Anyone who buys units in the same fund on more than one occasion within a General Investment Account eventually runs into a question that has a precise, if slightly counter-intuitive, HMRC answer: which specific units were sold, and at what original cost, when only part of a holding is disposed of? Rather than tracking each individual purchase separately, HMRC requires most holdings of identical shares or fund units to be pooled together using a mechanism known as Section 104 holding rules, which calculate a single average cost base for Capital Gains Tax purposes. Understanding how this pooling works is essential for anyone trying to calculate their own CGT liability accurately outside a tax-advantaged wrapper.
Why pooling exists
Without a pooling mechanism, calculating the gain or loss on selling part of a holding built up through multiple purchases at different prices would require choosing which specific units were being sold — the earliest ones bought, the most recent, or some other selection — each of which could produce a different taxable gain. Section 104 pooling removes this ambiguity by treating same-type holdings (for CGT purposes, "identical" shares or units in the same fund and class) as a single, blended pool with one average cost per unit, regardless of the actual order in which individual purchases were made.
How the pool is calculated
Adding to the pool
Each time additional units of the same fund and share class are bought, the amount paid is added to the pool's total cost, and the number of units bought is added to the pool's total unit count. The average cost per unit for the whole pool is then the total cost divided by the total number of units held.
Removing units from the pool on a sale
When units are sold, the cost attributed to those units for CGT purposes is calculated using the pool's current average cost per unit at the time of sale, multiplied by the number of units sold — not the price originally paid for whichever specific units might have been bought first or most recently. Both the total cost and the total unit count in the pool are then reduced proportionately to reflect the units sold.
A simple worked illustration
Suppose an investor buys fund units on three separate occasions:
| Purchase | Units bought | Price paid | Running pool total cost | Running pool total units |
|---|---|---|---|---|
| 1 | 500 | £2,500 | £2,500 | 500 |
| 2 | 300 | £1,800 | £4,300 | 800 |
| 3 | 400 | £3,200 | £7,500 | 1,200 |
After these three purchases, the pool holds 1,200 units at a total cost of £7,500, giving an average pooled cost of £6.25 per unit. If the investor then sells 600 units for £6,000, the cost attributed to those 600 units for CGT purposes is 600 × £6.25 = £3,750, producing a chargeable gain of £6,000 − £3,750 = £2,250, regardless of which specific purchase the units might be thought of as "belonging to." After the sale, the pool is reduced to 600 units remaining, with a total cost of £3,750 carried forward (also £6.25 per unit, since a proportionate reduction leaves the average unchanged). This is a simplified illustration excluding any transaction charges, and real calculations should reflect actual costs incurred.
The "same day" and "30-day" rules that sit alongside pooling
Section 104 pooling is the default matching rule for most holdings, but two specific exceptions take priority over it and must be checked first when a sale and a purchase happen close together in time.
Same-day rule
If units of the same fund are both bought and sold on the same day, those transactions are matched against each other first, before looking at the wider Section 104 pool, using the actual price paid and received on that specific day rather than the pool's average cost.
30-day rule (the "bed and breakfasting" rule)
If units are sold and then units of the same fund are repurchased within the following 30 days, the sale is matched against that later repurchase rather than against the Section 104 pool. This rule specifically prevents an investor from selling units to realise a loss or use their CGT annual exempt amount, and then immediately buying back the same fund to restore their original position — a practice sometimes referred to as "bed and breakfasting." Only after applying the same-day and 30-day matching rules does whatever remains fall back into the ordinary Section 104 pool calculation.
Order of matching rules
| Priority | Rule | What it covers |
|---|---|---|
| 1st | Same-day rule | Purchases and sales of the same fund on the same day |
| 2nd | 30-day rule | Sales matched against a repurchase of the same fund within the following 30 days |
| 3rd | Section 104 pool | Everything else, using the average pooled cost |
Why this matters practically for GIA investors
Record-keeping becomes essential
Because the pool's average cost changes with every purchase and sale, accurately calculating a gain requires a complete record of every transaction in a given fund — the date, number of units, and amount paid or received — rather than relying on memory or a single recent contract note. Missing even one earlier purchase can distort the calculated average cost and produce an inaccurate gain figure.
