Swaps are the interest that is earned/paid on your trade if it is held overnight. Swaps vary depending on the financial instrument you are trading because each instrument will have its own interest rate.
To find out the swap rate for an instrument, please consult the Rollover policy page and scroll down. You will see two rates: Short and Long. If you have a Buy trade, use the Long rate; if you have a Sell, use the Short rate.
To calculate the swap for your trade, use the formula:
Current Long/Short rate * number of lots = Swap amount in the Quote currency
Example:
Let's say your account currency is USD, and you have a 2-lot Sell (Short) trade on of EURGBP. We check the Contract specifications page and see today's swap Short rate is -1.08. So, let's put all that in the formula:
-1.08 * 2 = -2.19
As GBP is the Quote currency in the EURGBP pair, this is -£2.19. Then convert to USD using the current exchange rate.