Every net worth tracker can tell you what your shares are worth today. Almost none can tell you what they cost in the way HMRC counts it — and that is the number that decides your tax bill when you sell.
See it with example data £5.99/month · free trialMost people assume that when they sell shares they bought in batches, they can choose which batch they sold — the cheap ones or the expensive ones. For UK capital gains tax on shares of the same class in the same company, you generally cannot.
Instead the holdings sit in what HMRC calls a Section 104 holding: one pool of shares with one pooled cost. Every purchase adds to both the number of shares and the total cost. When you sell, your cost basis is the average across the whole pool, in proportion to what you sold.
A worked example. You buy 100 shares at £10 (£1,000), then later 100 more at £20 (£2,000). The pool is 200 shares costing £3,000 — an average of £15 each.
You sell 100 at £25. Instinct says you sold the cheap ones and made £1,500. The pooled cost says otherwise: 100 × £15 = £1,500 of cost against £2,500 of proceeds, so the gain is £1,000.
The remaining 100 shares stay in the pool, still carrying £1,500 of cost. Get this wrong in either direction and you either overpay tax or under-declare it.
There are rules that sit in front of the pool — disposals matched to same-day acquisitions first, then to anything bought in the following 30 days (the "bed and breakfasting" rule). Only what is left over is matched against the Section 104 pool. OwnWorth models the pool; the sequencing rules are worth reading about at HMRC's helpsheet HS284.
Because value and gain are different questions, and only one of them tells you what you can spend.
| What most trackers show | Today's market value of the holding. |
|---|---|
| What OwnWorth also holds | The pooled cost, the number of units, and therefore the unrealised gain sitting inside that value. |
| Why it matters | A £60,000 holding with £45,000 of cost is not the same asset as a £60,000 holding with £8,000 of cost. One is nearly all spendable; the other has a tax bill attached. |
This is the same idea as the distinction between paper value and spendable money that runs through the whole app. A number you cannot actually access is not the same as one you can.
OwnWorth is not tax software and does not file anything. It keeps the pool so that the gain inside your net worth is honest, and so you know roughly where you stand before a sale rather than in the following January. It does not apply your annual exempt amount, does not know your income tax band, does not handle share reorganisations, rights issues or accumulation-unit equalisation, and does not produce a capital gains computation for HMRC.
For a real disposal of any size, an accountant is worth the fee. What this does is stop you being surprised.
Only if you sell and then buy the same holding back within 30 days. If you do, the disposal is matched against that repurchase instead of the pool, which usually removes the tax advantage you were hoping for. It is worth knowing the rule exists before you act on a "sell and rebuy to use my allowance" idea.
No — gains inside an ISA or a pension are not taxable, so pooling has no tax consequence there. OwnWorth still tracks cost inside those wrappers, because knowing what you paid is useful even when nobody is taxing it.
Accumulation units complicate the real HMRC position, because reinvested income you have already been taxed on can be added to your cost base. OwnWorth does not model that automatically. If you hold accumulation funds outside a wrapper, treat the pooled cost here as a good estimate, not a filing.
Yes. Each holding shows value, pooled cost and unrealised gain separately, and the same split rolls up to the portfolio.
Yes. Everything is on one plan at £5.99 a month, and the trial needs no card.
There is a free Section 104 pooling calculator on this site — enter your buys and sells, see the pooled cost and the real gain. No signup, and nothing leaves your browser.
Try it with the example portfolio Tracking wealth as a couple →
OwnWorth is a record-keeping and planning tool, not financial, legal or tax advice, and we are not regulated to give any. Figures you enter are your own; projections are estimates based on them.