FiscalFox compares available market prices with the Section 104 pooled cost built from your imported transaction history. It then highlights positions that appear to be below cost and shows the potential reduction in gains if those losses were validly realised.
Calculate first
Import your taxable-account history so the UK pooled cost can be reconstructed.
Review estimates
See portfolio value, unrealised gains and positions currently below pooled cost.
Plan carefully
Compare possible losses with current-year gains and check the share-matching rules.
What the calculator estimates
The portfolio review gives a rough current picture rather than pretending to predict an exact future disposal. It estimates current value, unrealised gains and unrealised losses using available end-of-day prices. Positions below their pooled cost are grouped into a tax-planning view for the current tax year.
- Current portfolio value and allocation by holding.
- Estimated unrealised gain or loss against Section 104 pooled cost.
- Potential capital-loss offset compared with gains already realised in the selected tax year.
- A clear split between holdings with a supported price and holdings that still need review.
Illustrative 2026/27 example
Suppose you have £7,250 of realised gains and investments currently showing £1,460 below their pooled cost. If qualifying disposals produced the full £1,460 allowable loss, the estimated gains after in-year losses would be £5,790. Applying the £3,000 annual exempt amount would leave £2,790 of taxable gains.
This is an illustration, not a recommendation to sell. GOV.UK explains that allowable losses are deducted from gains and that unused losses may be carried forward when properly claimed. See the official guidance on losses for the reporting rules and deadlines.
Why portfolio profit and loss is not the tax result
A broker app often compares today’s price with a simple purchase price. UK share calculations can be different because disposals are matched first with same-day acquisitions, then acquisitions in the following 30 days, and only then with the Section 104 pool. Fees, corporate actions, ownership and transactions across taxable accounts can also change the result.
Market prices never rewrite your tax calculation
Completed disposals and reports continue to use the imported transaction evidence. If a price is unavailable, only the optional portfolio estimate is incomplete.
HMRC’s Helpsheet 284 describes the share-identification rules used in a UK Capital Gains Tax calculation.
Use the review before the tax-year deadline
The planning view is most useful after you have calculated gains for the current tax year and while there is still time to consider your options before 5 April. A loss shown today can change with the market, and a later purchase can change the tax treatment, so confirm the position again before acting.
Start with the UK Capital Gains Tax calculator. Paid customers can then open the portfolio review from the calculation review stage; it is not added to the main navigation or mixed into the core calculation.
Tax-loss harvesting questions
Is tax-loss harvesting allowed in the UK?
Yes. Selling an investment at a genuine loss can create an allowable capital loss, but the disposal must follow UK share-matching rules and the loss normally needs to be reported to HMRC before it can be carried forward.
Does an unrealised loss reduce Capital Gains Tax?
No. A fall in market value is only an estimate until the investment is disposed of. The tool highlights possible loss positions; it does not treat them as realised losses in your tax calculation.
Can I sell a share and buy it back?
You can, but a purchase of the same class of share on the same day or during the following 30 days can change which acquisition cost is matched to the disposal and therefore change the gain or loss.
Do missing market prices affect my FiscalFox tax calculation?
No. Market prices are used only for the optional portfolio estimate. FiscalFox calculates completed disposals from your transaction history, so a missing live price does not change the tax calculation or supporting report.
Is the portfolio review personal tax or investment advice?
No. It is an indicative planning view based on imported records and available market prices. Check the records and current price before acting, and seek professional advice for unusual or material decisions.
Calculate the tax record before reviewing the market estimate
This keeps a rough portfolio planning view in the right context: after the evidence-backed transaction calculation, not in place of it.
Open the UK CGT calculator