Does the UK ever use FIFO for shares?

No. For an individual, HMRC matches a disposal first against shares acquired on the same day, then against shares acquired in the 30 days after the disposal, and then against the section 104 holding. The section 104 holding is a single pooled asset carrying an averaged cost, described in helpsheet HS284 and in the Capital Gains Manual at gov.uk/hmrc-internal-manuals/capital-gains-manual/cg51550. There is no first in first out concept and no ability to select individual shares.

Can I use my section 104 pooled cost on my US return?

No. IRS Publication 551 requires basis to be traced to the particular shares sold, and where you cannot adequately identify them, the basis of the securities you acquired first applies. Averaging is offered for mutual fund shares, not for ordinary shares in an operating company. The pooled sterling cost is a UK figure only, and the US return needs a separate lot-level dollar ledger maintained since the first purchase.

When must I tell my broker which shares to sell?

Before settlement. Publication 550 and the underlying Treasury regulation require you to specify the particular stock to the broker having custody at the time of sale, obtain written confirmation of that specification within a reasonable time, and make the identification no later than the earlier of the settlement date or the required settlement time. Deciding at tax return time is not adequate identification, and FIFO then applies to that disposal by default.

Why is my US gain bigger than my UK gain on the same sale?

Usually two reasons stacked together. First, FIFO charges the sale against your oldest and typically cheapest lot, while the UK pool relieves a blended average cost across every tranche. Second, US basis and proceeds are translated at their own separate transaction dates, so sterling movement feeds into the dollar result. A rising share price and a weaker pound at purchase widen the gap in the same direction.

How does a rights issue affect the pool and my US basis?

Differently on each side. Helpsheet HS285 says you add the cost of the further shares acquired in the rights issue to the cost of the original shares, and CG51620 treats that as an operative event increasing the pooled qualifying expenditure of the section 104 holding. On the US side the subscription generally creates a new lot with its own acquisition date, cost and holding period. One corporate action, one pool adjustment in the UK and one additional lot in the US.

Do I have one pool if I hold the same stock at several brokers?

Yes for the UK. The section 104 holding covers all shares of the same class in the same company held by the same person in the same capacity, regardless of which platform holds them, so positions across multiple brokers form a single pool. The US ledger works the other way, because identification is made to the custodian holding the stock and each broker tracks its own lots, so FIFO effectively runs account by account.

US UK Tax Returns: Share Pooling vs US Cost Basis

US UK tax returns preparation for an American investor living in Britain has to solve a problem no single piece of software handles cleanly: the same share sale must be computed twice, under two cost basis systems that were never designed to agree. HMRC merges every share of the same class in the same company into one section 104 holding carrying a single averaged cost. The IRS keeps every purchase as a separate lot and charges the sale against the earliest lot unless you adequately identified a different one at the time of the trade. The two answers rarely match in amount, and often not in the year they land, which is where foreign tax credit relief starts to leak.

This is not an edge case. It bites the moment a client buys the same holding in more than one tranche. In the returns we prepare for bankers, founders and private investors, the pooled sterling gain and the lot-based dollar gain on a single disposal routinely differ by a third or more, and occasionally point in opposite directions.

What is a section 104 holding and how does the UK pool actually work?

A section 104 holding is a single pooled asset. HMRC helpsheet HS284 states that from 6 April 2008 all shares of the same class, in the same company, are held together in a section 104 holding, and that each share in the holding is treated as if acquired at the same average cost. The Capital Gains Manual at gov.uk/hmrc-internal-manuals/capital-gains-manual/cg51620 is blunter: all the shares in the section 104 holding are regarded as indistinguishable parts of a single asset which grows or shrinks as shares are acquired or disposed of. There is no lot, and no acquisition date for any individual share.

When you sell part of the holding, HS284 gives the arithmetic directly: the allowable expenditure is a fraction of the pool of actual cost, and the fraction is the number of shares sold divided by the total number of shares in the holding. Buy at four pounds and later at nine, and the pool remembers neither price, only the blend. The pool is adjusted each time an operative event occurs. The helpsheet sits at gov.uk/government/publications/shares-and-capital-gains-tax-hs284-self-assessment-helpsheet.

