Currency movements can produce taxable gains even when the underlying investment loses value in the taxpayer's own spending terms. That is the core surprise buried in the foreign currency exchange gain tax rules of both the United States and the United Kingdom: gain and loss are measured in the taxpayer's functional currency, not in the currency of the transaction. Repay a foreign mortgage after the dollar strengthens, close a euro deposit at a favourable rate, or convert Swiss franc proceeds from a house sale — and a computation crystallises that has nothing to do with any economic profit in local terms.
This article walks through how personal currency gains arise, the specific US rules under Internal Revenue Code Section 988, the UK's narrower but not-quite-symmetric treatment, and the mortgage-repayment fact pattern that produces the largest phantom bills. Figures and thresholds are stated as of 2026 and can shift with legislation; verify any specific position with a qualified adviser in the jurisdiction concerned.
The functional currency principle
Every tax system needs a base currency to measure income and gain. In the United States, an individual's default functional currency is the US dollar; in the United Kingdom, an individual's functional currency is generally sterling. When a taxpayer acquires an asset, incurs a liability, or holds a bank balance denominated in a different currency, the tax computation converts each leg into the functional currency at the exchange rate on the relevant date. The difference between the functional-currency amount recognised on acquisition and the functional-currency amount recognised on disposal is the currency gain or loss — separate from any market movement of the underlying asset.
The consequence is uncomfortable: two entirely separate profit-and-loss ledgers run in parallel. A euro-denominated apartment can rise 10% in euro terms while losing 4% in dollar terms, or vice versa. On sale, the US tax code cares only about the dollar movement — but it computes that dollar movement from two distinct sources: the underlying asset gain and, in some cases, a separately identified currency component.
US Section 988: personal transactions, ordinary treatment
The US regime for non-functional currency transactions sits in IRC Section 988 and its regulations. The provision covers foreign-currency-denominated debt, deposits, forward contracts, and — importantly for expatriates — the acquisition and disposition of foreign currency itself. The default rule for a Section 988 transaction is that gain or loss is ordinary, not capital, and is generally sourced by reference to the taxpayer's residence.
Ordinary treatment matters. Long-term capital gains in the United States benefit from a preferential 0/15/20% rate structure and, above the applicable MAGI thresholds, a 3.8% Net Investment Income Tax. Ordinary income can be taxed at the taxpayer's marginal rate, which reaches 37% at the top federal bracket under the OBBBA-permanent TCJA structure — before state tax in states that impose one (0-13.3% additional depending on the state). Converting a euro deposit that has appreciated against the dollar produces ordinary income at those higher rates, not a capital gain at the preferential rate.
The Section 988(e)(2) personal-transaction de minimis
Section 988(e)(2) provides a narrow relief for individuals: a "personal transaction" that generates a foreign-currency gain of US$200 or less is excluded from taxable income. The relief is per transaction, not per year, and applies only to personal transactions such as spending money abroad on a vacation or converting leftover holiday currency. Business, investment, and trade-or-business transactions are outside the de minimis and are fully taxable from the first dollar of gain.
Two features of the relief regularly disappoint taxpayers. First, the US$200 ceiling has been unchanged for decades and is not indexed for inflation. Second, personal-transaction currency losses are generally non-deductible under the disallowance of personal losses at IRC Section 165(c) — the relief is one-way. A traveler who buys €5,000 for a European trip and returns to convert the remaining €1,000 back into dollars can recognise ordinary income if the dollar has weakened by more than US$200 on the unspent balance, but cannot claim a deduction if the dollar has strengthened.
The mortgage-repayment trap
The largest phantom bills tend to arise from Section 988 debt — specifically, foreign-currency mortgages held by US persons abroad. The tax code treats the borrower's obligation as a non-functional-currency liability. Each principal repayment, and any refinance or final payoff, is compared against the dollar equivalent of the original borrowing. If the dollar has strengthened against the loan currency in the intervening period, the borrower recognises ordinary Section 988 gain equal to the reduction in the dollar cost of extinguishing the debt.
