US UK Tax Returns Preparation: Capital Loss Carryforwards

In US UK tax returns preparation, one capital loss creates two carryforwards that never agree. The IRS measures it in dollars, caps the deduction against ordinary income and carries it forward automatically; HMRC measures it in sterling and recognises it only if claimed in time. Each needs its own schedule.

For a dual filer with a substantial portfolio, a large realised loss is not a single number. It is two numbers, computed in two currencies, under two matching conventions, for two different tax years, and then consumed at two different speeds. At Jungle Tax we prepare both returns side by side, and the reconciliation of capital loss pools is one of the places where self-prepared and single-country-prepared returns most often go wrong. This guide explains how each system computes, reports and carries forward a capital loss, why the figures drift apart, and how the two schedules are maintained and, where necessary, rebuilt.

Why do US and UK capital loss carryforwards never match?

There are five structural reasons, and any one of them is enough to break the link between the two returns:

• Currency. The US return computes every gain and loss in dollars; the UK return computes it in sterling. Exchange-rate movement between purchase and sale changes the size of the result and can change its sign.
• Share identification. The US works lot by lot. The UK pools shares of the same class and uses an average cost, after applying same-day and 30-day matching rules.
• Tax year. The US individual tax year ends on 31 December. The UK tax year ends on 5 April. A disposal in the first quarter of the calendar year belongs to different reporting periods.
• Rate of use. A US net capital loss can absorb unlimited capital gains plus a small annual amount of ordinary income. A UK loss is set only against chargeable gains, and brought-forward losses are preserved to the extent gains are already covered by the annual exempt amount.
• Existence. A US carryover arises by operation of law from the figures on the return. A UK loss is not an allowable loss until it has been notified to HMRC within the statutory time limit.

The result is that the carryforward shown on the US Schedule D and the losses carried forward on the UK capital gains pages are independent records. Treating one as a translation of the other is the root error.

How does the US compute and carry forward a capital loss?

Netting short-term and long-term on Schedule D

Each disposal is listed on Form 8949 and summarised on Schedule D (Form 1040). Assets held for one year or less produce short-term results; assets held for more than one year produce long-term results. Short-term gains and losses are netted against each other, long-term gains and losses are netted against each other, and the two net figures are then combined. A net loss in one category reduces a net gain in the other.

What is the annual limit against ordinary income?

If the combined result is a net capital loss, an individual may deduct it against other income only up to $3,000 a year, or $1,500 for a married person filing separately. The IRS summarises the rule in Topic no. 409, Capital gains and losses. The figure is fixed by statute and is not indexed for inflation, which is why a six- or seven-figure loss can take many years to exhaust if there are no later gains to absorb it. There is no ceiling on the amount of a carryover that may be set against capital gains of a later year.

Indefinite carryforward that keeps its character

The unused loss carries forward indefinitely for an individual. It is not a single pool: the short-term portion carries forward as a short-term loss and the long-term portion as a long-term loss, and each re-enters the netting process of the following year in that character. The split is calculated on the Capital Loss Carryover Worksheet, which appears in the Schedule D instructions and in Publication 550. The worksheet is not filed, so the only evidence of how the carryover was derived is the preparer's own copy. That is the document most often missing when a file changes hands.

Two further points catch dual filers. First, the amount treated as used each year is governed by the worksheet and not by whether the deduction produced a tax benefit, so a carryover can be eroded in a low-income year. Second, a married couple filing jointly has a combined carryover that must be allocated between them if the filing status later changes, for example where one spouse is not a US person and the couple move between joint and separate filing.

Dollar basis: a sterling loss can be a dollar gain

For US purposes, the cost of an asset bought in sterling is translated into dollars at the exchange rate on the acquisition date, and the proceeds at the rate on the disposal date. The two legs are translated separately. It is not correct to compute a sterling gain or loss and translate the net figure at a single rate.

