Direct answer
Rollover (often called swap) on high liquidity forex pairs is generally calculated from an interest-rate differential between the two currencies in the pair, then adjusted for the contract’s rollover convention and the provider’s pricing rules. Because providers can implement different formulas, quotes can show different rollover amounts even when the underlying interest-rate logic is similar.
The mechanism: what “rollover” means
In spot forex, trades are usually settled on a standard settlement cycle rather than on the same day. When a position is held overnight, the contract is “rolled” to the next settlement date. The cost or credit for doing so is expressed as rollover.
A simplified way to think about rollover:
- Identify the two currencies in the pair (the “base” and the “quote”).
- Determine the overnight interest component for each currency.
- Compute the interest-rate differential (one side benefits, the other costs).
- Apply the pair’s convention (which side is treated as receiving interest and how quoting is converted into the account currency).
- Apply the provider’s adjustment (for example, how the provider converts benchmark rates into tradable swap points and any markup/charges embedded in the quote).
For high liquidity pairs, the key mechanics do not change. The difference is mostly that the underlying benchmark rates and market depth are typically reflected more consistently in pricing than in less liquid instruments.
Interest-rate inputs (the “differentials” part)
Most rollover logic is driven by benchmark interest rates for the two currencies. The practical challenge is that providers must translate benchmarks into the tradable swap for the specific currency pair and settlement dates.
Common elements inside that translation include:
- Benchmark reference: the interest-rate measure used for each currency.
- Day-count and compounding convention: how interest accrues over the period.
- Tenor/settlement mapping: which rate corresponds to the overnight rolled date.
Because these inputs and conventions are not identical across providers, the same overnight market conditions can still produce different displayed rollover values.
Broker/provider adjustments (the “pricing” part)
Even if two providers start from the same benchmark rates, their displayed rollover can differ due to:
- How they convert interest-rate differentials into “swap points.”
- Any embedded adjustments in the tradable quote (for example, costs or spreads in the swap pricing).
- How their system treats position direction (long vs short). Typically, one direction earns where the other pays, but the exact sign and magnitude come from the provider’s swap formula.
Evidence and examples (with explicit assumptions)
Example A: directional interest differential
Assume:
- Currency A has a higher overnight interest rate than Currency B.
- You hold a long position whose exposure earns the interest differential from A minus B under the provider’s convention.
In that setup, rollover is often positive for one direction and negative for the opposite direction. The precise amount depends on the provider conversion and whether the provider charges additional costs.
Example B: triple-swap convention
Many rollover systems account for weekend/non-business days by applying a larger rollover on a specific day, often called a triple-swap. The idea is that the position is effectively carried over multiple calendar days rather than just one overnight period.
Assume a system performs:
- Standard rollover on business days.
- Triple rollover on the last business day before a weekend to cover three days of accrual.
If you compare rollover charges on different days, you may see larger swap on the “pre-weekend” rollover date. The exact timing (which day gets triple) can vary by provider, server timezone, and operational settlement rules.
Material limitation / failure mode
A key limitation is that rollover shown on a platform is not just a pure “interest differential.” It is the provider’s computed swap after applying its own conversion rules, conventions, and embedded adjustments. Therefore:
- The rollover you observe may not match a simple hand-calculation from benchmark rates.
- Changes in provider methodology or quote presentation can alter results without any change in the underlying benchmark rates.
Limitations and how to verify independently
Rollover outcomes vary with market conditions, execution details, costs, and the provider’s swap rules. Historical patterns do not guarantee future rollover behavior.