Central banks increase or decrease borrowing costs in accordance with their monetary policy. Each currency has its own interest rate set by the respective central bank. When you buy or sell currencies or assets, the swap is calculated based on the interest rate differential between the assets in the symbol. Swaps are calculated in pips or points based on the interest rate differential between these assets.

For example, suppose the US Federal Reserve (the FED) sets an interest rate of 5% annually, while the Bank of Japan (BOJ) decides upon a 0% interest rate. This means that the US Dollar would yield 5% interest each year, while the Japanese Yen would not yield anything. Borrowing US Dollars would incur a 5% interest rate while borrowing the Japanese Yen wouldn't incur any yearly interest. So by simultaneously selling the Japanese Yen to buy US Dollars, it would incur a positive swap differential of 5% per annum. However by simultaneously selling US dollars to buy Japanese Yen, it would incur a negative swap of 5% per annum.

The annual swap differential is then divided by the number of days in the year and converted to the pip or point equivalent and is applied to open trades daily.