Two rules sit in front of the pool. CG51560 confirms that shares acquired by the same person on the same day and in the same capacity are treated as acquired by a single transaction, and that acquisitions within the 30 days after a disposal are matched next, with priority over every other rule except the same day rule. Those shares never enter the pool. But for an investor not trading around a position, almost every disposal lands straight on the section 104 pool.

How does the US cost basis system differ, and when does FIFO apply?

The US does the opposite. It never averages ordinary shares. IRS Publication 551 states that the basis of stocks or bonds you buy is generally the purchase price plus any costs of purchase, such as commissions and recording or transfer fees, and that basis attaches to that specific purchase. Each tranche is a lot with its own dollar basis, acquisition date and holding period. Publication 551 then gives the default in plain terms: if you buy and sell securities at various times in varying quantities and you cannot adequately identify the shares you sell, the basis of the securities you sell is the basis of the securities you acquired first. FIFO is not a method you elect; it is what happens when you fail to do something else.

Be explicit about what the US does not offer, because clients assume otherwise. Averaging exists for mutual fund and REIT shares, which Publication 550 frames as choosing average basis for mutual fund shares. It is not available for ordinary shares in an operating company. No US election replicates the UK pool and no UK election replicates a US lot. The systems can only be reconciled. See irs.gov/publications/p551 and irs.gov/publications/p550.

What does adequate identification require, and by when?

This is the point most guidance gets wrong, and it is expensive. Adequate identification is not a box you tick on the return. Publication 550 and the underlying Treasury regulation at 26 CFR 1.1012-1(c) require that at the time of the sale the taxpayer specifies to the broker or other agent having custody the particular stock to be sold, and that within a reasonable time confirmation of that specification is set out in a written document from the broker. The regulation fixes the deadline too: an identification counts as made at the time of sale if made no later than the earlier of the settlement date or the time for settlement required by the relevant SEC settlement rule.

Read that against how a UK private client actually trades and the problem is obvious.

The instruction must reach the custodian at or before the trade, not in the following January when the preparer opens the file.
The confirmation must come back in writing from the broker, and the preparer must hold it. A screenshot of a portfolio screen is not a broker confirmation.
Many UK execution platforms have no lot selection facility, because UK tax never needs one. A standing written instruction with a written acknowledgement is often the only workable route, and it has to exist before the first disposal.
If the identification is missing or late, FIFO applies and no later reconstruction undoes it.
The UK computation is unaffected. The pool does not care which shares you told your broker to sell.

Worked example: US UK tax returns preparation when one sale produces two gains

The following is an illustration only. Every figure is invented to show the mechanic, exchange rates are assumed round numbers rather than published rates, and dealing costs and stamp duty are ignored so the arithmetic stays visible. Adrian Wolfe is a US citizen, UK resident, who has built a position in a UK-listed industrial group over seven years.

March 2019: buys 10,000 shares at GBP 4.00, cost GBP 40,000. Rate assumed 1.30, so US basis USD 52,000, or USD 5.20 per share.
June 2021: buys 6,000 shares at GBP 6.50, cost GBP 39,000. Rate assumed 1.40, so US basis USD 54,600, or USD 9.10 per share.
November 2023: buys 4,000 shares at GBP 9.00, cost GBP 36,000. Rate assumed 1.25, so US basis USD 45,000, or USD 11.25 per share.
Position: 20,000 shares. UK pooled cost GBP 115,000, an average of GBP 5.75 per share. Total US basis USD 151,600 across three lots.
May 2026: sells 8,000 shares at GBP 10.00, proceeds GBP 80,000. Rate assumed 1.35, so US proceeds USD 108,000. No acquisitions on the day of sale or in the 30 days after.

The UK computation is one line. Under HS284 the allowable cost is 8,000 divided by 20,000, applied to pooled cost of GBP 115,000, giving GBP 46,000. Proceeds of GBP 80,000 less GBP 46,000 produces a chargeable gain of GBP 34,000. The pool carries on with 12,000 shares and GBP 69,000 of qualifying expenditure, still GBP 5.75 per share, because a part disposal never changes the average.

