How rollover is calculated for pair session behaviour

What rollover means in pair session behaviour

Rollover is the accounting of interest when a foreign-exchange (forex) position is held across the end of the trading day (commonly called “overnight”). In pair session behaviour, rollover is part of the shift you may notice between a quote you see now and the effective value when the position remains open into the next session.

Conceptually, forex has an underlying interest-rate structure: currencies yield different interest rates. When a position stays open, the broker or trading venue applies an interest-rate differential to reflect that cost or benefit. In practice, the exact numbers depend on the venue’s rollover rules, the instruments’ conventions, and execution timing.

Mechanism: inputs and a simple independent check model

A straightforward way to describe rollover is as a two-step process:

1. Compute the interest-rate differential component.

   • Choose an assumed reference rate for each currency in the pair (often based on interbank benchmarks or similar market rates).
   • The differential is typically expressed as an annualized rate difference, then converted to a daily portion using a day-count convention.

2. Apply operational and provider conventions. These may include:

   • Position direction: whether you are effectively long or short each currency.
   • Point scaling: converting an interest amount into the platform’s quote units (often involving the contract size and the quote currency).
   • Swap rate sign: depending on direction, the differential may become a charge (debit) or a credit (credit).
   • Timing cutoffs: rollover is applied based on when the venue’s system considers the day change.

Simple example model (with explicit assumptions)

Assume a simplified model where a provider converts interest into a per-day swap expressed in quote-currency terms.

• Pair: Currency A / Currency B
• You hold a position overnight at the platform’s cutoff.
• Annualized reference rates (assumed): rA for A and rB for B.
• Daily fraction: we approximate daily interest as (annual_rate × 1/365).
• Direction: if you are long A and short B, the “differential” direction is proportional to (rA − rB); if you reverse, the sign changes.

A generic form is:

• Interest differential amount (conceptual) ≈ (rLongCurrency − rShortCurrency) × (1/365)
• Rollover per day ≈ differential amount × (position size and contract scaling)
• Then provider convention adjustments are applied to match the venue’s published swap-rate methodology (including any internal markups or rounding).

This model is intentionally generic: real platforms may compute in quote points, apply specific day-count rules, and incorporate their own conventions. Still, the independent check is the same: verify which currency you effectively “own” in the pair direction, and confirm how many days the provider charges for when you hold across the cutoff.

Pair session behaviour link: why it can feel “jumpier” around session changes

Pair session behaviour often describes how pricing and trading characteristics shift by session. Rollover contributes because it changes the effective value overnight. On days where rollover is applied for more than one day, the adjustment can look larger, even if the underlying interest-rate differential concept is unchanged.

Triple-swap conventions and the “extra day” effect

A common rollover convention is triple-swap: on specific rollover days, the provider applies a rollover amount intended to cover an extended period (for example, when markets are closed over a weekend). In the simplest explanation, triple-swap means:

• Normally, rollover covers one day.
• On certain days, rollover covers three days (conceptually one extra day beyond the usual daily application).

In an independent calculation model, that means multiplying the daily interest component by roughly three, then applying the same provider scaling and sign logic.

Material limitation: rollover is not a pure interest-rate calculator

Even with careful assumptions, several things can prevent you from matching rollover perfectly:

• Provider-specific day-count and cutoff rules: the “day” boundary may not match the calendar day you expect.
• Published swap vs. internally booked swap: platforms may show a number derived from their own methodology.
• Quote-unit conversion: swap may be published in points, then converted using contract size and quote currency.
• Cost layering: some venues embed additional adjustments beyond the pure interest differential idea.

These differences mean you should treat rollover as an applied convention, not only a theoretical interest differential.

Limitations, risks, and failure modes

1. Market-condition dependence. Interest-rate inputs and execution environment can differ from what you assume, especially when benchmarks move.