Consider two holdings sold by a US citizen resident in the UK:

• Holding A was bought for £400,000 when £1 bought $1.50 (dollar cost $600,000) and sold for £340,000 when £1 bought $1.25 (dollar proceeds $425,000). The UK loss is £60,000. The US loss is $175,000.
• Holding B was bought for £400,000 when £1 bought $1.20 (dollar cost $480,000) and sold for £380,000 when £1 bought $1.35 (dollar proceeds $513,000). The UK return shows a loss of £20,000. The US return shows a gain of $33,000.

Holding B is the case that produces genuinely irreconcilable schedules: an allowable loss carried forward in one country and tax on a gain in the other, from the same contract note.

Wash sales and personal-use assets

A loss is deferred under the wash sale rule where substantially identical securities are acquired within 30 days before or after the sale, with the disallowed amount added to the basis of the replacement holding. Separately, a loss on property held for personal use, such as a home, a car or a yacht, is not deductible at all, even though a gain on the same asset would be taxable. Personal-use losses therefore never enter the carryover.

Interaction with the foreign tax credit and NIIT

Capital losses also flow into the foreign tax credit limitation on Form 1116. In general terms, foreign source capital gains and losses are netted within and across the limitation categories, a foreign source capital loss can reduce the foreign source income on which the limitation is calculated, and adjustments are required where foreign and US source capital results have opposite signs or where gains are taxed at preferential rates. The instructions to Form 1116 and Publication 514 set out the worksheets. The practical consequence is that a large loss year can reduce the credit available for UK tax paid on other income, and a carryover used in a later year has to be traced back to its source.

The Net Investment Income Tax is a 3.8% charge on net investment income above modified adjusted gross income thresholds of $200,000 for a single filer and $250,000 for a married couple filing jointly. Capital losses reduce net gains included in the base, and carryovers are taken into account, but the computation on Form 8960 follows its own rules and is not simply a copy of the Schedule D total. The UK-US treaty position on crediting foreign tax against this charge is a separate question and is outside the scope of this guide.

How does the UK compute and carry forward a capital loss?

A UK loss must be claimed to exist

This is the single most important difference. Under UK rules a capital loss is only an allowable loss if it is notified to HMRC, and the claim must be made within four years of the end of the tax year in which the loss arose. HMRC's public guidance on Capital Gains Tax losses confirms the time limit. A loss realised in the tax year ended 5 April 2023 must therefore be claimed by 5 April 2027. Once validly claimed, the loss carries forward without time limit until used.

The claim is normally made by entering the loss on the capital gains pages of the Self Assessment return for the year of the loss and attaching a computation. A person who is not within Self Assessment may claim in writing, stating the amount. A US citizen who has never been asked for a UK return, or who files one without capital gains pages because no tax was due, can therefore hold a perfectly good US carryover alongside a UK loss that is quietly running out of time.

Order of set-off and the annual exempt amount

Losses of the current year are set against gains of the same year first, and in full. There is no ability to restrict a current-year loss to preserve the annual exempt amount, which is £3,000 for 2024-25 and later years. Brought-forward losses are treated differently: they are used only to the extent needed to reduce net gains to the annual exempt amount, and the balance continues to carry forward. Where gains are taxable at different rates, the taxpayer may allocate losses against gains in the most beneficial way.

No offset against income

There is no UK equivalent of the US deduction against ordinary income. A UK capital loss sits unused until there is a chargeable gain, however long that takes. The one narrow exception is share loss relief, under which a loss on qualifying shares subscribed for in an unquoted trading company may, on a claim, be set against general income instead.

Clogged losses on disposals to connected persons

A loss arising on a disposal to a connected person, broadly certain relatives, business partners and companies under the individual's control, is a "clogged" loss. It may be set only against gains on other disposals to the same connected person, made while the connection continues. It must be tracked in its own column and cannot be merged into the general pool. The US has its own related-party loss disallowance rules, with a different list of related parties and a different mechanism, so the same transaction can be restricted in one country and not the other. Disposals between spouses who live together take place on a no gain, no loss basis for UK purposes, so no loss arises at all.

