Capital losses do not cross the Atlantic

A capital loss arises when you dispose of an asset for less than it cost you. Both countries recognise the idea. However, each applies its own rules about what counts as cost, which disposals qualify, and what the loss can then be set against.

No. Each system runs its own calculation from its own records, so a British loss does not reduce an American gain and an American loss does not reduce a British one. Relief for double taxation works on tax paid, not on losses, which surprises people who expect symmetry.

That means you can genuinely have a loss in one country and a gain in the other on the same sale.

Currency is usually why. The American calculation runs in dollars from purchase to sale.

So a modest sterling loss can become a dollar gain if the pound moved against you over the holding period.

Capital losses first offset capital gains. Where losses exceed gains, the IRS allows you to set the excess against ordinary income up to a yearly cap. Topic 409 states that the amount is the lesser of $3,000, or $1,500 if you file as married filing separately.

Anything beyond that cap does not vanish.

The instructions confirm you carry the remainder forward to later years.

So a large loss relieves slowly, in annual slices, rather than all at once.

Because the cap is set per return rather than per person, and the separate figure is half the joint one. That matters here more than it does for most American taxpayers, since filing separately is the usual status for a US citizen married to a non-US spouse.

So this audience routinely gets the lower number without choosing it.

Our guide to married filing separately with a UK spouse covers why that status is so common.

In practice it means a loss takes twice as long to relieve as the headline figure suggests.

Losses reduce gains. There is no general equivalent of the American offset against ordinary income, so an ordinary capital loss with no gains to absorb it simply waits. GOV.UK explains how losses work and how they interact with the annual allowance.

That waiting is not a problem in itself.

Losses carry forward once claimed, so a future gain can use them.

However, the carry forward depends on having claimed the loss, which is the step people miss.

No, and GOV.UK says so directly. You do not have to report losses straight away, which gives you room if a disposal happens in a year you were not expecting to file anything at all.

The flexibility has a limit though.

You have up to four years after the end of the tax year in which you disposed of the asset.

Miss that window and the loss is gone, however real it was.

It expires quietly. There is no notice, no reminder and nothing on a statement to tell you that relief you were entitled to has lapsed. The asset was sold, the money was lost, and the tax system simply never heard about it.

That is the most common avoidable mistake in this area.

It happens most to people who were not in Self Assessment at the time.

GOV.UK notes you can write to HMRC instead of filing a return, which covers exactly that case.

Because the American calculation converts both the purchase and the sale into dollars, using the rates at each date. If the pound fell between the two, the dollar proceeds can exceed the dollar cost even when the sterling proceeds did not.

So the same transaction produces opposite signs in the two systems.

It works the other way too, which occasionally helps.

Our guide to which exchange rate to use sets out which rate applies where.

Only indirectly. Relief for double taxation works on foreign tax actually paid, so a loss that reduces your British tax also reduces the credit available against your American tax on the same income.

That is the familiar netting effect across the two systems.

It does not make the British claim pointless.

However, it means the benefit of a loss is smaller than the British figure alone suggests, and the two returns need looking at together.

Property follows the same principles, with its own reporting on the British side. A loss on a residential property disposal still needs claiming, and the usual British reporting window for residential property gains is short.

Meanwhile the American calculation applies its own basis rules, and GOV.UK sets out the rules for reporting UK property gains.

Required depreciation on rental property affects the American figure in ways the British one never sees.

So a property disposal can produce two quite different numbers from identical facts.

Funds complicate this considerably, because a British collective fund is usually a passive foreign investment company for American purposes. That regime has its own rules about gains and losses, which do not behave like ordinary capital treatment.

So the simple picture in this guide does not carry across to those holdings.

Our guide to whether your fund is a PFIC explains how to check, and the instructions for Form 8621 set out the regime.

If the holding is a fund, treat the loss question as a specialist one rather than a routine one.

Only by choosing when you sell, which is a real lever if a disposal is already planned. The year of disposal decides the year of the loss in both systems, though the two years do not align with each other.

A sale in February sits in one British year and one American year.

A sale in June sits in the next British year and the same American one.

So timing a disposal can place the loss where it does the most good, provided you look at both calendars before selling.

Purchase date, purchase cost, sale date, sale proceeds and the costs of acquisition and disposal. You need those in both currencies, because each system computes in its own. Reconstructing any of that years later is slow and often impossible, so the time to capture it is the week of the trade.

Keep the contract notes rather than relying on a platform summary.

Platform statements often show sterling only, which leaves the American calculation unsupported.

Our guide to tax records covers how long to keep them, and basis records outlive almost everything else.

In our practice the clients who find this painless are the ones who saved the contract note at the time. Everybody else reconstructs it from a platform export years later.

A sequence that keeps both reliefs alive.

Unused losses carry forward rather than lapsing after a set number of years, so the relief survives as long as you keep claiming it. The practical risk is not expiry but forgetting, because a carryforward only exists if somebody tracks it between returns.

That tracking is the part that breaks when advisers change.

A new preparer cannot use a carryforward they have never seen.

So capital losses carried forward belong on the short list of things to collect when you move firms.

The American system restricts a loss where you buy back substantially the same holding within a short window around the sale. Britain has its own rules aimed at the same behaviour, and the two do not match in scope or in timing.

So a repurchase that is fine in one country may be caught in the other.

That matters most when rebalancing a portfolio near a year end.

Where a sale is deliberately crystallising a loss, check both sets of rules before buying anything back.

Sometimes, and this surprises people. First, the American system treats certain foreign currency transactions as giving rise to gain or loss in their own right. Then it applies its own character rules to the result.

However, the British treatment of the same movement differs.

For example, repaying a foreign currency mortgage can produce an American gain with no British equivalent.

So currency is not only a conversion step in this area. In short, it can be the transaction itself.

Consider an American in Bristol who sells a holding at a sterling loss, having bought it several years earlier. In Britain the loss is real and worth claiming.

In dollars the picture differs, because the pound was weaker at purchase than at sale. The American calculation shows a small gain rather than a loss.

She was not in Self Assessment that year, so nearly missed the British claim entirely. Writing to HMRC within the four-year window preserved it.

These figures are illustrative. However, the pattern is ordinary, and the part people lose is almost always the unclaimed British loss.

The first assumes a loss in one country automatically helps in the other. It does not, because the systems do not share figures.

The second never claims a British loss, usually because no return was due that year. Four years later the relief has gone.

The third forgets the carryforward exists. A loss claimed in one year and then lost in a spreadsheet is the same as no loss at all.

The fourth applies ordinary capital treatment to a fund holding, where the passive foreign investment company rules apply instead.

We compute both sides from the same underlying records, so the sterling and dollar figures come out of one exercise rather than two.

We also track carryforwards between years, which is where most relief gets lost over time.

Our US federal returns service covers the American reporting, and fees stay fixed and agreed in writing beforehand. How we work sets out the process.

If you have sold something at a loss and are unsure what was claimed, talk to us before the four-year window closes.