100.00)