July 31, 2026·6 min read

Why single-currency FIRE calculators give expats the wrong number

Every standard FIRE calculator assumes you earn, spend, and retire in the same currency. For anyone whose life crosses borders, that assumption can put the FI number off by 30 to 60 percent. Here is where the error comes from and how to correct it.

Single-currency FIRE calculators give expats the wrong number because they bake in three assumptions: that you earn and spend in the same currency, that one inflation rate covers your whole life, and that you will retire where you live today. Break any one of those, and the FI number they hand you can be off by 30 to 60 percent, in either direction. If your life crosses borders, you break all three.

The three assumptions, and who breaks them

Assumption one: earn-currency equals spend-currency. The calculator takes your expenses, multiplies by 25, and quietly prices your entire retirement in the currency you typed. If you earn NOK in Oslo but send money to family in India every month, or hold your investments in EUR, part of your financial life already lives outside that currency. The calculator can’t see it.

Assumption two: one inflation rate. Your future expenses inflate at the rate of the country you will spend them in. Applying Norwegian inflation to a retirement you plan to spend in Spain, or US inflation to one in Portugal, compounds a small annual error into a large one over 20 years.

Assumption three: you retire where you live now. This is the big one. The 25× rule prices your retirement at your current city’s cost of living. If there is a real chance you retire somewhere cheaper, or more expensive, the number on the screen is answering a question you didn’t ask.

The same person, two very different numbers

Worked example. Someone in Oslo spends about 45,000 NOK a month and plans to retire in Valencia, where the equivalent comfortable life runs roughly €2,400 a month. A standard calculator says: 45,000 × 12 × 25 ≈ 13.5M NOK. Price the same retirement in its actual currency and location instead: €2,400 ≈ 28,000 NOK a month, so 28,000 × 12 × 25 ≈ 8.4M NOK. That gap is roughly five million kroner, which at a decent savings rate is somewhere between five and ten extra years of work the calculator just prescribed for no reason.

The error runs the other way too. Retiring from a cheap base to an expensive one, or carrying family obligations in a strong currency, means the standard number is too small, and that version of the mistake hurts more.

Neither number is precise. That’s fine.

Exchange rates move, and a 20-year FX forecast is astrology. The point is not precision. The point is that the single-currency number carries a large, systematic bias for anyone cross-border, and the corrected number at least starts from the right country, the right currency, and the right inflation. You can hold the remaining uncertainty honestly: convert at a conservative rate, and let a range replace the single tidy figure.

How to correct it yourself

Price your retirement expenses in the currency and country you actually plan to spend them in, using that country’s cost of living, not a converted version of your current budget.
Apply that country’s inflation to those expenses, and your portfolio’s real return separately.
Convert your savings into the retirement currency at a rate 5 to 10 percent worse than today’s, so FX risk is inside the plan instead of a surprise.
Keep locked pensions out of the liquid FI math until the age you can touch them.
If a move is genuinely on the table, run the number for each destination and compare, rather than averaging them into one meaningless figure.

Or let the math run with the right defaults

The free FIRE Number calculator on this site uses region-aware defaults (16 regions) and real-return math, so the inflation assumption matches where you live. Inside fjordFIRE, the full FIRE engine holds every account in its own currency, recomputes your FI date per destination, and folds in the one-time cost of leaving where that applies.

fjordFIRE is opening to everyone this August, free. No bank logins, no surveillance.

Go deeper: Lesson: Multi-Currency Basics · Lesson: Cost of Living Arbitrage.

Companion tools: FIRE Number calculator · Relocation Runway calculator.