Interest rate differentials and exchange rates: theory and practice

An interest rate differential is the difference between the interest rates of two currencies, for example a US rate of 4% against a euro-area rate of 2%. It matters for exchange rates because money tends to be drawn towards higher returns, so a widening differential in a currency's favour often supports it, other things being equal. In practice, what moves currencies is usually a change in the expected differential, and textbook theory predicts something quite different from what markets have often done.

This guide explains the two versions of interest rate parity (covered and uncovered), works through the arithmetic of forward prices, explains why high-yielding currencies have often not behaved as the theory predicts, and shows how traders follow differentials in practice through yield spreads and central-bank guidance. It ends with how the differential shows up in your overnight swap. It is educational; it does not forecast any currency.

What an interest rate differential is

Every currency has a set of interest rates attached to it: the central bank's policy rate, short-term money-market rates and government bond yields of different maturities. An interest rate differential compares the same kind of rate across two currencies.

When traders talk about a pair's differential they usually mean one of three things:

• The policy-rate gap. For example, the Federal Reserve's target rate against the ECB's deposit rate. The central banks guide explains how each is set.
• The short-dated yield gap. Commonly the two-year government bond yield of one country minus the other's. Two-year yields move with expectations for policy over roughly the next two years, which makes them a useful market-based summary.
• The longer-dated gap. Ten-year yields, which also reflect growth, inflation expectations and risk premia over a longer horizon. The yield curve connects these maturities.

Covered interest parity: how forward prices are set

Covered interest parity (CIP) is the principle that explains forward exchange rates. It says that you should not be able to earn the interest rate gap without risk by borrowing in one currency, converting it, depositing it in the other and locking in the conversion back with a forward contract. If you could, everyone would do it until the opportunity disappeared.

For a pair quoted as base/quote, the relationship for one year is approximately:

forward = spot × (1 + quote-currency rate) ÷ (1 + base-currency rate)

Worked example: a one-year EUR/USD forward

These are illustrative numbers, not current rates.

EUR/USD spot is 1.1000. The one-year US dollar rate is 4% and the one-year euro rate is 2%.

1. forward = 1.1000 × 1.04 ÷ 1.02
2. 1.1000 × 1.04 = 1.1440
3. 1.1440 ÷ 1.02 = about 1.1216

The one-year forward is about 1.1216, roughly 216 pips above spot. The euro, the lower-yielding currency, trades at a forward premium; the dollar, the higher-yielding one, at a forward discount. Someone who holds dollars earns 2 percentage points more interest but, if they lock in the conversion back to euros at the forward rate, gives that advantage back through the worse forward price.

CIP is a pricing relationship enforced by arbitrage, and in normal conditions it holds closely for the major currencies. Since the 2008 financial crisis economists have documented persistent small deviations, known as the cross-currency basis, linked to bank balance-sheet costs and demand for dollar funding. For a retail trader the key point is simpler: the forward price, and therefore the cost of holding a position over time, is set by the rate gap.

Uncovered interest parity: the theory that often fails

Uncovered interest parity (UIP) makes a stronger claim. It says that without any forward contract, the expected change in the spot rate should offset the interest rate gap. In the example above, UIP says investors should expect the euro to rise by roughly 2% against the dollar over the year, so that holding dollars at 4% and holding euros at 2% produce the same expected return.

Put simply, UIP predicts that the higher-yielding currency should depreciate, on average, by about the interest rate gap.

The evidence has not been kind to this prediction. A long line of academic research, often traced to work by Eugene Fama in the 1980s, found that higher-yielding currencies have frequently not depreciated as much as UIP implies, and over many periods have tended to appreciate. This is known as the forward premium puzzle. It is the reason the carry trade exists: borrowing in a low-yielding currency to hold a high-yielding one has, over many historical periods, earned the interest gap without giving it all back through exchange-rate moves.

The catch is how the carry trade has tended to lose when it loses. The returns have historically come with occasional sharp, sudden reversals, often during episodes of market stress when investors unwind positions at the same time. Traders describe this as carry going up the stairs and down in the lift. The unwinding of yen-funded positions in early August 2024 is a frequently cited recent example. Explanations for the puzzle include compensation for exactly this crash risk, along with shifting risk premia and expectations errors. The short version: UIP fails on average, but not in a way that makes high yields a free lunch.

How differentials move currencies in practice

The textbook relationships describe levels. Day to day, exchange rates react to changes in expectations.

Expectations, not decisions

By the time a central bank changes rates, the move is usually priced in. What moves a currency is a change in the expected path of rates: a data surprise, a shift in guidance, or a change in what markets think the next few meetings will bring. The economic data guide and the FOMC meetings guide explain how surprises and guidance work. Hawkish and dovish shifts in language can move a currency on days when rates do not change at all.

Yield spreads as a summary

Many analysts chart a pair against the difference between the two countries' two-year yields. When the spread moves in one currency's favour, the pair often moves in the same direction over weeks or months. The relationship is loose, breaks down for long stretches and is not a trading signal, but it is a quick way to see whether a currency's move is consistent with rate expectations or driven by something else.

Worked example: a narrowing spread

Illustrative numbers only.

The US two-year yield is 4.0% and the German two-year yield is 2.0%, a spread of 200 basis points in the dollar's favour. Over a month, weaker US data leads markets to price more US rate cuts, and the US two-year yield falls to 3.6%. Euro-area expectations barely change and the German two-year yield is still 2.1%.

1. The spread was 4.0 − 2.0 = 2.0 percentage points (200 basis points).
2. It is now 3.6 − 2.1 = 1.5 percentage points (150 basis points).
3. It has narrowed by 50 basis points against the dollar.

