The Cross-Border FIRE Math Trap: Why Your Retirement Number Is Wrong

Running a retirement number takes about a minute. Spend $60,000 a year, multiply by 25, and the 4% rule says you need $1.5 million.

For someone whose money sits in one country, that answer is roughly right. For someone whose money crosses borders, it can be wrong by a third.

This article shows where that gap comes from, using one illustrative portfolio throughout.

Why the standard FIRE math fails expats

The 4% rule comes from historical US market data. It assumes one currency, one tax regime, and money you can reach when you need it.

That holds for a teacher in Ohio. It does not hold for an engineer who earns in francs, invests in dollars, holds a pension in London and plans to retire in Portugal.

The rule itself is not broken. The inputs are. Four leaks appear once money crosses a border, and each one looks small on its own.

• FX drag: the cost of converting money across borders, every year, forever.
• Tax drag: withholding taken before the money ever reaches you.
• Locked accounts: money you own but cannot touch for years.
• Currency drift: exchange rates moving against your plan.

The illustrative portfolio

The figures below describe a composite example, not a real person. Round your own accounts the same way and the method still works.

On paper, this portfolio can retire. That is the moment most people stop calculating. Every leak below applies after this point.

Leak 1: FX drag, the quiet yearly tax

Plan to spend euros while your money sits in dollars, pounds and francs, and every withdrawal has to cross a border.

Convert roughly $36,000 a year at an average cost near 0.8%, and that is about $290 a year.

$290 sounds like nothing. It is not nothing.

That cost repeats every year for thirty years. Invested instead at 5%, those charges compound past $19,000 over a full retirement.

It never appears on a standard calculator, and it never stops. It is a permanent tax on spending across a border.

Leak 2: tax drag, money gone before you see it

Dividends are taxed at source. You never see that money, and you do not control the rate. Your account structure does.

The gap between the worst and best setup is about $6,000 a year. Even the middle case gives up $4,500.

Tax rules also shift under your feet. Portugal closed its long-standing expat regime in 2024 and replaced it with a narrower one. Residency rules are not fixed inputs.

Leak 3: locked accounts, your money behind a gate

The 4% rule assumes every dollar is available on day one. In this portfolio, about $550,000 is not.

For thirteen years that money does not exist for the budget. The whole of your forties has to be funded from taxable accounts alone.

That forces a larger cash buffer, and a larger buffer earns less. The cost is real even though nothing was spent.

Leak 4: currency drift

A cheap country gets expensive when the exchange rate moves. Major pairs have swung by more than 20% within a decade, more than once.

If your savings currency weakens against your spending currency, your target rises. You did nothing wrong, and the number moved anyway.

Where the third goes

Stack the four leaks and the safe rate drops.

At 2.9%, $60,000 of spending needs about $2 million, not $1.5 million. That is the 33% gap, and none of it came from bad investing.

Four rules that close the gap

• Model the target in the currency you will actually spend, not the one you earned in.
• Count only the money you can reach at your retirement age. Locked accounts fund the later years.
• Choose account structures for withholding before chasing returns. Tax drag is larger than most fee differences.
• Re-run the number yearly. Rates move, rules change, and plans firm up.

Run your own numbers

Start with the FIRE calculator for a baseline target, then adjust the withdrawal rate down to reflect your own drag.

Track the consolidated figure using the cross-border tracking method, and read the multi-currency guide for the currency mechanics.

Every figure in this article is illustrative and rounded. It is educational information, not financial advice, and it is not tax advice for any specific country.