Why the Standard FIRE Calculator Overstates Your Number in the Netherlands

Most FIRE calculators start with the same formula: take what you want to spend each year, multiply by 25, and that's your number. If you need €40,000 a year to live on, you need €1 million invested. The math rests on a simple assumption that almost never holds in the Netherlands: that your investment stash is tax-free. It isn't. Box 3 wealth tax takes a slice every year, regardless of whether you withdraw money or not — and that changes exactly how much you need to save before you can actually walk away from work.

The 25× shortcut and what it assumes

The "multiply by 25" rule comes from the 4% safe withdrawal rate — a research-backed principle that you can withdraw 4% of your invested capital each year and have high confidence it'll last through a 30-year retirement without running out. So €40,000 annual spending ÷ 0.04 = €1,000,000. Simple, portable, and works as a rough starting point everywhere.

But that rule was developed for tax environments where wealth itself isn't taxed — where you pay tax only on the income your investments generate or on the gains you realize when you sell. In those places, a €1 million stash earning a 4–7% actual return keeps pace with inflation and your withdrawals. In the Netherlands, you're paying Box 3 tax on the stash itself, based on a deemed return, regardless of what you actually earn or spend.

Box 3 taxes your wealth, not your withdrawal

Here's the mechanism that changes everything: Box 3 doesn't ask what your portfolio actually returned this year. The Belastingdienst calculates an assumed return on your wealth as of January 1st — different rates depending on whether the money's in cash or investments — and taxes that deemed amount at a flat 36% rate.

Above the tax-free allowance (€59,357 per person, single), every euro in your investment account is treated as if it earned a fictional 6% return. That fictional income is then taxed at 36%. So:

• Taxable wealth = your total stash minus €59,357
• Deemed taxable income = taxable wealth × 6% × 36%
• Annual Box 3 bill = that fictional income at the flat rate

This tax hits your account every January 1st, whether you're living on your withdrawals, reinvesting returns, or doing neither. It's a wealth tax, not an income tax. That distinction means your actual €1 million stash isn't €1 million available to spend — it's €1 million that has to cover both your living expenses and a growing annual tax bill.

Working example: single, €40,000 spending, 2026 rates

Let's walk through the math using 2026 Box 3 rates and a real scenario.

Your situation: you're single, targeting €40,000 annual spending, and you plan to have all your retirement wealth invested in a diversified portfolio (stocks and bonds, nothing in cash).

Your stash under the 25× rule: €40,000 × 25 = €1,000,000

Now add Box 3 tax:

• Taxable wealth = €1,000,000 − €59,357 allowance = €940,643
• Deemed return at investment rates = €940,643 × 6% = €56,439
• Box 3 tax = €56,439 × 36% = €20,318

The gap: your €1 million stash now has to cover €40,000 in living expenses plus €20,318 in Box 3 tax every year. That's €60,318 total annual need, or 6% of your starting stash. Your 4% safe withdrawal rate has become a 6% requirement.

The solution: you need a stash large enough that its annual withdrawal covers both your spending and the Box 3 bill that stash itself generates. That's not a flat percentage bump on top of €1 million — a bigger stash means a bigger taxable base, which means a bigger tax bill, which means you need the stash to be bigger still. Solving that directly (see the FIRE-number calculator for the exact math) puts the real number for this scenario at roughly €2.1 million — more than double the naive €1 million, not 50% more.

What number to actually calculate

The precise FI number depends on your specific situation — whether you're single or have a fiscal partner (which doubles the tax-free allowance), whether some wealth might sit in cash, what your actual spending pattern looks like. Rather than a one-size formula, use a calculator that accounts for your actual Box 3 situation.

The FIRE-number calculator on this site does exactly this: it takes your target annual spending and your expected wealth, applies 2026 Box 3 rates, and computes the stash size you actually need directly (a closed-form calculation, not trial and error). It shows you the gap between the naive 25× number and the real one.

Key insight: the bigger your stash, the bigger the Box 3 bill becomes — which is why the relationship between stash size and true annual need isn't linear the way the 4% rule suggests it is. A 4% withdrawal from €1 million is €40,000, but that stash also owes €20,318 in Box 3 tax, leaving only €19,682 — nowhere near enough. Because each larger stash generates its own larger tax bill, closing that gap takes more than a flat markup: for this single, all-invested, €40,000-spending scenario, the actual stash needed is roughly €2.1 million, not €1.5 million.

If Box 3 seems daunting, remember that it's one component of a broader tax picture in the Netherlands. For deeper context on how wealth tax actually works, see the complete Box 3 guide. And if you're not yet sure what "safe withdrawal rate" means or how it fits into a FIRE plan, start with the withdrawal rate section of the financial independence guide.

Revisit this annually

Box 3 rates — the tax-free allowance, the deemed return percentages, the flat tax rate — all change from year to year. The €59,357 allowance this year may be different next year. The 6% deemed return on investments could shift as the Netherlands transitions its Box 3 system. Your own circumstances change too: your target spending may adjust, you might add a fiscal partner, you might deliberately shift between cash and investments to optimize your Box 3 position.

This guide's worked example uses 2026 rates and will look different in 2027. Treat your FI number not as a finish line you hit once, but as a number to recalculate each year as part of your annual financial check-in. You've already internalized that Box 3 exists and shapes your math; the hard part of the calculation is done. The maintenance is just updating the rates and re-running the numbers. Verify the current Box 3 rates and rules on belastingdienst.nl before using any FI number for major financial decisions, and if your situation involves partnerships, inheritances, or substantial wealth shifts, a Dutch tax advisor can help you account for timing and optimization strategies specific to Box 3.