All else being equal, that would tend to weaken the dollar against the euro, pushing EUR/USD up. Whether it actually does depends on everything else going on, from risk sentiment to positioning to energy prices, and the size of any move cannot be calculated from the spread change.

Real rates

Nominal rates are only part of the picture. If one country has 5% interest rates and 6% inflation, its real rate (the rate after inflation) is negative. Investors care about returns after inflation, and a high nominal rate caused by high inflation may not support a currency at all. The inflation and currencies guide explains how inflation feeds into central-bank reactions and real rates.

When differentials do not drive the price

Rate gaps are one driver among several, and for long periods others dominate. The what moves currency prices guide covers the full list. Common situations where differentials matter less:

• Risk-off episodes. In a sharp sell-off, safe-haven currencies such as the yen and Swiss franc can strengthen even when their rates are lowest, because carry positions are closed and capital moves to safety.
• Intervention. Authorities sometimes trade in the market to slow a currency's move. Japan's Ministry of Finance bought yen in September and October 2022 after a long period in which a widening US–Japan rate gap had weakened the yen. The Japanese yen guide covers this history.
• Crisis and credit concerns. A high yield that reflects default or political risk may attract little capital.
• Positioning. When many traders already hold the same carry position, there may be few new buyers, and an unwind can move the pair sharply against the differential.
• Terms of trade. Commodity prices can dominate for currencies such as the Australian or Canadian dollar, as the commodity currencies guide explains.

How the differential reaches your account: swaps

For a retail trader holding a forex CFD overnight, the interest rate differential is not abstract. It sets the swap, the overnight financing charge or credit applied at the daily rollover. Holding the higher-yielding currency against the lower-yielding one tends to earn a credit before broker charges; holding the reverse tends to cost money. The swap and overnight financing guide explains the mechanics, including the triple charge on Wednesdays.

Worked example: the gross carry on one lot

Illustrative numbers only.

You hold a position with a notional value of $100,000, long a currency yielding 3 percentage points more than the one you are short.

1. Gross interest gap per year: $100,000 × 3% = $3,000.
2. Per night: $3,000 ÷ 365, about $8.22.

That is the theoretical gross figure. Brokers add a markup, so the credit you receive is lower than this and the charge on the opposite position is higher. On some pairs both long and short positions pay. The swap calculator estimates the overnight cost from your broker's swap rates, lots and number of nights.

Two practical points follow. First, a small swap credit is easily wiped out by an adverse exchange-rate move: $8 a night is less than a one-pip move on a standard lot of many pairs. Second, the position is leveraged, so the exchange-rate risk applies to the full notional while the interest is earned on it too. The leverage and margin guide explains why that magnifies losses as well as gains.

How to study differentials yourself

You do not need a forecast to use this framework. An educational approach:

1. Track the policy rates of the currencies you trade and the dates of their central banks' meetings, using the economic calendar guide as a starting point.
2. Chart a pair against its two-year yield spread over several years. Note the periods where they move together and the periods where they diverge, and look up what else was happening.
3. Compare your broker's swap rates with the policy-rate gap to see how much markup is applied.
4. Note reactions to surprises. When a data release changes rate expectations, record how far the yield spread and the pair moved. Over time you build your own picture of how sensitive each pair is.

If you build a rule from this, such as only holding positions where the differential is in your favour, test it like any other strategy, as the backtesting guide explains, and include the occasional sharp unwind in the test period.

Common mistakes

• Treating a high yield as a reason to buy. The interest earned is small next to typical exchange-rate moves, and high yields can carry high risks.
• Reacting to decisions that were expected. If the market priced the decision, the information was already in the price.
• Ignoring real rates. A high nominal rate eaten by inflation may not support a currency.
• Assuming UIP or carry relationships are stable. Both have held and failed for long periods; neither is a rule.
• Forgetting swap markups. The swap you receive is not the full differential, and the swap you pay is more.

Frequently asked questions

What is an interest rate differential in forex?

It is the difference between the interest rates of the two currencies in a pair, for example the US policy rate minus the euro-area rate for EUR/USD. It affects forward prices, the overnight swap on CFD positions and, through investors' search for returns, the exchange rate itself.

Does a higher interest rate always strengthen a currency?

No. Higher rates, or expectations of them, often support a currency, other things being equal. But if the move was already expected, if inflation is high enough to make real rates low, or if risk sentiment, intervention or political risk dominates, the currency can weaken despite higher rates.

What is the difference between covered and uncovered interest parity?

Covered interest parity says forward exchange rates adjust so the rate gap cannot be earned without risk with a forward contract; it is enforced by arbitrage and holds closely in normal markets. Uncovered interest parity says the expected change in the spot rate should offset the rate gap without any hedge; historically that prediction has often failed.

What is the forward premium puzzle?

It is the observation that high-yielding currencies have, over many periods, not depreciated by as much as uncovered interest parity predicts, and have sometimes appreciated. It is the main reason carry trades have earned returns historically, along with the risk of sharp losses during market stress.

Why do traders watch two-year yields?

Two-year government bond yields reflect expectations for central-bank policy over roughly the next two years, and they move continuously as data and guidance change. The difference between two countries' two-year yields is therefore a convenient market-based measure of the expected rate differential.

How does the interest rate differential affect my swap?

The overnight swap on a forex CFD is based on the rate differential between the two currencies, adjusted by the broker's markup. Holding the higher-yielding currency usually earns a smaller credit than the full differential, and holding the lower-yielding one usually pays more than it. Check the swap rates in your platform's contract specification.