Two countries, two thirty day rules, and they point in opposite directions

Figures relate to tax year 2025-26 (UK) · 2025 (US)

Selling a losing holding to bank the loss is an old and perfectly legal move. Both countries know about it, and both countries wrote rules to stop you doing it too easily.

The trouble is that they wrote different rules. One looks backwards as well as forwards. The other looks forwards only, and it does not disallow anything. So a trade that works cleanly in Britain can fail the American wash sale test on the same day.

Key takeaways

• The American wash sale rule blocks a loss if you buy back within 30 days either side.
• Britain simply matches the sale against the repurchase, so no loss appears.
• The American window runs 61 days in total. The British one runs 30 days forward.
• A disallowed American loss moves into the cost of the replacement holding.
• A purchase in a retirement account can destroy the loss permanently.
• Your spouse buying the same shares can trigger the American rule too.

What is the wash sale rule?

Sell a security at a loss and buy something substantially identical within thirty days either side, and the wash sale rule disallows the loss for that year. The rule sits in section 1091 of the code.

You do not lose the loss forever in most cases. It attaches to the cost of the replacement shares instead, so it surfaces when you finally sell those.

The holding period carries across too. That can turn a short term position into a long term one, which occasionally helps.

What the British rule does

Britain uses share identification rules rather than a disallowance. Sell shares and buy the same class back within the following thirty days, and the sale matches against that repurchase.

According to HMRC guidance, same day acquisitions match first, then acquisitions in the next thirty days, then the main pool. So the loss you hoped to crystallise simply never arises.

The effect resembles the American one. But the mechanics differ, and the difference shows up in the details.

Why the two windows do not line up

Britain looks only at what you buy after the sale. America looks thirty days before as well, which catches a purchase you made weeks earlier and then forgot.

That backward reach is the wash sale trap. Buy more of a falling holding on the first of the month, sell the lot on the twentieth, and America can deny the loss on shares you bought before you ever decided to sell.

Britain would ignore that earlier purchase for these purposes. So the same set of trades produces a British loss and no American one.

The two rules side by side

A worked example

The figures below are illustrative and use round numbers to show the mechanics.

Priya holds a global equity fund that has fallen. On 10 March she buys another £5,000 of it, because she thinks it looks cheap.

On 25 March she changes her mind and sells the whole holding at a loss of £8,000. She waits two months before buying anything similar.

Britain gives her the loss in full, because she bought nothing in the thirty days after the sale. America looks back at the 10 March purchase and applies the wash sale rule to that slice.

So her British return shows a loss and her American return shows part of it postponed. Same trades, two answers.

Does it apply across accounts?

Yes on the American side. The wash sale rule follows you, not the account, so a sale in one brokerage and a purchase in another still meet inside the window.

Automatic reinvestment causes most accidents here. A dividend reinvested into the same fund three weeks after a sale counts as a purchase.

Turn reinvestment off before harvesting losses. It is a two minute job that saves a real deduction.

What about a spouse?

The wash sale rule treats a purchase by your spouse as your own. Selling in your account while your husband or wife buys the same fund defeats the loss. Britain runs the opposite logic. Spouses are separate people for capital gains, and transfers between them happen without a gain or a loss.

So the classic British planning move of selling and having a spouse buy can be perfectly effective in Britain and useless in America.

What about an ISA or a pension?

This is where a loss can vanish altogether. Buying the replacement inside a retirement account triggers the wash sale rule, and the disallowed amount gets no home in the account basis.

So the loss disappears rather than deferring. That is a permanent cost, and a common one for people who sell in a taxable account and rebuy in a pension.

British wrappers bring their own American problems, which our guide to the Lifetime ISA and US tax covers in detail.

Does it catch funds and trackers?

The wash sale rule catches anything substantially identical. Two trackers following the same index from the same provider will usually qualify, so switching between them rarely escapes the rule. Two funds following different indices normally do not. A broad developed market tracker and a narrower domestic one differ enough in most views.

The safest route stays the simplest. Wait thirty one days, or buy something genuinely different in the meantime.

What about crypto?

America has historically treated digital assets as property rather than securities, which put them outside the wash sale rule. Britain applies its thirty day matching rules to crypto holdings directly.

So the asymmetry flips. A British investor can lose the benefit of a repurchase that America would have allowed.

