the issue of calculating the amount of interest credited or charged to your account at rollover time might not be such a trivial issue as it might seem at first.

The input information you need to have is:

- borrow interest % of the base currency if you are long / lend interest % of the base currency if you are short
- lend interest % of the quote currency if you are long / borrow interest % of the quote currency if you are short
- number of units of base currency in the position
- number of units of quote currency in the position
- exchange rates for the base currency/"home currency" and/or the quote currency/"home currency" if you want to know what the grand total interest is in your "home currency" (for GBP/JPY, you will probably want to know what's the interest charged/credited in USD, even though USD is not present in this FX pair, which means you need to know what GBP/USD and JPY/USD is, too)

So the total interest credited / charged is simply the sum of the following equation:

- (amount of borrow interest on your long currency) - (amount of lend interest on your short currency)

The amount of borrow interest on your long currency is determined by the following equation:

- (borrow interest %) * (units of long currency) * 1/365

Similarly, the amount of lend interest on your short currency is determined by the following equation:

- (lend interest %) * (units of short currency) * 1/365

As stated above, you will probably want to have the result in your "home" currency, which is mot likely the currency in which your trading account is denominated: USD. This means that you need to convert the amount of interest credited on the long currency / charged on your short currency to your home currency.

The interest credited / charged can also be recalculated to pips of the FX pair in question.

To make the long story short, I've set up the calculation in Excel:

The Excel sheet is attached below.

Michal