The US computation is a different animal. With no adequate identification, FIFO charges the sale against the 2019 lot: 8,000 shares at USD 5.20, or USD 41,600. Proceeds of USD 108,000 less USD 41,600 produces a long term gain of USD 66,400. Convert the sterling gain at the sale date rate purely for comparison and GBP 34,000 is about USD 45,900. Same shares, same day, and the US gain is roughly USD 20,500 larger.

What happens if a specific identification is made instead?

Now assume Adrian instructed his custodian before settlement to sell the November 2023 lot in full plus 4,000 shares from the June 2021 lot, and holds the written confirmation. US basis becomes 4,000 at USD 11.25 plus 4,000 at USD 9.10, which is USD 81,400, and the US gain falls to USD 26,600. The swing between the FIFO result and the identified result on one sale is USD 39,800, and the direction of the mismatch has flipped: the US gain is now smaller than the sterling gain rather than larger.

The UK figure does not move. It is still GBP 34,000, and the pool still stands at 12,000 shares and GBP 69,000. That asymmetry is the practical heart of the subject: the US side has a lever and the UK side has none. Choosing high basis lots to suppress the US gain looks efficient in isolation and can quietly strand the UK tax. Choosing low basis lots inflates the US gain and may soak up credit that would otherwise expire. Neither is right in the abstract, and both are irreversible after settlement.

Why does an unchanged sterling price still produce a US dollar gain?

Because the US return is a dollar return at both ends. IRS guidance at irs.gov/individuals/international-taxpayers/foreign-currency-and-currency-exchange-rates requires you to immediately translate into dollars all items of income, expense and tax that you receive, pay or accrue in a foreign currency, using the exchange rate prevailing when you receive, pay or accrue the item. Basis is translated at the acquisition date, proceeds at the disposal date, and nothing is retranslated afterwards.

Take 1,000 shares bought at GBP 6.50 when the rate was 1.20, giving basis of USD 7,800, sold years later at exactly GBP 6.50 when the rate was 1.45, giving proceeds of USD 9,425. Sterling gain nil, dollar gain USD 1,625. This is an ordinary share disposal whose two legs were measured with different rulers. The compliance consequence is the painful one: no UK gain means no UK tax, so there is no foreign tax on that gain to credit at all.

How do corporate actions feed the pool but adjust basis?

Corporate actions are where reconciling schedules quietly break, because the same event does structurally different things on each side. Helpsheet HS285 is the UK anchor: on a reorganisation, the issue of any new shares is not treated as an acquisition and the loss or alteration of any old shares is not treated as a disposal. It sits at gov.uk/government/publications/share-reorganisations-company-takeovers-and-capital-gains-tax-hs285-self-assessment-helpsheet.

Bonus issue: UK side, share count rises with no new expenditure, so pooled cost per share falls across the whole holding at once. US side, basis is spread within each affected lot, so the historic differences between lots survive.
Rights issue: HS285 says you add the cost of the further shares to the cost of the original shares, and CG51620 treats that as an operative event increasing pooled qualifying expenditure. On the US side the subscription generally opens a new lot with its own date and cost.
Reorganisation or takeover for shares: the UK pool continues uninterrupted, while US lot identity and holding period generally carry over, so the ledger keeps its internal structure rather than resetting.
Scrip or stock dividend: not a free issue on either side. UK rules can give the new shares a base cost by reference to an amount treated as income, and US treatment turns on whether the distribution was taxable. Confirm in the Capital Gains Manual at gov.uk/hmrc-internal-manuals/capital-gains-manual and in Publication 550 first.
Capital distribution: this can reduce pooled expenditure on the UK side rather than trigger a disposal, while the US side reduces basis in the specific lots. Post it to both ledgers in the same sitting or the records drift permanently.

What does the mismatch do to the foreign tax credit?

