Direct answer
Rollover (often called swap or swap rate) represents the interest-related cost or credit applied for holding a foreign-exchange position past a daily cut-off. For an account in a “base currency,” the rollover you see is calculated from the interest-rate differential of the two currencies in the traded instrument, then expressed in your account base currency using an agreed conversion method.
Because providers can differ in how they define inputs (what exact benchmark rates they use), how they apply signs (which currency is charged), and which days get special treatment, the only reliable way to confirm the exact figure is to compare the provider’s documented swap rates and the daily rollover convention against the dates you held the position.
Mechanism: definition and the key inputs
A simplified conceptual model is:
- Identify the two currencies of the instrument (Currency A and Currency B).
- Determine the interest-rate inputs for each side (how “interest” is represented in the swap). These inputs are usually derived from benchmark short-term rates (for example, interbank or central-bank-linked reference rates), plus any standardized adjustments.
- Compute an interest differential that reflects the relative cost/credit of the position.
- Apply the provider’s convention for the trade’s direction (whether you are effectively long Currency A or short it).
- Convert the resulting rollover amount into your account base currency.
What “interest-rate differential” means in practice
In many FX rollover frameworks, the swap is constructed so that it approximates the financing cost of one currency versus the other. The calculation uses agreed-upon reference rates and standard assumptions (such as how the “financing” period is represented), rather than using the exact interest you would receive or pay in a real bank deposit/loan.
How the account base currency enters
Your account base currency matters because the swap rate you see is presented in that currency. Conceptually, providers compute the swap in terms that match the traded instrument and then convert using a documented FX conversion approach.
Conversion can depend on the price reference used for conversion and on the timing of conversion (for example, at or near the daily cut-off). That is why the same instrument position might show different rollover amounts when your account base currency changes.
Evidence or example (with explicit assumptions)
Assume an instrument that references Currency A and Currency B, and assume the following (example-only) convention:
- The provider computes a daily rollover in the instrument’s quote terms before conversion.
- The sign follows a direction rule: holding long Currency A versus Currency B yields a net credit, while the opposite yields a net debit.
- Rollover is applied at the daily cut-off.
Let the computed net daily swap be 0.50 units of Currency B (credit) before conversion, and assume your account base currency is Currency C. To display the amount in Currency C, the provider converts Currency B to Currency C using a specified conversion rate.
If the conversion rate used is 1 Currency B = 2.0 Currency C, then the displayed rollover in Currency C would be:
- 0.50 × 2.0 = 1.0 Currency C.
If instead the net daily swap is negative (a debit), the same conversion logic applies, but the sign reverses.
Triple-swap convention (why one day can be larger)
A common rollover failure mode is assuming “one day held = one identical daily swap.” Many rollover conventions treat certain calendar periods differently (often a larger amount applied on one day so that the financing effect spans a weekend or other non-business period). Under such a rule, the rollover on that special day can be approximately three times the usual daily amount, but the exact multiplier and which day it applies to are convention-dependent.
Limitations and risks (material failure modes)
- Provider-specific conventions: The exact formula, the benchmark inputs, and the sign/direction rule can vary. You may see different swap behavior for the same instrument across providers. 2) Conversion timing and reference: If conversion uses a specific reference price at cut-off, slight differences in timing can change the displayed base-currency amount. 3) Date and calendar effects: Daylight saving time, cut-off time definitions, and special-day multipliers (triple-swap-like behavior) can change the total rollover for a holding window.