Losses of non-resident years

An individual who is not UK resident is generally outside the charge to UK capital gains tax, apart from disposals of UK land and certain related assets. It follows that a loss on, say, a US brokerage portfolio realised in a year of non-residence is not an allowable UK loss and cannot be brought into the UK pool on arrival. Losses on UK land realised while non-resident are within the UK regime and are reported under the non-resident rules. Where an individual is only temporarily non-resident and returns within the statutory period, gains and losses on assets held before departure may be treated as arising in the year of return. For former remittance basis users, foreign losses of earlier years may not be allowable at all unless an election was made at the relevant time, which needs to be confirmed from the historic returns.

Reporting on the SA108

The capital gains pages (SA108) ask for the total losses of the year, the brought-forward losses used in the year, and the losses available to carry forward, by reference to the category of asset. Computations are submitted as an attachment. UK residential property disposals that give rise to tax must also be reported within 60 days of completion, and that in-year return has its own rules on which losses can be taken into account.

US vs UK capital loss rules compared

Feature	United States (IRS)	United Kingdom (HMRC)
Currency of computation	US dollars, each leg translated at the rate on its own date	Sterling, each leg translated at the rate on its own date
Tax year	Calendar year to 31 December	6 April to 5 April
Share identification	Lot by lot; specific identification or first in, first out	Same-day rule, 30-day rule, then the Section 104 pool at average cost
Holding period distinction	Short-term and long-term netted separately	None
Is a claim required?	No; the carryover arises from the return	Yes; within four years of the end of the tax year of loss
Offset against other income	Up to $3,000 a year ($1,500 married filing separately)	None, apart from share loss relief on qualifying shares
Tax-free threshold	None; a 0% rate band may apply to long-term gains	Annual exempt amount of £3,000
Use of brought-forward losses	Against all gains, then the annual ordinary income limit	Only down to the annual exempt amount
Carryforward period	Indefinite, retaining short-term or long-term character	Indefinite once claimed
Repurchase within 30 days	Loss deferred into basis of replacement shares	Disposal matched to the reacquisition
Connected or related party sale	Loss disallowed under related-party rules	Loss clogged; usable only against gains with the same person
Personal-use assets	Loss not deductible	Loss generally not allowable on exempt assets and wasting chattels
Where reported	Form 8949 and Schedule D; carryover worksheet retained	SA108 capital gains pages with computation attached

Why does the same sale produce two different loss figures?

Currency

The worked figures above show the principle. In practice the effect compounds, because every purchase in a long-held portfolio has its own historic exchange rate. A dollar-denominated US portfolio held by a UK resident has the mirror-image problem: the US computation is straightforward, while the UK computation requires every purchase and sale to be translated into sterling. A US fund that fell 10% in dollar terms can show a sterling gain if the dollar strengthened over the holding period. Foreign currency held in its own right is a further layer, with each country applying its own rules to currency gains and losses on bank balances.

Pooling versus lot identification

Suppose 10,000 shares were bought in three tranches at rising prices and 4,000 are sold. The US computation uses the cost of the specific lots identified as sold, or the earliest lots if none are identified. The UK computation first asks whether any shares of the same class were acquired on the same day or in the following 30 days, and then takes 4,000 shares' worth of the average cost of the whole pool. The US result might be a loss on the most recent high-cost lot. The UK result, using the blended cost, might be a smaller loss or a gain. Neither is wrong, and the difference reverses only when the entire holding is sold.

Timing of the tax year

A loss realised on 20 February 2026 falls in the UK tax year 2025-26 and the US tax year 2026. A gain realised on 20 November 2025 falls in the same UK year but in the earlier US year. On the UK return the two are netted as current-year items. On the US returns they fall in different years, so the gain is taxed in 2025 and the loss carries forward from 2026 with no ability to carry it back. The mismatch also determines which year's foreign tax is available for credit, which is why the two returns have to be planned as a pair at preparation stage and not reconciled after the event.

Rate of use: a two-year illustration

Take Holding A in isolation. On the US side, the $175,000 loss in the first year is deducted as to $3,000, leaving a $172,000 carryover. If the following year brings $40,000 of gains, the carryover absorbs them and a further $3,000 is deducted against ordinary income, leaving $129,000. On the UK side, the £60,000 loss is claimed and carried forward in full. If the following UK year brings £30,000 of gains, £27,000 of the brought-forward loss is used to reduce them to the £3,000 annual exempt amount, leaving £33,000. After two years the schedules read $129,000 and £33,000, and no exchange rate connects them.

