Direct answer
Rollover (often called “swap”) is a periodic cash adjustment that reflects the interest-rate difference between the two currencies in a forex position, adjusted by the provider’s specific financing convention. For JPY-linked trades, the “JPY Reaction” part usually refers to how the JPY leg is treated inside that general framework: the rollover is still built from an interest-rate input, then modified by the provider’s pricing model and any special day-count rules such as triple-swap over a rollover window.
The basic model (definition and assumptions)
A position held open past the broker’s daily cut-off typically earns or pays rollover. Conceptually, the swap comes from two pieces:
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Interest-rate differential between the base and quote currencies. In a simplified educational view, if one currency carries a higher interest rate than the other, the position structure that is “long the higher-rate side” tends to receive financing, while the opposite structure tends to pay. The exact “interest rate” used is not the same as retail savings rates; providers usually rely on interbank rate references and their own day-count and settlement conventions.
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Provider adjustments and conventions. Even with the same interest-rate differential, the final swap amount can differ because providers apply:
- their own formula structure,
- spreads or markups to financing (a cost component may be embedded),
- operational timing (server time cut-off), and
- instrument-specific contract details that affect how the cash amount is computed.
Because these adjustments are provider-specific, the only way to compute an exact number independently is to use the provider’s published swap/financing methodology for the exact instrument and the exact holding date.
How it is calculated in practice (mechanics)
A common educational approach is to separate the steps, while clearly stating assumptions:
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Choose the interest references. Assume a provider uses a reference rate for JPY and a reference rate for the other currency (for example, a short-term interbank benchmark family). The direction matters: the swap sign follows which side is notionally funded versus received.
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Apply the day-count and holding convention. Swap is usually calculated per day (or per relevant holding period) and then compounded or summed according to the provider’s convention.
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Apply instrument scaling. Many providers convert the theoretical interest differential into a cash amount per contract size. This includes mapping “points/pips” exposure to a monetary value using the instrument specification.
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Apply provider markups and operational factors. The final displayed swap often reflects additional provider costs or margins, and can change when the provider updates its swap rates.
Triple-swap convention (the special case)
Most retail forex systems apply a triple-swap on a specific rollover event (commonly associated with the weekend). The general idea is that the interest for multiple calendar days is charged or credited at once to cover the period where the market settlement cycle spans additional days.
In an educational example, if a provider normally applies one daily swap, then during the triple-swap window the calculation effectively uses three days’ worth (or another multi-day equivalent) rather than one. The limitation is that the exact number of days and the exact time/day that triggers the triple swap can vary by provider and by the instrument’s convention.
Limitations and failure modes (what can go wrong)
Several material limitations can make a “back-of-the-envelope” calculation mismatch the actual swap credited or charged:
- Provider-specific methodology differences: even if you know the currency interest differential, the provider may use different rate references, day-count conventions, or rounding rules.
- Embedded financing costs: what you compute from interest rates may exclude markups/fees that providers include in the displayed swap.
- Timing and cut-off issues: if your position changes (open/close) near the daily rollover cut-off, the swap that applies can differ from what you expected.
- Triple-swap triggering uncertainty: assuming “triple” is always three days may fail if the provider uses a different multi-day mapping or if the trigger depends on server time.
- Market-condition variability: the provider’s swap rules can update, so historical relationships between rates and swap amounts do not guarantee future amounts.