Direct answer
Rollover (also called swap) for a forex position is generally calculated as the interest-rate difference between the two currencies in the pair, adjusted by overnight financing conventions. “USD Concentration” means your overall exposure is weighted toward positions involving USD, so the USD-related financing component tends to dominate the rollover impact you experience.
Mechanics: what “USD concentration” changes in rollover
A forex trade is exposed to two things during holding periods: (1) the exchange-rate movement and (2) carry/financing, reflected in rollover. The rollover portion is designed to represent the cost or benefit of financing the currencies until the position is closed.
Step 1: Define the carry direction
Rollover depends on whether you are long or short the pair. Intuitively, holding a position means you are effectively “borrowing one currency” and “lending the other.” If the currency you effectively borrow has the higher interest rate, the rollover tends to be negative for that side. If it has the lower interest rate, it tends to be positive. Exact signs follow the contract’s long/short definition.
Step 2: Use the interest-rate differential (core input)
In a simplified educational model, the rollover is linked to:
- the interest rate for currency A
- the interest rate for currency B
- the size of the position
- the time convention for “one night”
The conceptual center is the differential: rate(A) − rate(B). The forex swap reflects this differential, scaled into a daily or per-day amount.
Step 3: Apply overnight conventions (day-count and rollover timing)
Real systems do not always treat “one day” as the same interval. A common convention is that some rollover events occur at a specific server time, and certain days can include extra time (for example, when moving from a weekend). This is why swap can be larger on some rollover days even if the interest rates are unchanged.
Step 4: Account for broker/platform adjustments
Providers often do not pass through a pure theoretical interest differential. They can add adjustments such as:
- a dealer markup/markdown or internal cost
- how the swap is computed for buy vs sell
- how they round or cap values
- their specific triple-swap or multi-day convention when applicable
So, even if you can correctly compute an interest-rate differential, the displayed rollover may still differ because provider-specific rules translate that differential into their final swap charge/credit.
Where “USD concentration” fits
USD concentration does not change the underlying mechanics of rollover for any one position. Instead, it changes the overall effect you feel across positions, because more of your exposure relates to USD financing. If many of your open positions involve USD as either the base or quote currency, then the USD-related carry component tends to be the largest contributor to the net rollover you accumulate.
Evidence or example (educational, with explicit assumptions)
Consider a simplified educational setup for a USD-involving pair, ignoring taxes, fees outside swap, and any provider markup.
Assumptions for the example:
- You hold a position overnight that triggers exactly one standard rollover event.
- The swap value is proportional to the interest differential: rate(A) − rate(B).
- Time scaling is constant (no extra multi-day rollover in this example).
- We only illustrate direction and relative magnitude, not exact provider numbers.
Example logic:
- If you are positioned so that you effectively receive the interest of the currency with the higher interest rate and pay the lower one, the net rollover is conceptually positive.
- If the effective borrowing currency has the higher rate, the net rollover is conceptually negative.
- For “USD concentration,” if USD has the higher relevant rate compared with the other currency in your dominant exposures, then positions with that structure tend to contribute more to positive or less negative rollover for the side that benefits from receiving USD financing (and the opposite side experiences the charge).
Even in this simplified model, the key point is that your net rollover outcome is a weighted sum across your open positions’ financing directions. USD concentration mainly affects those weights, not the basic rule.
Limitations and failure modes
- **Interest rates are not the only driver. ** Swap/rollover is commonly impacted by provider-specific adjustments, not just the pure market interest differential.