Box 3 is the Netherlands' tax on wealth - savings, investments, and (secondary) property - rather than income. Since 2025, you can choose between the standard notional-yield method or, if it works out cheaper, being taxed on your actual returns instead.
The Belastingdienst assumes your savings, investments, and debts each earned a fixed, government-set rate of return, taxes that assumed (notional) yield at 36%, regardless of what you actually earned that year.
If your assets genuinely underperformed the notional assumption - a bad year for investments, or a high-interest debt - you can opt to be taxed on what you actually earned instead. It only helps if your real return was lower than the notional one; the Belastingdienst won't apply it automatically.
A portion of your Box 3 assets (higher for fiscal partners filing jointly) is exempt from tax entirely each year before either method is applied.
Which method should I use?
Whichever results in less tax for your specific numbers - the linked calculator lets you compare both using the figures you enter.
Does Box 3 cover my primary residence?
No - your main home falls under Box 1 (eigenwoningforfait), not Box 3. Box 3 covers savings, investments, and any secondary property.
What if I have debts?
Debts above a threshold reduce your taxable Box 3 base, though the notional deemed rate on debt is different from the rate on savings or investments.