A forex carry trade aims to earn the difference between interest rates in two currencies. The trader takes exposure to the higher yielding currency and funds it through the lower yielding currency, hoping that exchange rate movements do not wipe out the income.
The attraction is straightforward: a position can generate income even when its exchange rate barely moves. The risk is less comfortable. Interest builds gradually, while a currency loss can arrive in minutes. For UK traders, the distinction between a headline interest rate differential and the overnight adjustment actually credited to a trading account is especially important.
How a Forex Carry Trade Works
In a traditional carry trade, an investor borrows one currency, converts it into another and buys an interest bearing asset. The borrowing currency is the funding currency; the currency receiving the investment is the target currency. Derivatives can create similar exposure without a separate cash loan, as set out in the BIS analysis of carry trade structures.
Suppose an investor can borrow currency A at 2% annually and invest in currency B at 5%. The gross interest rate differential is three percentage points. If the exchange rate stays unchanged for a year, that difference produces a positive return before transaction costs and other charges.
However, the debt remains payable in currency A. If currency B falls sufficiently against A, converting the investment back will leave a loss despite the interest earned. A carry trade therefore combines a funding decision with a directional currency position. It is not an interest account with a more interesting name.
Which Direction Does the Trade Take?
Consider a hypothetical GBP/JPY trade where sterling offers the higher interest rate. Buying GBP/JPY means buying pounds and selling yen, which is the direction associated with positive gross carry under those assumptions. Selling GBP/JPY takes the opposite exposure.
The order depends on how the pair is written, not on which currency sounds more attractive. In forex quotes and currency pairs, the first currency is the base currency. Buying the pair buys that currency against the second one. A pound denominated account does not automatically make sterling the funding currency for every trade.
Calculating Carry and Total Return
A useful starting estimate is:
Gross carry ≈ position value × annual interest rate differential × holding period in years.
This is a planning approximation, not a broker payment formula. It assumes stable rates and broadly stable conversion values. The actual result depends on the instrument, funding terms, exchange rate and charges.
For a hypothetical £20,000 equivalent position with a three percentage point annual differential, estimated gross carry over 90 days is:
£20,000 × 0.03 × 90 ÷ 365 = £147.95.
Compare that income with the position’s currency exposure, not just the cash deposited as margin. A 1% adverse move on £20,000 represents approximately £200, already more than the estimated carry for those 90 days.
A Worked Cash Carry Example
Assume an investor borrows ¥2,000,000 for one year at 1%, converts it into £10,000 at GBP/JPY 200, and earns 4% on the sterling investment. These are illustrative rates, not current market quotes. Ignore fees and assume both rates remain fixed.
After one year, the sterling investment is worth £10,400 and the yen debt is ¥2,020,000. The closing exchange rate determines the result:
| Closing GBP/JPY | Sterling investment converted to yen | Profit or loss after repaying debt |
|---|---|---|
| 210 | ¥2,184,000 | ¥164,000 profit |
| 200 | ¥2,080,000 | ¥60,000 profit |
| 195 | ¥2,028,000 | ¥8,000 profit |
| 190 | ¥1,976,000 | ¥44,000 loss |
The break even closing rate is approximately 194.23, calculated as ¥2,020,000 divided by £10,400. Sterling can fall about 2.88% from the opening rate before the interest advantage disappears. That cushion is slightly less than the three percentage point differential because the exchange rate also affects the sterling interest received.
Why Retail Overnight Credits Differ From Policy Rates
For a retail rolling forex position, do not treat the difference between two central bank rates as the amount you will receive. Check the provider’s actual overnight adjustment for the pair, direction and position size. A positive theoretical differential does not guarantee a positive account credit.
Provider pricing matters. The FCA review of CFD pricing and overnight funding, which includes rolling spot forex, found wide differences in funding charges. In some cases, firms charged for positions that other providers would have credited. It also highlighted charges calculated on full underlying exposure rather than simply the margin deposited.
Before estimating income, check the adjustment’s units, conversion into your account currency, rollover cut off, and treatment of weekends and holidays. Confirm whether the displayed figure already includes the provider’s markup. Otherwise, a spreadsheet can look precise while counting the same cost twice.
Suppose a platform quotes a net credit of £1.20 per chargeable day for your proposed position. Over an assumed 30 chargeable days, that would produce £36. If opening and closing costs total £18, only £18 remains before any currency movement. On those assumptions, the first 15 days of credits merely recover trading costs.
Keep that estimate separate from a forecast. Recheck the actual terms rather than assuming one displayed overnight rate will remain available throughout a long holding period.
