Your savings rate is half FX. Here's the version that isn't.
The savings rate is the single most-quoted FIRE metric and the most broken for anyone with money in more than one currency. Here's why your reported number is lying, and the three-term decomposition that tells you the truth.
For anyone holding assets in more than one currency, the standard savings-rate proxy (net-worth change ÷ income) silently combines three different things: real saving (income − spending), market returns, and FX movement. The number can swing 10+ percentage points based on currency moves you did not make. Here is the honest decomposition, and why your reported 38% might really be 26%, or 50%.
Related tool: our Currency Conversion Cost calculator shows the other half of the same problem: what you lose every time you actually move money between currencies. The savings-rate distortion and the FX-conversion cost are two sides of the multi-currency tax most calculators ignore.
What “savings rate” is supposed to mean
The honest definition: the percentage of your income that you didn’t spend. If you earned 80k and spent 50k, you saved 30k. Your savings rate is 30 ÷ 80 = 37.5%. Clean.
The practical proxy most people use: change in net worth ÷ income. If your net worth grew by 30k this year and you earned 80k, you assume the 30k came from savings. Good enough for one currency, one country.
Why the proxy lies for multi-currency lives
Net worth doesn’t only change because you saved. It changes because the markets moved your investments, because interest accrued, and, the silent one, because exchange rates moved.
If you hold accounts in EUR while your dashboard reports in NOK, a 5% EUR/NOK move can shift your net worth by tens of thousands of kroner. Without doing anything. You can have a great savings year and a bad-looking savings rate. Or vice versa: bad savings, flattering FX, dashboard says you’re crushing it.
The three-term decomposition that fixes it
Financial planners use a method called Brinson attribution to separate portfolio performance into its real drivers. The same approach works for net worth changes between two months:
- Real savings. Balance changes held at constant exchange rates. What you actually did.
- FX movement. Rate changes applied to constant balances. What currency markets did to you.
- Residual. The interaction term: both balance and rate moved. Small in most months, meaningful in volatile FX periods.
Sum: total change. Decompose: honest insight. Your savings rate is the first component over your income, not the total over your income.
Why this matters for decisions
People make real decisions on bad savings-rate numbers. They tighten their belt when they were already saving plenty. They loosen up when they shouldn’t. They feel guilty after a strong month or smug after a weak one. Both wrong reactions, both directly traceable to a metric that includes FX noise.
The fix isn’t to ignore FX. It’s to see it separately. Currency moves are real wealth changes: they affect your purchasing power, your runway, your FI date. They’re just not savings. Mixing the two destroys both signals.
How fjordFIRE handles this
fjordFIRE runs the three-term decomposition every time you check in. The Net Worth Trends page surfaces real-savings vs FX-impact as separate lines, with a per-currency breakdown showing exactly which currency contributed what to the total change. Each period-over-period delta is labelled: organic growth (what you did), FX (what the market did to you), and the residual.
The same attribution feeds the Cross-Pillar Cascade insight after each check-in, so when your dashboard says “net worth up 78k,” the explanation underneath tells you whether to be proud of yourself or to thank the kroner.
Go deeper: Lesson: FX Attribution.
Companion tools: Savings Rate calculator · Currency Conversion Cost calculator.