Consider a US citizen who borrows £400,000 at an exchange rate of $1.50 per pound to buy a London flat. In dollar terms, the initial liability is $600,000. Some years later, the borrower refinances or sells and pays off the remaining principal — say £250,000 — at a rate of $1.25 per pound, extinguishing $312,500 of dollar liability against a £250,000 principal originally recorded at $375,000. The $62,500 difference is Section 988 gain, taxed at ordinary rates, even though the borrower has not received a single dollar of new income. Layer this on top of a foreign-home-sale computation (see selling a foreign home as a US expat) and the combined bill can materially exceed any local capital-gains liability.
The corresponding loss — when the dollar weakens against the loan currency between borrowing and repayment — is also a Section 988 transaction. For an individual, personal-use mortgage loss on the primary residence has historically been characterised as a non-deductible personal loss, mirroring the asymmetry of the personal-transaction de minimis. Investment-property mortgage losses are treated differently, and the analysis is heavily fact-specific; a qualified US tax adviser should confirm characterisation before relying on any deduction.
Foreign currency accounts and the UK approach
The United Kingdom takes a structurally different route. Historically, foreign currency held in a personal bank account was a chargeable asset for capital gains tax purposes, and every conversion or spending event was in principle a disposal. That regime produced routine phantom gains and record-keeping burdens for expatriates and returnees alike.
Finance Act 2012 amended section 252 of the Taxation of Chargeable Gains Act 1992 with effect from 6 April 2012, removing foreign currency bank accounts held by individuals from the scope of capital gains tax. The relief applies to non-sterling amounts standing to the credit of a bank account held by an individual, the trustees of a settlement, or the personal representatives of a deceased person — but does not extend to physical foreign currency held outside a bank account, nor to currency held in the course of a trade. As a practical matter, most personal foreign-currency exposure of UK residents runs through bank accounts and is therefore outside CGT.
What the exemption does not do is remove currency movement from the CGT computation on the underlying foreign asset. When a UK resident sells a euro-denominated apartment, the gain is computed in sterling: the sterling equivalent of the sale proceeds less the sterling equivalent of the acquisition cost, each converted at the spot rate on the relevant date. Currency movement between acquisition and disposal is baked into the sterling gain and taxed at the CGT rates in force at disposal — from the 30 October 2024 Budget, the main rates are 18% within the basic-rate band and 24% above it, with 24% applying to residential-property gains at both bands. There is no separate currency-only computation for UK residents on a directly-held foreign asset; the gain is a single sterling number.
Foreign-currency mortgages: the UK borrower position
For UK-resident individuals, the picture on foreign-currency debt is more benign than the US Section 988 rule. Movement in the sterling value of a personal foreign-currency mortgage is not, in the ordinary case, a chargeable event for the individual borrower — because a personal-use debt is not itself a chargeable asset in the borrower's hands, and any corresponding gain accrues to the lender rather than the borrower. The mortgage-repayment phantom-gain fact pattern that dominates US expat planning does not have a direct UK-resident analogue in the personal context.
The picture changes if the debt is held in the course of a trade, or by a company (where different rules under the loan relationships regime apply), or where the foreign-currency loan is itself a "debt on a security." Expatriates with mixed structures — for example, a UK resident with a US-dollar mortgage on a US rental property held personally — should confirm characterisation with a UK adviser, because the interaction between s.252 TCGA, the loan relationships regime, and the rental-income computation is not intuitive.
Why the same transaction can be taxed twice
The most disorienting outcome for cross-border individuals is that identical facts can trigger tax under one country's currency rules but not the other's, or under both simultaneously without symmetrical relief. Consider a US citizen tax-resident in the United Kingdom under the post-6 April 2025 arising basis (the non-dom regime having been abolished; qualifying new residents may still elect into the four-year Foreign Income and Gains regime — see UK non-dom abolition):
- On repayment of a euro mortgage, the United States imposes Section 988 ordinary income on the dollar-strengthening component.
- The United Kingdom does not tax the same event, because the borrower's currency movement on personal debt is not a chargeable event.
- Because no UK tax is paid on the transaction, no UK foreign tax credit is available to relieve the US ordinary-income liability.
