The rule
Belgium's 10% capital gains tax applies to gains realized from 1 January 2026. Gains you accumulated before that date are not in scope.
The mechanism that delivers this is the step-up: for listed assets held before 2026, the acquisition value used to compute your gain is the last closing price of 2025, not the price you originally paid.
What that means in practice
Suppose you bought a world ETF at €52 in 2019. At the end of 2025 it closes at €118. You sell in 2026 at €112.60.
- Measured against what you paid, you made €60.60 per share.
- Measured against the step-up value, you made −€5.40 per share — a loss.
Only the second figure matters for this tax. Six years of growth are simply outside the regime.
When the purchase price is used instead
If your evidenced historical cost is higher than the 31/12/2025 close, you may use it instead — an option reported as available until 31 December 2030. This is the case for anything bought near a peak and still under water at the end of 2025.
"Evidenced" is doing real work in that sentence: keep the contract notes.
Two parts of this remain unconfirmed in our source rules, and we flag them rather than present them as settled. The first is the option itself: that the higher historical cost may be substituted at all is reported consistently, but we have not seen it in the administration's own text. The second is what the 2030 date actually bounds — the date of disposal, the date of acquisition, or the tax year. Those readings do not give the same answer for a sale made close to the deadline. Confirm both with the FPS Finance or your accountant before relying on either.
Why a missing 2025 price costs you money
If a tool cannot find the 31/12/2025 close for one of your holdings, it has to fall back to your purchase price. For a long-held position that price is usually much lower — so the computed gain, and the tax on it, come out too high.
Belfolio names every instrument it could not price rather than quietly showing you a larger number.
The ten-year rule
Separately, gains on assets held continuously for at least ten years are exempt under the general regime. That is measured per parcel, from the actual acquisition date — which is why per-lot records matter and a single average purchase price is not enough.
How Belfolio applies this
Belfolio keeps both figures on every lot: your evidenced historical cost and the 31/12/2025 value. Where the rule lets the higher evidenced cost stand in, both numbers are there to compare.
The reference price itself is resolved in a fixed order. A figure you can defend — a year-end broker statement, or a price you typed in with the statement in hand — always outranks anything we fetch. A fetched close is shown with its source and can be replaced, never silently trusted.
The split between the taxable part and the exempt part is then computed per disposal, and the ten-year clock keeps running per lot — averaging the pre-2026 cost does not erase the acquisition dates.
What a sale yields against a given step-up value, the capital gains tax simulator computes without you uploading anything.
Always verify your figures
Belfolio computes and presents these amounts for information only. This is not tax advice. Verify them with the FPS Finance or your accountant before filing.
Published 2 Aug 2026
Frequently asked questions
- Does Belfolio apply the 31/12/2025 step-up automatically?
- Yes. Every lot acquired before 2026 carries both its evidenced historical cost and the 31/12/2025 reference value, and the taxable and exempt parts are computed from whichever the rule calls for. Reference prices come first from your own evidence — a year-end broker statement or a price you typed in — and only then from a fetched close, which is shown with its source and can be replaced.
- What happens when the 31/12/2025 price for one of my holdings is missing?
- Belfolio names the instrument it could not price rather than silently falling back to your purchase price — a fallback that would usually overstate both the gain and the tax on a long-held position.
- Can I use my real purchase price instead of the 2025 close?
- Reportedly yes, when it is higher and evidenced, until 31 December 2030 — but parts of this option remain unconfirmed in the administration's own texts, and Belfolio flags them as unsettled rather than presenting them as certain. Keep your contract notes either way.