Quick answer
Idle cash loses purchasing power at roughly the inflation rate every year, compounding — at 7% annual inflation, cash left idle for 10 years loses about half its real purchasing power. The only way to counter this is to earn a return rate higher than the inflation rate, so the money compounds faster than prices rise.
The number in a savings account doesn't change just because prices go up — but what that number can actually buy does. That gap between the figure on your statement and what it's genuinely worth is purchasing-power erosion, and like interest, it compounds.
Why erosion compounds, not just adds up
A second year of 7% inflation doesn't just subtract another 7% — it takes 7% off an already-shrunk base, the same way compound interest works in reverse. That's why purchasing power erodes faster the longer cash sits idle, not at a steady linear rate.
What idle cash is really worth after inflation, at a few common rates
| Years idle | At 5% annual inflation | At 7% annual inflation | At 10% annual inflation |
|---|---|---|---|
| 5 years | ≈78% of original value | ≈71% of original value | ≈62% of original value |
| 10 years | ≈61% of original value | ≈51% of original value | ≈39% of original value |
| 20 years | ≈38% of original value | ≈26% of original value | ≈15% of original value |
Formula used:
Idle Cash (Real Value) = Starting Amount ÷ (1 + Inflation Rate) ^ Years
Countering erosion: the growth side of the equation
If a savings or investment return rate is higher than the inflation rate, purchasing power doesn't just stop shrinking — it actually grows in real terms, because the money is compounding faster than prices rise. Monthly top-ups compound the same way a standing savings contribution would.
Formula used:
Saved Amount (Future Value) = [Starting Amount × (1 + Return Rate) ^ Years] + Compounded Monthly Top-Ups
Run both scenarios side by side with your own numbers in the Savings & Inflation Calculator — it compares idle cash against a chosen return rate and monthly top-up, over whatever time horizon you're planning for.
A planning lens, not a forecast
Both the inflation rate and the return rate in any projection like this are assumptions you choose yourself, held constant for the whole period — real inflation and real investment returns both vary year to year, sometimes sharply. Treat this as a way to understand the mechanics, not a guarantee of what will actually happen to your specific currency or investment.
For educational and informational purposes only — not financial, investment, trading, legal or tax advice. See our full Financial Disclaimer.