- The Foreign Earned Income Exclusion of US$132,900 for 2026 is available only against earned income and does not shelter Section 988 gain.
Compare the mechanics of relief in FEIE vs Foreign Tax Credit and the wider fact pattern in US citizens moving abroad. The result is that the US citizen can bear full ordinary-rate US tax on a currency movement that produced no economic gain in either country's ordinary measure — a structural artefact of citizenship-based taxation applied to a system that treats currency as separately taxable.
The comparison at a glance
| Situation | US-resident individual | UK-resident individual |
|---|---|---|
| Gain on converting a foreign-currency bank balance | Ordinary income under §988; US$200 per-transaction de minimis for personal transactions | Outside CGT for individuals under s.252 TCGA 1992 (from 6 April 2012) |
| Gain on repayment of a foreign-currency mortgage | Ordinary Section 988 gain on the dollar-strengthening component; losses on personal-use mortgages generally non-deductible | Not a chargeable event for the individual borrower in the ordinary case |
| Gain on sale of a foreign-currency-denominated investment asset | Currency movement folded into US capital-gain computation on the underlying asset | Currency movement folded into sterling CGT computation on the underlying asset (CGT rates 18% / 24% from 30 Oct 2024) |
| Personal-use currency loss | Generally non-deductible | Outside CGT if in a bank account (no gain, no loss) |
What records taxpayers actually need
Section 988 computations, and any UK CGT computation involving foreign-currency-denominated assets, cannot be reconstructed after the fact from year-end balances. The information a taxpayer needs to retain, contemporaneously, includes:
- The exchange rate on the date each foreign-currency asset was acquired or each foreign-currency debt was drawn down
- The exchange rate on the date of each principal repayment, conversion, or partial disposition
- The source of the exchange rate used — spot rate on the transaction date is the general rule for individuals; the US Treasury publishes reporting rates for FBAR/FATCA purposes, but these are period averages and are not the correct rate for a Section 988 computation on a specific transaction
- Loan statements showing beginning principal, each repayment, and ending principal in the loan currency
- Bank statements for any account into which a large foreign-currency inflow was received and later converted
These records overlap with — but are not the same as — the records needed for FBAR and FATCA reporting. Reporting-basis exchange rates cannot be reused for gain-computation purposes without introducing distortion.
Planning levers, within the rules
Currency-gain risk cannot be eliminated for a taxpayer holding foreign-currency assets or debts, but its magnitude can be managed:
- Sequencing. Refinancing a foreign-currency mortgage in the same currency crystallises Section 988 gain or loss on the extinguished debt just as fully as sale of the property. Timing the refinance during a period of weaker dollar can reduce or defer gain — subject to broader financial considerations.
- Currency choice on borrowing. A US citizen buying a foreign home has a real economic decision to make: borrow in local currency (interest-rate advantage, currency risk on the debt) or borrow in dollars against US assets (no Section 988 exposure, no currency risk on the debt). The tax-neutral option is often the dollar loan.
- Bank account structure. UK residents can consolidate foreign-currency exposure into a personal bank account structure that falls within the s.252 TCGA exemption, avoiding CGT drag on incidental currency movement.
- Coordinated sale timing. When a foreign home is sold, the sale-proceeds conversion, the mortgage payoff, and the associated Section 988 computations can compound. Sequencing conversions across tax years, where market conditions permit, can spread ordinary-income recognition.
See also capital gains timing when moving abroad and sending money abroad tax implications for related planning surfaces. None of the above is a substitute for jurisdiction-specific advice on the individual's fact pattern.
Where to go next
For the country context underlying this article, start with the United States and United Kingdom profiles, then compare rate regimes on the country comparison tool. Related expat computations are covered in selling a foreign home as a US expat, rental property abroad tax, and streamlined filing for US expats. Broader residency mechanics are set out in how tax residency works, and common cross-border questions are collected in the TaxAtlas FAQ. Currency taxation is highly fact-dependent, and the amounts at stake in a single mortgage repayment can be significant — engage a qualified adviser in both the source and residence jurisdictions before making structural decisions.