Why Hedging Does Not Preserve a Free Interest Spread
An obvious question follows: why not earn the higher interest rate and lock in the future exchange rate, removing the currency risk?
Under covered interest parity, the forward exchange rate adjusts for the interest rate difference between comparable assets. The forward price is therefore not normally the same as the spot price. The Bank of England discussion of interest parity and carry trades sets out this relationship and distinguishes it from leaving currency exposure unhedged.
Using the earlier assumptions, the theoretical one year GBP/JPY forward rate is:
200 × 1.01 ÷ 1.04 = approximately 194.23.
Selling the final £10,400 at that forward rate produces ¥2,020,000, exactly enough to repay the borrowing. The apparent interest advantage disappears in this simplified example before costs.
This does not mean a forward rate predicts where the spot rate will finish. It is a pricing relationship under stated assumptions. Nor does buying a currency forward remove exchange rate risk from an otherwise open speculative position: what matters is the combined exposure of all the transactions.
Interest Rate Differentials Can Change
A carry trade should not be selected by ranking headline policy rates alone. Relative interest rates influence demand for currencies, but other forces also affect their value. The Reserve Bank of Australia’s explanation of exchange rate drivers separates interest rate effects from factors such as commodity prices and changing appetite for risk.
For trade planning, distinguish between the income available now and the income assumed over your intended holding period. Suppose currency B yields 5% and currency A costs 2%, but your scenario assumes B’s rate falls to 3% halfway through the year. Applying a three percentage point differential to the entire year would overstate the projected carry.
Run at least a stable rate case and a narrowing differential case. Also ask what would happen if the funding rate rose while the investment rate fell. That combination reduces both sides of the interest advantage.
For sterling positions, the relevant comparison involves both central banks, not just the UK rate. Our guide to how Bank of England decisions affect sterling covers the distinction between a policy announcement and the expectations already reflected in market prices.
What Happens During a Carry Trade Unwind?
A carry trade unwind occurs when traders close positions by selling investment currencies and buying back funding currencies. If many participants do this together, their transactions can reinforce the price moves that caused the exits.
August 2024 provided a clear example. Currency carry positions came under pressure as markets reassessed economic conditions and investors reduced borrowed exposure. Yen appreciation and margin pressures helped amplify the moves, documented in the BIS analysis of the August 2024 carry trade unwind. Markets subsequently stabilised, but that did not remove losses already realised by traders forced to exit.
The practical issue is timing. A position may have an attractive projected annual income and still suffer an unaffordable loss next week. Planning around the eventual interest receipt does not solve an immediate cash requirement.
Consider a portfolio holding long GBP/JPY, AUD/JPY and NZD/JPY positions. The currency names differ, but all three positions sell yen. For a stress test, assume they move against you together rather than treating them as independent bets. This is a direct application of correlation and hidden concentration across trades.
Position Size Matters More Than the Advertised Yield
Suppose £5,000 of account equity supports a £25,000 equivalent currency position. A 4% adverse move would produce an approximate £1,000 trading loss before funding and costs: 20% of the original equity.
By comparison, a 3% annual net carry assumption on that exposure produces about £61.64 over 30 days. The £1,000 price loss is more than sixteen times that monthly income. These are simplified figures, but they show why a carry projection belongs beside a loss scenario, not on its own.
Work backwards from an affordable loss rather than forwards from a desired daily credit. If doubling the position makes the income appealing only by making a plausible currency move unaffordable, the position is too large for that risk budget.
Use pip values and forex position sizing to translate adverse price moves into account currency. Include room for transaction costs and test outcomes worse than the planned exit. Do not count future carry as money available to absorb a loss today.
Evaluating a Carry Strategy Before Committing Money
A useful carry assessment should answer four questions:
- What is the net income? Use executable funding terms, not just policy rates.
- What currency move erases it? Calculate the break even move over the intended holding period.
- What changes invalidate the trade? Set conditions for reviewing rates, costs and currency exposure.
- Can the account withstand the stress case? Assess combined positions and cash requirements.
For a historical test, require funding inputs that belong to the period being tested. Applying one present day overnight credit across years of price data would test an invented income stream. Where historical funding data is missing, label the assumption and repeat the calculation using less favourable credits or outright charges.
Separate exchange rate profit from carry income in the results. A profitable currency trend can conceal an unhelpful funding arrangement, while steady credits can make a losing position look productive. The process for building and testing a forex strategy should account for both, alongside costs and drawdowns.
The decision is not whether one currency pays more than another. It is whether the net income offers enough compensation for the currency exposure, funding uncertainty and potential losses. Positive carry helps a trade’s arithmetic; it does not make the trade safe.