Manual

Getting started

There are three ways to add trades to the calculator:

• Manual entry — use the trade form to add one trade at a time. Suitable for a small number of trades or corrections.
• CSV import — upload a CSV file with your trades. See the import format section below for the required columns.
• Paste from spreadsheet — copy rows from Excel or Google Sheets and paste them directly into the import dialog. The columns must match the CSV format.

Trade fields

Understanding results

Once you have entered trades, the calculator applies HMRC matching rules and produces:

• Tax year summaries — total gains, total losses, net gain/loss, and whether the Annual Exempt Amount (AEA) covers it.
• Disposals — each sell or transfer, showing proceeds, allowable cost, and the gain or loss. You can expand each disposal to see exactly how it was matched.
• Acquisitions — each buy showing cost and how it feeds into the pool.
• Section 104 pool state — the running pool for each symbol showing quantity and cost at each point in time.

Matching rules explained

HMRC mandates three matching rules applied in strict priority order:

1. Same-day rule — a disposal is first matched against any acquisitions of the same security on the same day. If you buy and sell on the same day, they are matched together regardless of order.
2. Bed & breakfast rule (30-day) — any remaining unmatched disposal quantity is then matched against acquisitions in the following 30 days, taking the earliest first. This prevents selling at a loss and immediately rebuying to crystallise the loss.
3. Section 104 pool — any quantity still unmatched after the above rules is matched against the Section 104 pool, which is a weighted average of all prior unmatched acquisitions.

The rules are applied in this order for every disposal. A single disposal can be partially matched by different rules (e.g. 20 shares same-day, 30 shares from the pool).

Stock splits

To record a stock split, add a split entry specifying the date, symbol, and ratio (e.g. 4:1). The calculator adjusts the pool quantity by the split ratio while preserving the total cost basis. This means the average cost per share decreases but the total pool value stays the same — exactly as HMRC requires.

Add the split on the effective date. All subsequent trades should use post-split quantities and prices.

Transfers

A transfer represents a no-gain/no-loss disposal, typically a transfer to a spouse or civil partner. The disposal proceeds are deemed to equal the allowable cost, so no gain or loss arises.

Transfers participate fully in matching rules. If you buy shares on the same day as a transfer, the same-day rule applies and the transfer is matched against that day's acquisition — not the Section 104 pool. Similarly, if you buy shares within 30 days after a transfer, the bed & breakfast rule applies. This can produce surprising cost bases for the recipient.

Simulated trades

The simulator lets you add "what-if" trades to see how they would affect your capital gains position without committing them as real trades. Simulated trades are shown in a separate colour and are included in the calculation but clearly marked in the results.

Use this to plan disposals: check how selling a specific quantity at a target price would affect your gains for the current tax year before you execute the trade.

Optimise gains

The AEA optimiser analyses your current pool and suggests disposals that would use up your remaining Annual Exempt Amount without exceeding it. This is useful near the end of a tax year when you have unused allowance.

The optimiser considers all securities in your pool and finds combinations of disposals that maximise use of the AEA. The suggestions are added as simulated trades so you can review the full impact before acting.

Export / PDF

You can download a PDF report for each tax year. The report includes all disposals with matching details, pool state, and a summary of gains and losses. This is suitable for attaching to your self-assessment or sending to an accountant.

Select the tax year you want to export from the results tab, then click the download button.

Import formats

The CSV importer expects the following columns in this order:

Example: