Since 2026, Belgium has taxed capital gains on financial assets at 10%, on gains realized from 1 January 2026 onward. Behind this single rate lies a little-known choice with heavy consequences: opt-in or opt-out. And depending on what you choose — or what happens by default — you could be advancing money to the tax office that you never had to pay.
From 1 June 2026, your online bank or platform established in Belgium starts withholding 10% on your capital gains, directly at source. This is the default regime, the opt-in. Convenient, on the face of it: you have nothing to do.
The problem is what this withholding does not do. The intermediary withholds indiscriminately, transaction by transaction, account by account. It does not apply the €10,000 exemption per person that you are entitled to each year. It does not offset the capital losses you realized elsewhere. Each platform withholds in its own corner, without any overall view of your situation.
The result is mechanical: you pay too much, and you tie up cash. This overpaid money is not lost, but you only get it back much later — when you file your return, around 2027 for 2026 income. In the meantime, it is the tax office that holds your cash, not you.
Let’s lay out the two options clearly.
The distinction matters. Under opt-in, you bear a flat withholding that you then have to claim back. Under opt-out, you take back control: you declare the amount actually due, after the exemption and after offsetting your losses.
Note: if you invest through a foreign platform, no Belgian intermediary withholds for you. You are in self-declaration anyway — in other words, effectively in opt-out.
Three concrete reasons tip the balance.
The €10,000 exemption recovered straight away. The €10,000 exemption per person per year (indexed, up to €30,000 for a couple) is simply not applied at the withholding stage. Under opt-in, you only recover it afterward, via your return. Under opt-out, it applies immediately: you only declare the capital gain that actually exceeds this threshold.
Offsetting losses across accounts. Withholding is done by each Belgian intermediary separately. If you gain on one platform and lose on another, opt-in ignores your losses. Self-declaration, by contrast, lets you offset your capital losses against your gains.
Cash flow preserved. No flat withholding to claim back later. Your money stays your money until the tax is actually due.
Take an illustrative example. You realize +€12,000 of capital gains on one platform and −€3,000 of losses on another. Under opt-in, 10% is withheld on €12,000, that is €1,200 advanced to the tax office, with no account taken of the loss or the exemption. Under opt-out, your real base is €9,000 — below the €10,000 exemption. Result: nothing to advance. (Simplified example, to be confirmed for your situation.)
The framework for this tax rests on a law adopted on 3 April 2026 and published in the Belgian Official Gazette on 21 April 2026. But the subject is young, and the practical arrangements keep evolving. This instability is, in itself, an additional argument in favor of opt-out: keeping control of your return gives you the flexibility to adjust your position as the framework becomes clearer, rather than being subject to an automatic withholding that is hard to correct mid-year.
Two 2026 deadlines come up often — and they are easily confused.
31 May 2026 = end of the transitional period. From 1 January to 31 May 2026, no withholding at source is applied; withholding by Belgian intermediaries starts on 1 June 2026.
31 August 2026 = the deadline to notify your opt-out, that is, to signal that you will declare your capital gains yourself.
These are two different things. The first concerns when the withholding machine switches on. The second, your window to take back control. Letting 31 August pass without acting means staying in opt-in by default — with everything that implies for your cash flow.
Deciding between opt-in and opt-out requires an overall view that few investors have at hand: how much in gains, how much in losses, across how many accounts, and ultimately how much is really due.
This is precisely what Benchmarkr puts at your disposal. By aggregating your accounts, the app computes your capital gains and the tax actually due, provides a consolidated annual view and helps you concretely compare the two scenarios so you can decide with full knowledge of the facts. Assistance and computation — not tax advice or investment advice, but the clear figures on which to base your decision.
To explore the mechanism in detail, see our reference page: Capital gains tax 2026: opt-in or opt-out?.
Before 31 August, ask yourself the question while there is still time to act. Compare opt-in and opt-out or create your Benchmarkr account to see your capital gains and the tax due.
This article is provided for information purposes only and does not constitute tax advice.
Aggregate your accounts, compute your TOB and capital gains, and anticipate your deadlines — without paying more than necessary.
Discover Benchmarkr