The formula used to calculate the swap charges depends on the instrument group that is being traded.
Forex: Pip Value X Swap Rate X Number of nights /10
Spot Metals, Energies & Indices: Lot Size (i.e., 0.1 or 1.0) X Swap Rate X Number of nights
Swap charges occur daily for all instruments (triple charge on Friday), and are reviewed on a weekly basis & updated accordingly.