Forward exchange rates are not determined by subjective market guesses about where a currency’s value will head in the future. Instead, they are calculated mathematically using the pricing standard of Interest Rate Parity (IRP).
The Classical Covered Interest Rate Parity Equation
The IRP model proves that the forward premium or discount of a currency matches the interest rate differential between the two countries involved, preventing risk-free financial arbitrage. The plain text mathematical formula is written as follows:
Forward_Rate = Spot_Rate * ((1 + (Foreign_Interest_Rate * (Days / 360))) / (1 + (Base_Interest_Rate * (Days / 360))))

Where:
  • Forward_Rate = The calculated nominal forward exchange rate for the designated delivery date.
  • Spot_Rate = The current market exchange rate for instant T+2 delivery.
  • Foreign_Interest_Rate = The annualized nominal interest rate available in the quote/foreign currency country.
  • Base_Interest_Rate = The annualized nominal interest rate available in the base currency country.
  • Days = The exact calendar duration of the forward contract lifecycle.

Forward Pricing Calculation Example
If the current EUR/USD Spot_Rate is 1.1000, the annualized US Dollar interest rate (Foreign_Interest_Rate) is 5.0% (0.05), the annualized Euro interest rate (Base_Interest_Rate) is 2.0% (0.02), and a treasurer requires a 180-day forward contract, the plain text calculation is:
US_Interest_Factor = 1 + (0.05 * (180 / 360)) = 1 + (0.05 * 0.5) = 1 + 0.025 = 1.025
EUR_Interest_Factor = 1 + (0.02) * (180 / 360)) = 1 + (0.02 * 0.5) = 1 + 0.01 = 1.01
Forward_Rate = 1.1000 * (1.025 / 1.01) = 1.1000 * 1.01485 = 1.1163

By setting the forward contract rate at exactly 1.1163, the interbank system ensures that an investor cannot borrow funds cheaply in Europe, convert them to dollars, and invest them at higher rates in the US without having the yield advantage offset by the forward exchange rate adjustment.
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