US Person UK Capital Gains: Selling Shares at a Loss Under Two 30-Day Rules

US person UK capital gains: an introduction

The US person UK capital gains problem is sharpest when you sell shares at a loss and buy them back. Britain and America both have a 30-day anti-avoidance rule, but they work in different directions. As a result, one sale can be matched to different shares in each country. Your loss may survive in one return and vanish, for now, in the other.

This article supports our guide to the US-UK double taxation treaty. It is written for US citizens and green card holders who live in the UK and hold shares in a general investment account. We cover selling at a loss, buying back, and the currency effect that can turn a loss into a gain.

Why US person UK capital gains need two calculations

A US person is a US citizen or green card holder, taxed by the US on worldwide income wherever they live. If you are also UK resident, HMRC taxes your gains too. Each country computes the gain in its own currency, with its own matching rules. Therefore, you always run two calculations for one sale.

What this article leaves out

Non-UK funds, including US-listed exchange traded funds, raise separate offshore fund and PFIC questions. UK funds held by a US person are usually PFICs, which our article on PFICs and UK funds explains. Property gains are covered in our piece on a US person selling UK property. Here we stay with ordinary company shares.

How does the UK match a sale of shares?

The UK matches a sale first with shares bought the same day, then with shares bought in the next 30 days, and only then with your pooled holding. That middle step is the UK 30-day rule. It looks forward from the sale, never back.

The three UK matching steps

HMRC's helpsheet HS284, Shares and Capital Gains Tax, sets out the order. A section 104 holding is the pool of identical shares you already own, held at an average cost. Shares bought on the same day as the sale come first. Next come shares bought within the following 30 days. Only what is left is matched with the pool.

Who the 30-day rule catches

HMRC's Capital Gains Manual at CG51560 explains that the rule matches shares bought by the same person in the same capacity. Its own example says a sale made personally and a purchase made as a trustee would not be matched. Consequently, a purchase by your spouse is generally outside the rule, because your spouse is a different person.

The ISA question

Many investors sell in a general account and buy back inside an ISA, often called "bed and ISA". It is widely treated as outside the 30-day rule. However, HMRC's helpsheet and the manual page above do not address it directly. For a UK-only taxpayer, that is a small point. For a US person, the US answer is very different, as the next section shows.

How does the US wash sale rule differ?

The US wash sale rule disallows a loss if you buy substantially identical shares in the 30 days before or after the sale. It looks both ways, and the disallowed loss is added to the cost of the new shares. So the loss is postponed, not lost.

The rule in the Code

The rule sits in section 1091 of the Internal Revenue Code. A wash sale is a loss sale matched by a purchase of substantially identical stock or securities within the window. The Code does not define "substantially identical". In addition, the holding period of the old shares carries over to the replacement, under section 1223.

An ISA is just an account to the IRS

The IRS does not recognise an ISA as a tax shelter. To the US, an ISA is an ordinary brokerage account that you own. Therefore, buying back inside your ISA is a purchase by you, and the wash sale rule can apply. Our article on ISA reporting on a US return covers the wider treatment.

Purchases inside a US retirement account

IRS Publication 550 lists a purchase for your IRA or Roth IRA as one that triggers a wash sale. Revenue Ruling 2008-5 adds a harsh twist. The loss is disallowed, and the IRA's basis does not go up. In that case the loss is gone for good, not merely deferred.

What happens to US person UK capital gains when you buy back?

Usually, both countries defer part of the loss, but by different amounts and through different shares. The UK matches the sale with the new purchase. The US keeps the original sale but moves the loss into the new shares' cost. These two methods rarely give the same figure.

A worked example

Consider a typical case. Emma, a fictional US citizen in Leeds, bought 1,000 shares in a UK listed company in February 2025 for £10,000. We assume the pound bought $1.25 then, so her dollar cost was $12,500. In March 2026 she sold all 1,000 shares for £8,000. We assume $1.35 to the pound on that day, so her proceeds were $10,800. Fifteen days later, she bought 1,000 shares back for £8,200 in the same general account.

On the UK side, the 30-day rule matches her sale with the buy-back. Her UK loss is just £200, which is £8,000 less £8,200. Her original £10,000 shares are treated as still held. On the US side, her loss is $1,700, which is $12,500 less $10,800. However, the wash sale rule disallows it and adds it to the cost of her new shares.

The two results side by side

Today, Emma has a £200 UK loss and no US loss. Later, when she finally sells, the UK will measure her gain against the £10,000 pooled cost. The US will measure it against the new dollar cost plus the $1,700 deferred loss. These are the US person UK capital gains records she must keep, and they will differ for years.

If she had bought back in her ISA

Suppose instead that Emma bought back inside her ISA. On the common view, the UK 30-day rule would not apply, so she would keep a £2,000 UK loss. The US result would barely change. The wash sale rule would still disallow the $1,700, now attached to shares in an account that the UK ignores. That single choice makes the two returns tell very different stories.

Can a UK loss be a US gain?

Yes. Each country converts at its own rate on its own dates, so currency moves can flip the result. A sterling loss may be a dollar gain, and the reverse is also possible.

How each country handles currency

HMRC's manual at CG78310 says foreign currency amounts are converted to sterling when incurred or received. For a UK share bought in pounds, there is no currency step at all. By contrast, IRS Publication 54 requires every amount on a US return to be stated in dollars. In practice, cost is converted at the purchase date and proceeds at the sale date.