How should two loss schedules be maintained?

Good practice in US UK tax returns preparation is a single transaction ledger feeding two separate loss schedules. For every acquisition and disposal the ledger should hold the trade date, quantity, price in the currency of the transaction, incidental costs, and the dollar and sterling values at a consistently sourced exchange rate for that date. From that ledger:

• The US schedule records, for each calendar year, the net short-term and net long-term result, the amount deducted against ordinary income, and the short-term and long-term carryover to the next year, agreed to the Capital Loss Carryover Worksheet. It should also note the source of each loss for Form 1116 purposes and any wash sale basis adjustments still embedded in holdings.
• The UK schedule records, for each year to 5 April, losses arising, the date and method by which each was claimed, losses used against current-year gains, brought-forward losses used, and the balance carried forward. Clogged losses and losses relating to UK land in non-resident years are kept in separate columns, together with the Section 104 pool cost for each holding.

The two schedules should be reviewed together each year, not to force agreement, but to confirm that every difference is explained by currency, identification method, tax year or rate of use. An unexplained difference is usually a missed transaction or a loss that was never claimed. Individuals with larger portfolios will find the high net worth reporting issues that sit alongside this, including broker reports issued on a calendar-year, single-currency basis that suit neither return without adjustment.

How is an untracked carryforward rebuilt on late or self-prepared returns?

We regularly see carryforwards that were dropped when software was changed, when a preparer was replaced, or because a return was never filed for a loss year. The approach differs by country.

US. The carryover is a matter of computation, not election. It is rebuilt by starting from the year the loss arose and working the Capital Loss Carryover Worksheet forward year by year, using the taxable income actually reported, including years that are now closed. If the loss year was never filed, that return has to be prepared so that the figures exist. Where a carryover was simply omitted from later open-year returns, those returns can be amended. Where the missing years are part of a wider pattern of non-filing by someone resident outside the US, the reconstruction is usually done within a catch-up submission prepared by IRS streamlined filing specialists, so that the opening carryover on the first compliant return is supportable.

UK. The computation can always be reconstructed, but the loss itself survives only if it is claimed within the four-year window. Losses from years still in time can be claimed on a late return, by amending a filed return where the amendment period is open, or by a standalone written claim. Losses from years outside the window are, as a general rule, no longer available, and the UK schedule must start from a lower figure than the economic loss would suggest. We cover that deadline in detail in a separate guide in our guides library; the point for present purposes is that the US and UK pools can diverge permanently for no better reason than a missed claim.

In both countries the rebuilt schedule should be supported by contract notes or broker statements, the exchange rates used, and a short note of the method, kept with the return file.

Common preparation errors we correct

• Translating the net sterling loss into dollars at an average rate instead of translating cost and proceeds separately.
• Copying the US carryover onto the UK return, or the UK figure onto Schedule D, at a spot rate.
• Using broker-reported lot-level results for the UK return without applying the same-day, 30-day and pooling rules.
• Omitting the capital gains pages in a UK loss year because no tax was due, so that the loss is never claimed.
• Restricting a UK current-year loss to preserve the annual exempt amount, which the rules do not allow.
• Carrying forward a US loss as a single figure without its short-term and long-term split.
• Including personal-use asset losses, or UK losses from non-resident years, in the respective pools.
• Merging clogged losses into the general UK carryforward.
• Ignoring the effect of foreign source capital losses on the Form 1116 limitation.

Speak to a cross-border preparation team

Capital loss schedules are long-lived records: an error made in the year of the loss is carried into every later return until the pool is exhausted. Our US tax return and UK tax return teams prepare both sides from one ledger, document the reconciliation, and rebuild carryforwards where the history is incomplete. If your US and UK returns show loss figures you cannot explain, or you hold losses that have never been reported in one country, contact our cross-border team to arrange a confidential consultation with US-UK tax accountants who prepare both returns together.