Interactive Brokers and Belgian taxes: which export to import, and what follows

What IBKR does — and does not — settle for a Belgian resident

Interactive Brokers is established outside Belgium and is not part of the Belgian withholding system. For a Belgian resident, across the three taxes, that means:

• the stock-exchange tax (TOB) is not withheld — you file it yourself, for each month in which you traded, via MyMinfin → Miscellaneous taxes → DivTax;
• the 30% Belgian withholding tax on dividends is not withheld, and no Belgian tax certificate is issued — dividends go in the annual return;
• the 10% capital-gains tax on gains realised from 2026 is not withheld — it too is calculated and declared annually, by you.

What does not happen at source has not disappeared: it simply becomes your work. How that compares with other platforms is covered in which brokers withhold which taxes. This page is about the step before the returns: getting the right file out of IBKR, so those three filings rest on something.

The export: an Activity Flex Query in CSV

The file that brings everything in one go is the Activity Flex Query:

1. In the Client Portal, open Performance & Reports, then Flex Queries.
2. Create an Activity Flex Query and tick at least the Trades, Cash Transactions and Open Positions sections.
3. Choose the CSV format.
4. For the period, take it from your first purchase — not just the current year.
5. Run the query and download the CSV file it produces.

The Open Positions section is no small print: it carries the dated valuation of your securities, which is what supplies your values on 31 December 2025 without you having to enter them. For positions held before 2026, the gain is measured from that step-up value, not from your original purchase price.

The classic Activity Statement in CSV — the annual statement, for instance — is read as well. A year-end statement is moreover your evidence of the 31 December 2025 value, dated by the statement itself.

Why the full period? A single-year export contains disposals whose matching purchases live in an older file. Without a purchase date there is no cost to match — and no way either to establish how long you held a position.

Why naming your broker matters on upload

MEXEM and LYNX operate on Interactive Brokers' infrastructure and export exactly the same file — byte for byte identical to IBKR's. The file itself therefore does not say who your broker is, and an upload without a broker name is treated as an IBKR import by default. The import tells you so, too.

Tax-wise, these are nonetheless three different situations:

• IBKR withholds nothing — everything above applies in full.
• MEXEM does withhold the stock-exchange tax at source. Naming Interactive Brokers on upload would lead you to self-declare — and pay a second time — a tax MEXEM has already remitted.
• LYNX withholds the stock-exchange tax and the dividend withholding, but runs the opt-out system for capital gains: that 10% remains yours.

Same file, three different lists of what is left for you to do. The broker name you pick on upload is therefore no administrative detail: it decides which taxes are treated as settled and which as yours.

What the import produces

Once the file is read, the import produces the three totals in the form the returns expect:

• the TOB total per month in which you traded, for the monthly DivTax return;
• the dividend lines — gross foreign dividend and foreign withholding already applied — for the annual return;
• the realised capital gains, measured from the 31 December 2025 step-up value where it applies and matched lot by lot for later purchases.

Which return receives each total, and by which deadline, is what the self-declaration walkthrough covers. The figures are reported; the decision to file them, and the check against your own statements, remain with you.

From file to figure

The calculator reads the Flex Query and the Activity Statement directly and computes the three totals across all your trades. To price a single order first, the stock exchange tax simulator shows the TOB on one transaction.

Verify your figures

Belfolio describes these steps for information only. This is not tax advice. Check your situation with the FPS Finance or your accountant before filing.