What it means
The idea rests on a relationship called covered interest parity. In a market with no frictions, the forward exchange rate should differ from today's spot rate by exactly the interest rate difference between the two currencies, otherwise borrowing in one currency to lend in the other would produce free money.
When the quoted forward rate departs from that fair level, an arbitrage exists. The trade has four legs executed at the same time: borrow in one currency, convert at spot, invest for a fixed term at the other currency's rate, and sell the future proceeds forward at the quoted forward rate.
Every price is known at the outset, so the outcome is not a forecast but a calculation. The catch, and the reason this is not a route to easy money, is that these opportunities are usually tiny and short-lived.
Dealing spreads, transaction costs, credit limits and the capital a bank must hold against the position typically absorb the difference, and any gap large enough to see is normally arbitraged away within seconds by automated systems. That said, persistent gaps do appear, and they are informative rather than free.
Since the global financial crisis, measurable and sustained deviations from covered interest parity have been observed in several currency pairs, generally reflecting balance sheet constraints on banks or year-end funding pressure rather than a genuine free lunch. The distinction from uncovered interest arbitrage matters.
In the uncovered version there is no forward contract, so the investor is betting that the spot rate will not move against them, which is speculation rather than arbitrage and carries real risk of loss.
In practice
Real-world examples.
Example
A bank's treasury desk spots that the three-month forward on a currency pair is out of line with the deposit rates it can actually transact at. It executes all four legs within seconds and books a small, certain margin on a large notional amount.
Example
A corporate treasurer with surplus dollars needs the cash in nine months. Rather than leave it on deposit, he places it in a higher yielding currency and covers the conversion back with a forward, which is the same mechanic applied to real operating cash.
Example
An analyst notices the forward premium on a pair widening sharply every December. The pattern reflects banks shrinking balance sheets over the year end rather than a free profit, and the gap closes again in January.
Formula
Calculation
The fair forward rate, quoted as units of currency A per unit of currency B, = Spot rate x (1 + interest rate on A) / (1 + interest rate on B). An arbitrage exists when the actual quoted forward rate differs from this fair rate.
Suppose the spot rate is $1.2500 per pound, the one-year dollar interest rate is 2%, and the one-year sterling interest rate is 5%. Fair one-year forward = 1.2500 x 1.02 / 1.05 = 1.2143 dollars per pound. But the market is quoting a one-year forward of 1.2300, so sterling is being sold forward at a better price than parity implies.
Start with $1,000,000. Convert at spot: $1,000,000 / 1.2500 = 800,000 pounds. Invest for a year at 5%: 800,000 x 1.05 = 840,000 pounds. At the same moment, sell 840,000 pounds forward at 1.2300, which delivers 840,000 x 1.2300 = $1,033,200 in a year's time.
Compare that with simply investing the $1,000,000 at the dollar rate of 2%, which returns $1,020,000. The arbitrage profit is $1,033,200 - $1,020,000 = $13,200, or 1.32% of the starting amount, locked in at the outset with no exposure to where the exchange rate actually goes. In a real market, dealing spreads on four separate transactions would consume much of that.Case study
Seen in the real world.
Bridgewater Mills is an invented manufacturer used here as an illustrative example, with no connection to any real business. It held $1,000,000 of cash earmarked for a machinery payment due in twelve months, sitting in a dollar deposit paying 2%.
The treasurer noticed that sterling deposits paid 5% while the one-year forward rate stood at 1.2300 dollars per pound, above the 1.2143 that covered interest parity implied. Working with the company bank, she converted the $1,000,000 at 1.2500 into 800,000 pounds, placed it on a one-year sterling deposit reaching 840,000 pounds, and simultaneously sold that 840,000 pounds forward at 1.2300 for $1,033,200.
The board asked the obvious question: what if the pound collapses? The answer was that the forward contract had already fixed the conversion, so the $1,033,200 was contractually certain regardless of the spot rate a year later, leaving only the credit risk of the deposit bank and the forward counterparty. Against the $1,020,000 the dollar deposit would have produced, the fictional company gained $13,200, and the treasurer noted that the real work had been in checking the rates she could actually transact at rather than the ones on the screen.
Watch out
Common mistakes.
- Comparing interest rates between two currencies and concluding the higher one is better. Without accounting for the forward rate, that comparison ignores the currency movement the market is already pricing in.
- Leaving the position uncovered. Skipping the forward contract turns a locked-in arbitrage into a currency bet, and the exchange rate move can easily exceed the interest advantage.
- Using screen rates rather than dealing rates. Bid-offer spreads across four legs, plus fees and credit charges, routinely eliminate an apparent profit that looked real on mid-market quotes.
Questions
People also ask.
Why does covered interest parity hold most of the time?
Because any meaningful gap is traded away immediately by banks and funds with the systems and balance sheet to execute all four legs at once.
What is the difference between covered and uncovered interest arbitrage?
The covered version fixes the return currency rate with a forward contract, while the uncovered version leaves it floating and is therefore speculation.
Can an ordinary company do this?
In principle yes, using deposits and forwards through its bank, but retail and corporate dealing spreads usually make the arithmetic unattractive unless the amounts are large.
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