Interest rate parity
The rule by which the difference between two currencies' interest rates is built into the forward rate.
Formula
F = S \cdot e^{(r_{q} - r_{b}) t}S is the current rate and F the rate for the term; r_q and r_b are the rates of the quote and base currencies, t the term in years.
How to read the number
It follows that the rate difference offers no risk-free gain: it has already been paid for in the price of the future rate.
When the metric lies
The rule holds while capital moves freely between currencies. Under restrictions on moving funds the forward rate drifts away from parity and stays there for a long time.
Also known as: covered interest parity