What is changing
Today, Box 3 taxes a fictive yield on your wealth on 1 January. Stocks and ETFs are taxed at a fixed assumed return (about 6%/year) regardless of what they actually did. The Wet werkelijk rendement box 3 replaces this with a tax on actual returns.
For most investments — including shares and ETFs — the bill introduces a vermogensaanwasbelasting (capital-growth tax). It taxes the realised income (interest, dividends, rent) plus the change in market value over the year, whether you sold or not. Real estate and start-up shares fall under a separate vermogenswinstbelasting and are only taxed on realisation.
How current Box 3 works
Each year on 1 January the Belastingdienst values your wealth and splits it into three buckets: spaargeld (savings), beleggingen / overige bezittingen (investments and other assets) and schulden (debts). A different fictive yield applies to each bucket.
The taxable base is your total wealth minus the heffingsvrij vermogen (€57,684 for 2025). The fictive return is then applied to the taxable share, and the result is taxed at 36% (in 2024 and 2025).
How Box 3 2028 will work
For shares the calculation each year becomes: income = (value on 31 Dec − value on 1 Jan) − money you put in + money you took out + dividends received − costs. That income figure can be negative.
A flat €1,800 per taxpayer per year is exempt. Anything above is taxed at 36%. Losses carry forward indefinitely (subject to a €500 minimum threshold), but cannot be carried back.
What this means for DeGiro investors
If you mostly buy and hold a diversified portfolio that compounds steadily, the new system tends to be cheaper in flat or down years (because there's no fictive return on paper) and more expensive in big up years (because the actual gain may exceed the assumed yield). Across a typical investing decade the two systems can be remarkably close — but with very different year-to-year volatility.
The dashboard's Box 3 page replays both rule sets against your actual DeGiro transaction history so you can see, year by year, what you would have paid under each. Everything runs in your browser — your transactions never leave your device.
See the comparison on your own data →
When will the new Box 3 take effect?
The cabinet aims for 1 January 2028, but the bill still needs to clear the Eerste Kamer (Dutch Senate). Treat the calculator as a planning tool, not a fixed forecast.
Will I be taxed on unrealised gains?
Yes — for shares and ETFs the new system is a yearly mark-to-market. The change in market value over the year is part of taxable income whether you sold or not.
What happens in a losing year?
Negative income carries forward indefinitely (subject to a €500 threshold) and reduces taxable income in future profitable years. Losses cannot be carried back.
What about my savings account?
Bank deposits keep being taxed under similar mark-to-market rules (interest plus FX changes). FolioInsights only models stock activity from DeGiro, so the comparison here assumes DeGiro is your only Box 3 wealth.
Is the calculator tax advice?
No. It is a simulation based on public rates and the published bill. For an actual filing decision, talk to a Dutch tax advisor.
Sources
Rijksoverheid: Plannen werkelijk rendement box 3. Belastingdienst: Income in box 3. Deloitte (technical brief on the bill): Wet werkelijk rendement box 3.