What is Interest Rate Parity (IRP)?
Interest Rate Parity (IRP) is the “no-free-lunch” relationship between interest rates in two countries and the FX forward rate. In simple terms: if one currency offers a higher interest rate than another, that currency should typically trade at a forward discount (or the lower-yield currency at a forward premium) so that hedged returns end up broadly comparable.
IRP is foundational for pricing and interpreting a forward exchange rate and for understanding how the interest rate differential between two countries translates into forward pricing. When markets are calm and funding is easy, pricing often sits close to the covered form of IRP; when funding gets stressed, deviations may show up as a cross-currency basis.
Covered vs. Uncovered IRP (quick context)
- Covered IRP (CIP): FX risk is hedged using a forward. This is the core “pricing” relationship behind hedged currency positions and is closely tied to Covered Interest Arbitrage.
- Uncovered IRP (UIP): FX risk is not hedged; it’s closer to an expectations story about future spot rates. In reality, risk premia and behavior often dominate, which is why UIP is frequently discussed alongside the Carry Trade.
Interest Rate Parity Formula
Where:
- = forward exchange rate
- = spot exchange rate
- = domestic interest rate
- = foreign interest rate
Important: keep compounding and timing consistent. If you’re using annualized rates for a shorter tenor (e.g., 1M/3M/6M), convert them to the same period and convention. This is where concepts like Effective Annual Rate (EAR) matter because compounding assumptions can change the implied forward.
Forward Premium or Discount
- If the result is positive, the currency is at a forward premium.
- If the result is negative, it’s at a forward discount.
In real FX quoting, you’ll often see the premium/discount expressed as forward points rather than a percentage, especially for short tenors.
Why IRP matters in practice
IRP is what makes “rate shopping” across currencies non-trivial: the forward market is supposed to offset the yield advantage implied by different interest rates. That’s why investors look at IRP when deciding whether to hedge FX exposure, and why traders watch IRP-like relationships when evaluating whether the forward curve reflects macro forces such as Purchasing Power Parity (PPP) or Fisher Equation dynamics.
Example
Given:
, ,
So, the domestic currency trades at a 1.46% forward discount.