Proposals to extend the American rule to digital assets surface regularly. Check the position for the year you are filing rather than assuming last year's answer.

How the cost pools differ

Britain averages. Shares of the same class sit in a single pool with one blended cost, so every sale uses that average.

America identifies lots. You can choose which shares you sold, which lets you pick the highest cost ones and shrink the gain.

Corporate actions widen the gap further. A rights issue or a share split feeds into the British pool as one adjustment, while America tracks it lot by lot.

That single difference means the two countries almost never produce the same gain on the same trade. Expect a mismatch and plan for it rather than fighting it.

What counts as substantially identical?

The same company's ordinary shares, clearly. Options and convertible instruments over the same shares can count as well, which surprises people who use them to keep exposure. Shares in a different company in the same sector do not count. Nor do two funds with genuinely different mandates, whatever they hold on a given day.

Brokers take their own view when they report adjustments, and that view is not binding on you. Where you disagree, keep a note of why and be ready to explain it.

There is no published list, so judgement matters. Document the reasoning when you make a close call.

What happens to a disallowed loss?

It increases the cost of the replacement shares, and it waits there. When you sell those shares without repurchasing, the benefit finally arrives. Brokers report adjustments on the American side, but they only see one account. Where you traded across two providers, nobody makes the adjustment for you.

So keep your own record. A short spreadsheet of harvested losses and replacement lots pays for itself the first time somebody asks.

How to harvest a loss safely

1. List every purchase of the holding in the previous thirty days.
2. Turn off dividend reinvestment across all accounts holding it.
3. Check whether a spouse or a retirement account holds the same asset.
4. Sell the position and note the exact date.
5. Avoid anything substantially identical for thirty one days.
6. Record the British pool position and the American lot position separately.
7. Claim the British loss on your self assessment return within the time limit.

What if you trade frequently?

Then the arithmetic gets heavy quickly. Every sale needs a wash sale check against the sixty one day window, and a busy account can generate dozens of adjustments in a year.

Software helps on the American side but rarely handles the British pool at the same time. Most dual filers end up keeping a parallel record.

If your trading is active, agree a method with your adviser in January rather than reconstructing it in October.

What about currency on top?

Add another layer. America measures the purchase and the sale in dollars at the rates on their own dates, so a sterling loss can shrink or grow before any of this applies.

Occasionally a sterling loss becomes a dollar gain. The rule then has nothing to disallow, and the position reverses entirely.

Run the dollar figures before deciding whether a harvest is worth doing. Our guide to UK dividends on a US return covers the same currency point for income.

Does the rule apply to gains as well?

No, and that asymmetry is worth knowing. Sell at a profit and buy straight back, and both countries simply tax the gain. Britain even uses that feature deliberately. Investors realise gains inside the annual exempt amount and repurchase later, which resets the pool cost upward.

An American filer gets no such allowance, so the same move just brings the tax forward. Check both sides before using it.

What if the loss is on a foreign fund?

Then the punitive rules for offshore funds may matter more than any thirty day window. A British fund held by an American filer often falls into that regime. Losses inside it behave differently, and an election can change the treatment entirely. Our guide to the purging election explains one route out.

So check the fund classification first. Harvesting a loss on a holding taxed under those rules rarely does what you expect.

Mistakes and penalties we see with loss harvesting

The first is the forgotten reinvestment. A small automatic purchase quietly cancels a large planned loss, and nobody notices until the statements arrive.

The second is assuming a broker statement covers everything. Brokers adjust within their own account and cannot see the rest of your holdings.

The third is missing the British claim. A capital loss in Britain generally needs claiming within four years, and an unclaimed loss simply expires.

None of these carries a penalty by itself. They just cost money, which is worse in its own quiet way.

How US UK Tax Accountants helps

We run both calculations on the same trades, flag where a planned harvest fails one test, and keep the British pool and the American lots reconciled year to year. Then we tell you the date you can safely buy back.

Where losses have already gone astray, we check whether a claim can still be made. Our US federal return service covers the reporting on the American side.

If you are planning to sell before the year end, get in touch first. The thirty day clock only helps you if it starts on the right day.

Last reviewed 24 September 2026. This article is general information and not personal tax advice. Every situation turns on its own facts, so take advice on yours before acting.