An example of the flip

For example, say you buy shares for £10,000 when the pound is worth $1.20, a cost of $12,000. You sell for £9,500 when the pound is worth $1.35, giving $12,825. HMRC sees a £500 loss. The IRS sees an $825 gain. With US person UK capital gains, you can owe US tax on a sale that lost you money in pounds.

Why credits may not help

The foreign tax credit relieves US tax only where UK tax was paid on the same income. In the flip case, there is no UK tax, because there is no UK gain. As a result, the US tax on the currency gain usually has nothing to absorb it. Our article on Form 1116 and UK tax explains how the credit is calculated.

How do loss rules and rates compare?

The UK lets current-year losses absorb gains in full, then carries any surplus forward with no cap. The US lets losses offset gains, then allows only a limited amount against other income each year. Timing rules differ as well.

The UK side

According to GOV.UK's page on capital losses, you can claim a loss up to four years after the end of the tax year of the sale. Losses from the same year are deducted from that year's gains first. Earlier losses are used only to bring gains down to the tax-free allowance. For 2026 to 2027 that allowance is £3,000, and gains on shares are taxed at 18% or 24%, as HMRC's rates table shows.

The US side

Under section 1211, net capital losses can reduce other income by up to $3,000 a year, or $1,500 if married filing separately. Section 1212 carries the rest to the next year, keeping its character, and so on year by year. IRS Topic 409 explains the long-term rates of 0%, 15% and 20%, with thresholds that change every year.

The net investment income tax

The net investment income tax is a 3.8% US tax on investment income above set income levels. The IRS says, in its questions and answers, that foreign tax credits cannot reduce it. Some taxpayers have challenged that position under other treaties. We treat it as contested and follow the IRS view unless you choose to file a disclosed position.

Which shares did you sell, in each country?

In the UK, you rarely choose: the matching rules decide. In the US, you can often choose which lot you sold, if you identify it properly. That freedom can help a US person reduce friction between the two returns.

Specific identification in the US

Treasury Regulation 1.1012-1 treats the earliest shares as sold unless you adequately identify the lot. Identification normally means telling your broker which lot to sell at the time. A UK broker may not record lots this way. Therefore, keep your own dated note of each lot sold, in dollars, at the time of sale.

The UK pool

The UK pool removes choice. Every share of the same class is averaged into one cost, and part of that cost goes with each sale. Consequently, the UK cost of a sale will often sit between your US lots. Neither figure is wrong, but they will not match.

Keeping two ledgers

In our experience, the cleanest answer is two parallel ledgers. One records the UK pool in pounds. The other records US lots in dollars, with any wash sale adjustments. Both should be updated on the day of every trade, not rebuilt from statements at year end. That pair of ledgers is the backbone of US person UK capital gains reporting.

What should you check before you sell at a loss?

Look at both windows, both currencies and both claim rules before you trade. Five minutes of checking can save a deferred loss or an unexpected US bill. This is a filing checklist, not investment advice.

Before the sale

First, check whether you bought the same shares in any account in the last 30 days, including your ISA. That purchase can trigger the US rule even though the UK ignores it. Next, work out the dollar result, not just the sterling one. A sterling loss may be a dollar gain.

After the sale

Then, note any purchase of the same shares in the 30 days after. Both rules look forward, but each treats the purchase differently. Record the dollar loss that the wash sale moves into the new cost. Finally, keep the contract notes, because both tax authorities may ask for them.

At year end

Claim your UK loss on your Self Assessment return, well within the four-year limit. On the US side, the sale goes on Form 8949 and Schedule D, with any wash sale adjustment shown. Check that UK tax paid on gains is matched to the right US year for the credit. Our treaty relief service covers that matching.

Which US person UK capital gains mistakes are most common?

The commonest are using one ledger for both returns, ignoring ISA trades and forgetting the currency step. Each one produces US person UK capital gains figures that look right in one country and wrong in the other.

Copying the UK figure onto the US return

Many people take the result from their UK computation, convert it at one rate and enter it on Form 8949. That shortcut ignores the different matching and the different cost. For US person UK capital gains, the US figure must start from the dollar cost of the actual lot sold. Converting a sterling result is almost always wrong.

Treating the ISA as invisible

Because HMRC ignores ISA gains, people often leave ISA trades out of their US records altogether. However, every ISA sale is a US sale, and every ISA purchase can be a wash sale replacement. Consequently, the ISA needs its own US ledger, just like the general account.

Missing the US gain on a sterling loss

A loss in pounds feels like a loss, so it rarely gets a second look. Yet a stronger pound can turn it into a dollar gain. In our experience, this is the US person UK capital gains error that is easiest to miss, because nobody sends you a form showing the dollar figure.

Leaving the loss claim too late

Finally, a UK loss that is never claimed cannot be used. The four-year limit sounds generous, but it passes quickly when the loss is small and the paperwork is dull. Claim each loss in the return for the year it arose, and keep the records for both countries together.

How US UK Tax Experts can help

US UK Tax Experts prepares the US and UK returns from one file, so that the two agree. For US person UK capital gains, we rebuild the UK pool and the US lots side by side. We also apply both 30-day rules and work out the dollar result of each sale. See our US federal returns service and our UK Self Assessment service.

Conclusion

Selling at a loss is simple in one country and tricky across two. The UK looks forward 30 days and averages your cost. The US looks both ways, moves the loss into the new shares and measures everything in dollars. Keep two ledgers, check both windows before you trade, and your US person UK capital gains reporting will hold together.

Contact us

If you have sold shares at a loss this year, email hello@usuktaxexperts.com or use our contact page. Tell us the dates of each sale and purchase and the accounts used. We reply within one working day.