How this calculator works
A forward contract fixes an exchange rate for a future date. Covered interest parity says the forward rate must equal the spot rate adjusted for the interest you can earn in each currency over the contract: otherwise one could borrow in the low-rate currency, convert, lend in the high-rate currency and lock in a risk-free profit. The adjustment uses each currency's annualized interest rate for the contract period. This tool uses an actual/360 day count, the usual convention for most pairs, and assumes no transaction costs, credit risk or capital controls. Those frictions are why real forward rates can drift slightly from the theoretical price.
Formula
Forward = spot × (1 + i price × d/360) / (1 + i base × d/360). Rearranged, spot = forward × (1 + i base × d/360) / (1 + i price × d/360) and i price = ((forward / spot) / (1 + i base × d/360) - 1) × 360/d. The annualized forward premium is (forward / spot - 1) × 360/d.
Worked example
A 90-day contract with a spot rate of 1.10, a 4.5% price-currency rate and a 3% base-currency rate gives a forward of about 1.10409. The price-currency rate for the period is 1.125% and the base-currency rate is 0.75%.
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Frequently asked questions
Which currency is the base?
The base currency is the one being bought or sold, usually the first in the pair. The price is quoted as units of the price currency per one unit of the base currency, so a spot of 1.10 means 1.10 price-currency units buy one base-currency unit.
Why does the day count matter?
Interest accrues over the contract, and the interest rates are annualized. Using 360 instead of 365 for a 90-day contract changes the adjustment by roughly 1.4%, enough to move the forward rate in the fourth decimal place.
Is the forward rate a forecast?
No. It is the price at which banks agree to trade today, set by the interest differential and by supply and demand. It can differ from the expected future spot rate, and the difference is the currency risk premium.