Belgium spent decades as the country where private investors paid no capital gains tax. That ended on 1 January 2026. Gains you realize from now on are taxed at 10%, above a €10,000 annual exemption, and every position you already held got a new starting price: its value on 31 December 2025.
That last part is the one people miss, and it is good news. The tax is not retroactive. Twenty years of gains you were sitting on at the end of 2025 are not suddenly taxable. Only what your portfolio does from 2026 onwards is.
What follows is how the calculation works in practice. It is the arithmetic, not advice, and the fine print of your own situation belongs with your accountant.
The photo: your cost basis resets to 31 December 2025
For anything you already owned, the price you actually paid stops mattering for tax. What matters is what the position was worth on 31 December 2025. Belgians have taken to calling it the photo.
Say you bought 100 units of an index fund in 2019 at €80 each, so €8,000. On 31 December 2025 they were worth €150 each, so €15,000. You sell the lot in September 2026 at €190, for €19,000.
Your taxable gain is €4,000, not €11,000. The €7,000 you made before 2026 sits below the photo line and is not taxed.
This is why the photo value is worth getting right for every position you hold. It is the number the whole calculation stands on, and nobody is going to compute it for you across every account you own.
The higher-of rule, for positions that were underwater
A reset cuts both ways. If a position had fallen below what you paid by the end of 2025, resetting its basis down to the photo value would manufacture a taxable gain out of a real loss.
Belgium's transitional rule, which runs until the end of 2030, prevents that: where your actual acquisition cost is higher than the photo value, the acquisition cost is used instead.
An example. You bought a position for €12,000 in 2021. On 31 December 2025 it was worth €9,000. You sell it in 2026 for €11,000.
- On the photo value alone: a €2,000 gain, taxable.
- With the higher-of rule: basis is your €12,000 cost, so you have a €1,000 loss, and that loss is deductible against your other gains.
You are still down €1,000 on the trade. The rule just stops you paying tax as though you were up.
The €10,000 exemption applies to the year's net gain
The exemption is not per sale or per account. It applies once, to your net result for the whole year: gains minus losses.
Suppose across 2026 you realize €26,000 of gains and €2,000 of losses.
- Net gain: €24,000
- Less the exemption: €14,000
- Tax at 10%: €1,400
If your net result for the year is a loss, there is no tax. And if your net gain lands under €10,000, there is no tax either, which for a lot of ordinary portfolios is the practical outcome most years.
Unused exemption carries forward. A year in which you do not use up the base exemption adds €1,000 to the following years' exemption, for up to five years, so the exemption can reach €15,000. A buy-and-hold investor who realizes nothing from 2026 through 2030 and then starts selling in 2031 has €15,000 of exempt gains that year, not €10,000. It works the other way too: realizing past the base consumes what you carried. For anyone planning withdrawals around the exemption, this is worth a full extra year of tax-free selling over a decade.
Note that dividends and interest are a separate matter and always were: those keep being taxed at the standard 30% withholding rate, not at 10%. Three different kinds of income, three different rates, and only one of them is new.
Portfolios held inside a company are also a different regime again, taxed as corporate profit at 25% with no personal exemption.
Why this is harder than it looks
The rules above fit on a page. Applying them to a real portfolio is where it falls apart, for four reasons.
You need a 31 December 2025 valuation for every position. Not a portfolio total. A per-position value, because each one gets its own basis. If you hold thirty positions, that is thirty numbers to establish and keep.
Your broker will not do it for you. Statements show what you paid and what it is worth today. The photo value on one specific historical date, per position, is not a standard report, and if you use more than one broker, no single statement sees your whole picture anyway.
Partly sold positions get messy. If you have sold some units of a holding since, the cost attributable to the units you still hold is not the lifetime figure your broker shows. Getting this wrong in the wrong direction means overstating your gain and overpaying.
Then there is everything outside a broker. Bank savings, money-market funds, and on-chain holdings all need the same treatment, and the further you get from a mainstream Belgian broker, the less likely anyone hands you a clean historical valuation.
None of this is hard arithmetic. It is bookkeeping, and it is the kind that punishes you a year later when you cannot reconstruct where a number came from.
Getting the number out of your own records
This is the problem our tax engine was built for, and the Belgium preset implements exactly the rules above: the 10% rate, the €10,000 exemption, the 31 December 2025 reset, and the higher-of transitional rule.
The approach is that the tax figure is a replay of your own ledger rather than something you assemble in a spreadsheet each spring. Every sale is matched to the specific lots it came from, using FIFO or weighted average, and each figure stays traceable back to the trades that produced it. Click any gain and you see the lots behind it, which of them used the photo value, which used acquisition cost under the higher-of rule, and what per-unit price each contributed.
Where a photo value is genuinely unknown, the engine says so rather than guessing, and flags the position under Needs attention until you declare one. Unknown cost is counted as zero, which overstates the tax rather than quietly understating it. You would rather find out now than in a letter.
Positions can be synced read-only from brokers, EU banks and exchanges, so the ledger stays current without you maintaining it by hand, and the forecast reads the same tax settings, which means you can see what a planned sale costs you before you make it rather than after.
The feature documentation covers the mechanics in full, including declaring photo values and the escape hatch for when a computed basis is itself wrong.
What this article does not cover
Deliberately: which precise assets fall inside the regime for your situation, how substantial shareholdings are treated, how losses carry across years, and how your particular broker reports to the tax authorities. Those are real questions with answers specific to you, and they belong with an accountant.
What a good tool gives you is the arithmetic done consistently and traceably from your own records, so the conversation with your accountant starts from a number with its working attached instead of a shoebox.
You can try the calculation on your own portfolio during the 14-day trial, or look at the live demo first to see the tax report on example data.
If you are still tracking all of this in a spreadsheet, we wrote about where that breaks down. The 2026 rules moved that line considerably.
Thomas Heremans is the founder of Krosos and built the tax engine described here.
As always: these are estimates computed from your own assumptions and records, meant for planning and for your declaration. Not tax advice.