What does opt-out mean for the Belgian capital-gains tax?

That your intermediary does not withhold the 10% at source. The tax does not disappear: you compute it yourself and settle it through your own return. Anyone investing through a foreign broker — DEGIRO, Interactive Brokers, Trade Republic, Revolut, Trading 212 — is in that position by default, without ever having chosen it.

Is opt-out cheaper or more expensive than withholding at source?

The rate is 10% either way. The difference is timing and the exemption: a withholding at source does not know your annual €10,000 exemption and takes no account of losses you realise later in the same year. Under opt-out you build both into the figure instead of reclaiming them afterwards.

Which code do I use on my return for the capital-gains tax?

It has not been published yet. FPS Finance has a page on the tax online, but the annexes carrying the boxes and codes for assessment year 2027 are still missing from it. We will fill that in once the administration publishes it; until then, any number circulating elsewhere as 'the code' is guesswork.

You are on the opt-out side of the capital-gains tax: what lands on you

What opt-out actually is

The 10% capital-gains tax applies to net gains you realise on financial assets from 1 January 2026. Who collects it is not fixed: an intermediary established in Belgium withholds it at source in principle, but an investor can ask it not to. That is opt-out.

Opt-out does not remove the tax. It only moves who computes it, who declares it and who pays it — from your broker to you.

Two groups are in that position:

Those who chose it with a Belgian intermediary.
Those who never chose it but are in it anyway. A foreign broker does not withhold the Belgian tax. That covers DEGIRO, Interactive Brokers, Trade Republic, Revolut and Trading 212. It also covers at least one Belgian player: LYNX Belgium does withhold the stock-exchange tax at source, but runs an opt-out system for the capital-gains tax. Withholding TOB therefore does not imply withholding this tax.

If you are in that second group you signed nothing, and it is easy to assume there is nothing to do. That is the expensive mistake.

Three jobs move to you

1. Computing the gain yourself

With no withholding at source, no intermediary establishes the figure for you. You start, lot by lot, from an acquisition value — and for everything you held before 2026 that is not what you once paid: it is the value of the asset on 31 December 2025.

One nuance is routinely skipped. If that reference value is lower than your original purchase price, the original price applies — and the loss that follows from it cannot be used to offset other gains.

See the step-up value at 31 December 2025.

2. Adding it up across brokers

The exemption is per person per year, not per account. That is what makes opt-out awkward as soon as you invest with more than one intermediary, because none of them sees your full year.

The first €10,000 of realised gains is exempt (assessment year 2027). Unused exemption can be carried forward, but published sources disagree on the mechanism — one reading has an unused year accruing +€1,000, another a running balance of unused exemption, both up to a total of €15,000. Those produce different numbers for the same investor, so treat any carry-forward figure as a working assumption until it is confirmed against a primary source. See the €10,000 exemption.

The mixed case is the hardest: one broker has already withheld 10% without knowing your exemption, another has withheld nothing. Both have to land in the same annual total. See multiple brokers, one Belgian return.

3. Declaring and paying

This is the step that cannot be finished today. See below.

What FPS Finance has not published

FPS Finance has a page on the capital-gains tax online. It confirms the rate, the €10,000 exemption for assessment year 2027, and the 31 December 2025 reference value.

What is not on it matters just as much:

The box and the codes the gain goes under on the assessment-year-2027 return. The annexes to that page are announced but absent.
The exact date from which Belgian intermediaries must withhold. Sources contradict each other.
The cut-off date by which opt-out could still be elected.
How gains from the first months of 2026 are settled, in the window where the tax was already running but withholding was not yet possible.

We will only put those figures here once they come from a primary source. That is not hedging: on this specific point, dates that differ by weeks are circulating right now, and a wrong date costs more here than a missing one.

What you can do now

The return itself only arrives in 2027. The file that makes it possible is built now, and cannot be reconstructed later.

Fix the 31 December 2025 value lot by lot, with the source and the date you looked it up. End-of-2025 closing prices get harder to retrieve each year.
Keep the date, quantity and proceeds of every sale. A broker that withholds nothing usually issues no Belgian tax certificate either.
Record which account is on opt-out and which is not. That determines whether an amount is already settled.
Total your year across all accounts, never account by account.

One risk that sits apart from the 10%

The 10% rate assumes your transactions fall within the normal management of private wealth. If your pattern falls outside it — through frequency, borrowed money, or scale relative to your wealth — the characterisation changes and 10% is no longer the figure.

That distinction long predates 2026 and your opt-out choice does not affect it. What does change: with no intermediary withholding, your return becomes the only place that pattern is visible. See speculator or professional investor.

What the two routes mean for an expected year — withheld at source against self-declared, reclaimable part included — the opt-out simulator puts side by side.

What a given sale would leave withheld, or for you to declare, the capital gains tax simulator works out.

This is general information on Belgian taxation, not tax or investment advice. Figures and dates missing here are missing because they are not yet confirmed by a primary source.

Published 11 Aug 2026

Frequently asked questions

What does opt-out mean for the Belgian capital-gains tax?: That your intermediary does not withhold the 10% at source. The tax does not disappear: you compute it yourself and settle it through your own return. Anyone investing through a foreign broker — DEGIRO, Interactive Brokers, Trade Republic, Revolut, Trading 212 — is in that position by default, without ever having chosen it.
Is opt-out cheaper or more expensive than withholding at source?: The rate is 10% either way. The difference is timing and the exemption: a withholding at source does not know your annual €10,000 exemption and takes no account of losses you realise later in the same year. Under opt-out you build both into the figure instead of reclaiming them afterwards.
Which code do I use on my return for the capital-gains tax?: It has not been published yet. FPS Finance has a page on the tax online, but the annexes carrying the boxes and codes for assessment year 2027 are still missing from it. We will fill that in once the administration publishes it; until then, any number circulating elsewhere as 'the code' is guesswork.