Accumulation units add a further layer
For accumulation units, where income is automatically reinvested rather than paid out as cash, the reinvested amount is generally added to the Section 104 pool's cost base each time it is treated as reinvested for tax purposes, even though no new units are physically purchased with fresh money by the investor. This detail is often overlooked and can affect the final calculation of pooled cost if not accounted for correctly, since it increases the cost base without a corresponding cash purchase transaction to reference.
Interaction with the CGT annual exempt amount
Once a chargeable gain has been calculated using the Section 104 pool, it is set against the investor's Capital Gains Tax annual exempt amount, which stands at £3,000 for 2025/26, with any gain above that exempt amount taxed at 18% for basic rate taxpayers or 24% for higher and additional rate taxpayers on investment gains, figures that can change and should always be checked against current HMRC guidance.
Common situations that complicate the pool
Fund mergers and share class conversions
Funds occasionally merge with other funds, or a platform moves investors from one share class to another — for example, from an older, higher-charging share class into a newer, lower-charging one. HMRC generally treats certain of these events, such as a straightforward share class conversion within the same fund carried out for administrative reasons, as not triggering a disposal for CGT purposes, meaning the original pooled cost simply carries over into the new holding rather than resetting. However, the precise tax treatment can depend on the specific circumstances of the merger or conversion, and it is worth checking the details of any such event rather than assuming a particular treatment applies automatically.
Transfers between spouses or civil partners
Assets, including fund holdings in a GIA, can generally be transferred between spouses or civil partners without triggering an immediate CGT charge, with the receiving spouse effectively inheriting the same pooled cost base as the transferring spouse. This can be a useful planning tool where one spouse has unused CGT annual exempt amount or is in a lower tax band, though it requires careful record-keeping to track the original pooled cost through the transfer.
Partial platform transfers "in specie"
Where a fund holding is transferred between platforms without being sold and repurchased — known as an "in specie" transfer — the Section 104 pool and its associated cost history generally need to transfer along with the units themselves. In practice, this depends on the receiving platform accurately recording the historical cost information, and gaps or errors in this handover are a common source of future difficulty when calculating gains, making it worth retaining independent records of purchase history rather than relying solely on a platform's transferred records.
Practical tools for tracking the pool
Many investment platforms now calculate an indicative pooled cost and unrealised gain automatically within account statements or online dashboards, which can be a helpful starting point. However, these figures are not always guaranteed to be complete or fully accurate for tax purposes, particularly where a holding has been transferred from another platform, held for many years, or involves reinvested accumulation unit distributions. Keeping an independent spreadsheet or ledger recording every purchase, sale, and reinvestment date and amount provides a useful cross-check and a fallback record if a platform's own figures are ever queried or found to be incomplete.
Why ISAs and SIPPs avoid this complexity entirely
Investors holding the same funds within a Stocks & Shares ISA or a SIPP do not need to perform any of this pooling calculation, since gains within these tax-advantaged wrappers are not subject to Capital Gains Tax at all. Section 104 pooling is specifically a feature of unwrapped holdings, such as those in a General Investment Account, which is one of the practical reasons many investors prioritise using their ISA and pension allowances fully before holding significant fund positions in a GIA where this ongoing calculation becomes necessary.
Key takeaways
- Section 104 pooling combines all purchases of the same fund and share class into a single pool with one blended average cost per unit for CGT purposes.
- Selling part of a holding uses the pool's average cost at the time of sale, not the price of any specific historical purchase.
- The same-day rule and the 30-day "bed and breakfasting" rule take priority over Section 104 pooling and must be checked first for sales followed by a nearby repurchase.
- Accurate record-keeping of every purchase, sale, and reinvested distribution is essential to calculate the pooled cost correctly.
- Gains calculated this way are set against the CGT annual exempt amount (£3,000 for 2025/26) before tax applies at 18% or 24% depending on the investor's tax band.
- Holding funds within an ISA or SIPP avoids this pooling calculation altogether, since gains inside these wrappers are not subject to CGT.