For positions still open at "cut-off time" (usually 5pm New York EST), there is a daily rollover interest rate that the trader either pays or earns, depending on your established margin and position in the market. If you do not want to earn or pay interest on your positions, simply make sure they are all closed before 5pm EST, the established end of the market day.
Since every currency trade involves buying one currency and selling another then interest rollover charges are part of forex trading. Interest is paid on the currency that is sold (borrowed), and earned on the one that is bought. If a client is buying a currency with a higher interest rate than the one he/she has sold (borrowed), the net differential will be positive (i.e. USD/JPY) - and the client will earn interest as a result. Ask your client sales representative about specific details regarding rollover if you are unsure.
You are long 100,000 EUR/USD. The EUR/USD at rollover is trading at 1.1800, EUR short-term interest rate is 2.25% and the USD short-term interest rate is 4.00%, the theoretical rollover calculation would be as follows:
contract notional value x (base currency interest rate - quote currency interest rate) / 365 days per year x current base currency rate = daily rollover interest debit/credit
Therefore: 100,000 x (2.25% - 4.00%) / 365 x 1.1800 = daily rollover interest debit/credit
Further: 100,000 x -1.75% / 365 x 1.1800 = USD -$5.66 rollover debit to your account
Since you are long a base currency (EUR) bearing a lower interest rate than the quote currency (USD), you will pay for that rollover.
Likewise if you are long 100,000 NZD/JPY. The NZD/JPY at rollover is 89.50, NZD short term interest rate is 7.50% and the JPY short term interest rate is 1.25%, the theoretical rollover calculation would be as follows:
Therefore: 100,000 x (7.50% - 1.25%) / 365 x 89.50 = daily rollover interest debit/credit
Further: 100,000 x 6.25% / 365 x 89.50 = 1532 JPY or NZD17.12 rollover credited to your account.
Since you are long a base currency (NZD) bearing a higher interest rate than the quote currency (JPY), you will earn interest for that rollover.
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