A Swap is a financial derivative in which two parties agree to exchange cash flows based on the underlying value of an asset. The most common types of swaps are interest rate swaps and currency swaps.To calculate gains on a swap, you will need to determine the difference between the value of the swap at the time it was entered into and its value at the time it was closed or terminated. This difference represents the gain or loss on the swap. The gain or loss can be calculated by determining the present value of the cash flows being exchanged and comparing the present value of the cash flows received to the present value of the cash flows paid.For example, if a party entered into an interest rate swap and received fixed rate payments while paying floating rate payments, and the fixed rate payments were higher than the floating rate payments, the party would have a gain on the swap. The gain would be calculated as the present value of the difference between the fixed rate payments and the floating rate payments.