What put-call parity is measuring
Put-call parity links a call and a put with the same strike and the same expiry to the underlying asset and the present value of the strike price. In its simplest form, the relationship shows that a call minus a put should equal the prepaid cost of holding the underlying minus the discounted strike. If quoted prices deviate from that relationship, the difference is commonly described as the parity gap.
This calculator can either verify the gap directly or solve for one missing variable when the others are known. That makes it useful for checking whether a call premium looks too rich or too cheap relative to the paired put, or for inferring a parity-consistent spot, strike, call, or put value from the rest of the entered assumptions.