Interest Rate Parity  (IRP) Calculator Notes
Spot Rate, S Spot exchange rate
Base Currency Annual Interest Rate (%)   Use annual interest rate in home country, such as 5(%); id = annual rate * (Days of Forward Contract / 360)
Quoted Currency Annual  Interest Rate (%)  Use annual interest rate in foreign country, such as 5(%);if = annual rate * (Days of Forward Contract / 360)
Days till Maturity of the Forward Contract Could be 90 days, 180 days, 270 days, or 360 days, etc. Use a 360-day year to calculate the partial interest rate. For example, 90 days is 1/4 of a year
Forward Rate, F -- Forward exchange rate based on Interest Rate Parity

Notes:

The formula to determine the forward foreign exchange rate is the following:

F = S * ((1 + iq) / (1 + ib)

A simpler version: F = S*(iib

Where:

• ib is the interest rate in the  base currency (for partial interest rate, use ((days to maturity / 360) * annual interest rate) to get id )
• iq is the interest rate in the  quoted currency (for partial interest rate, use  ((days to maturity / 360) * annual interest rate) to get if )
• S is the current spot foreign exchange rate.
• F is the forward foreign exchange rate.

Forward Premium = F/S -1 = (F-S)/S

Examples: USD/GBP, the US Dollar is the base currency; EUR/USD, the EURO is the base currency

GBP/USD = 2, interest rate in UK is 10%, interest rate is US is 5% ==> F = 2*(1+5%) / (1+10%) = 1.9091

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