It attacks the credit in two directions at once: amount and year. Where the US gain is larger, UK tax paid on the smaller sterling gain cannot cover the US liability on the bigger dollar one, and residual US tax falls due on economics the client believes were already taxed in Britain. Where the US gain is smaller, UK tax goes unabsorbed. Publication 514 confirms the limitation is computed separately for each category of income, that Certain Income Re-Sourced by Treaty is one of those categories, and that unused foreign taxes carry back one year and forward ten. Gain on a share sale is not automatically foreign source in US hands, so whether a credit exists at all must be settled against the treaty and Publication 514 at irs.gov/publications/p514.

On year, the calendars do not line up. GOV.UK confirms at gov.uk/self-assessment-tax-returns/deadlines that the UK tax year ran from 6 April 2025 to 5 April 2026, with an online filing deadline of 31 January 2027, while the US return is a calendar year filing. A May disposal sits in one UK year, one US year, and produces a UK payment date later still. Where the credit is claimed on a paid basis, the tax and the income can land in different US years, and a credit in the wrong year behaves like no credit at all until a carryback or carryforward rescues it.

Why the gap only truly closes on full liquidation

Here is the point almost nobody makes. Across the life of a holding the total cost relieved is identical under both systems: the UK relieves GBP 115,000 of pooled expenditure and the US relieves USD 151,600 of lot basis, no more and no less. The difference between pooling and lot selection is therefore a timing difference in substance, not a permanent one. But it unwinds completely only when the last share is sold. In Adrian's case the USD 20,500 excess US gain in 2026 is borrowed from later years.

For an investor who holds core positions for decades and sells in slices, that reversal may never arrive inside the ten year carryforward window Publication 514 allows. The credit consequence of pooling versus FIFO is therefore structural, and should be modelled across the whole disposal programme rather than one sale at a time.

One pool, several brokers: why the two ledgers cannot live in one place

The second point competitors miss is the shape of the records. The UK pool is defined at the level of the person, not the account. CG51560 frames the matching rules by reference to shares acquired by the same person in the same capacity, and the section 104 holding at gov.uk/hmrc-internal-manuals/capital-gains-manual/cg51550 is a pool of expenditure for a class of share, not for a custody arrangement. Hold the same stock through a UK platform, a US brokerage account and a nominee account, and HMRC still sees one pool.

The US side is naturally the opposite. Identification is made to the broker having custody, and each custodian tracks and reports its own lots, so FIFO runs independently in each account. The same investor has one UK pool and three separate US lot ledgers. Aggregate the brokers and the UK answer goes wrong in the opposite direction from the way the US answer goes wrong by keeping them apart.

How does a preparer build a reconciling schedule across both systems?

The workable answer is a permanent dual ledger per security, maintained continuously rather than rebuilt each filing season, with one transaction feeding two independent running balances.

One row per transaction: trade date, settlement date, quantity, sterling consideration, dealing costs, exchange rate used and its source.
A UK column set carrying the running pool: shares held, pooled qualifying expenditure and pooled cost per share, aggregated across every broker and recalculated at each operative event.
A US column set carrying open lots: acquisition date, quantity remaining, dollar basis, custodian and holding period, kept separately by account.
An identification log recording, per disposal, whether an identification was given, when relative to settlement, and where the broker confirmation is filed.
A reconciliation line per disposal showing the sterling gain, the dollar gain, the difference, and that difference split between method and currency.
A credit tracker linking UK tax on each gain to the US year in which it is claimed, with any carryback or carryforward flagged as it arises.
Form 8949 notes: where a custodian reported an incorrect basis, the instructions at irs.gov/instructions/i8949 require you to enter the reported basis and correct it through the adjustment column with code B. Disposals with no Form 1099-B, the norm for UK brokers, go in the boxes for transactions not reported to the IRS.

Built once and kept current, that ledger puts the identification decision in front of you before settlement rather than after, and shows either revenue authority that the other return was prepared consistently.

If you hold multi-tranche share positions on both sides of the Atlantic, this is the layer of US UK tax returns preparation where accuracy is won or lost. You can see how we scope dual-country reporting at us-uktax.com/cross-border-tax-planning, the US filing side at us-uktax.com/us-tax-services and the Self Assessment side at us-uktax.com/uk-tax-services. Where earlier years were filed on a single-country basis and the pool was never maintained, the correction route runs through us-uktax.com/irs-streamlined-filing.