Why share pooling isn't as simple as "cost ÷ shares"
Most people who bought the same company's shares more than once assume they can just average the purchase prices. HMRC's actual rule — Section 104 of the Taxation of Chargeable Gains Act 1992 — is close to that, but with two catches that trip people up: purchases and sales in the same day, and purchases made within 30 days after a sale ("bed and breakfasting"), are matched separately and never enter the pool. Only what's left forms the Section 104 holding, and everything in it shares one average cost, regardless of when it was actually bought.
This calculator handles the pooling arithmetic. It deliberately does not attempt same-day/30-day matching — that requires your full disposal history, not just your buy lots, and is genuinely complex enough that it deserves its own dedicated tool rather than a shortcut here.
Source: TCGA 1992 s.104 · Calcul déterministe — no AI, no arbitrary estimate
Section 104 pooling formulas
total_shares = Σ shares_bought (each qualifying lot)
total_cost = Σ (purchase_price + acquisition_costs) // broker fee, SDRT 0.5%
— Average cost —
avg_cost_per_share = total_cost / total_shares
— On a later disposal —
allowable_cost = shares_sold × avg_cost_per_share
gain = sale_proceeds − allowable_cost − selling_costs
Excludes same-day and 30-day matching, which must be applied before any remaining shares enter the pool.
Common pool scenarios
| Scenario | Pool treatment |
|---|---|
| Bought 100 shares in Jan, 100 more in June at a different price | Both lots pool together — one average cost applies to any future sale |
| Sold 50 shares, then bought 50 more within 30 days | Matched under the 30-day rule, not the pool — different cost basis for CGT |
| Bought and sold same shares same day | Matched same-day first — never touches the pool |
| Received a bonus issue (extra shares, no cost) | Add to pool as a lot with £0 cost — dilutes average cost per share |
| Reinvested dividends (DRIP) bought more shares | Each DRIP purchase is a new pool lot at its own cost |
Your pooled average cost per share only gets you halfway. It tells you the allowable cost — it doesn't tell you what you actually owe HMRC, and it says nothing about a tax year where you also sold a property or crypto alongside your shares.
That's exactly what mycgttax.co.uk is built for. A few concrete cases:
- Single share disposal this year: take the average cost per share from above, plug the sale proceeds and shares sold into mycgttax's shares calculator and get your gain and tax owed instantly.
- Sold property and shares in the same tax year: most calculators (including exact-match domains) apply your £3,000 annual exempt amount and rate bands to each disposal type separately — silently overstating your allowance. Use mycgttax's combined multi-asset calculator, which allocates the allowance once, the way HMRC's Self Assessment CGT summary actually assesses it.
- Unsure if you're a basic or higher-rate taxpayer this year: mycgttax factors in your other taxable income to apply the correct 18%/24% split automatically.
- Sold crypto too: the site's crypto satellite covers disposal, swap, and gift treatment under HMRC's crypto guidance — feed those gains into the same combined calculator.
- Not sure how much allowance you have left: check the allowance calculator before you dispose of anything else this tax year.
mycgttax.co.uk covers property, shares, crypto, and combined multi-asset disposals against 2026/27 HMRC rates, updated for the October 2024 rate alignment.
Calculate your UK Capital Gains Tax →FAQ
What is a Section 104 pool?
HMRC's method for tracking cost basis when you've bought shares in the same company at different times. Rather than tracking each purchase separately, qualifying shares are pooled and given one average cost per share.
How do I calculate my Section 104 pool cost?
Add up the total cost of all qualifying purchases (including dealing costs), divide by total shares held. That average applies to whatever quantity you sell later.
Does this calculator handle same-day and 30-day matching?
No. It only pools long-term holdings under Section 104. Same-day and 30-day bed-and-breakfasting disposals are matched separately by HMRC's rules, before anything reaches the pool.
What costs can I include?
Purchase price plus incidental acquisition costs — broker commission, Stamp Duty Reserve Tax (0.5% on UK share purchases).
How do I work out my actual tax once I have my pool cost?
Multiply average cost per share by shares sold, subtract from proceeds for your gain, then apply your annual exempt amount and rate. A combined calculator does this in one step, including other disposals in the same tax year.
Do stock splits or bonus issues affect the pool?
Yes — add the resulting shares as a lot with £0 cost. This lowers your average cost per share correctly.
Is this tool free and does it store my data?
Yes, free, no signup. All calculations run in your browser — nothing is sent to a server or stored.