ReviewedEducational article · Updated Oct 2026
Key takeaways
- Inflation reduces how much a dollar buys over time — purchasing power compounds downward.
- A future price can be estimated with the same compound-growth math used for savings.
- Real return ≈ nominal return minus inflation (a useful approximation).
- Long horizons make even “small” inflation rates material.
Inflation is a general rise in prices. When prices rise, each unit of currency buys less. That erosion of purchasing power compounds the same way investment growth does — quietly at first, then more obviously over decades.
The flip side of compounding
If prices rise at a steady 3% per year, something that costs $100 today costs about $134 in 10 years and about $181 in 20 years, all else equal. The math is identical to compound interest: multiply by (1 + i)t.
Approximate future price of a $100 item after 20 years at a steady 3% inflation rate — before taxes, quality changes or substitutions.
A dollar in the future is not the same dollar as today — not because of interest, but because of prices.
Future cost and present value
Two related questions:
- Future cost: what will today’s expense cost in t years at inflation rate i? →
Cost × (1+i)t - Purchasing power: what is a future sum worth in today’s dollars? → divide by
(1+i)t
Both are rearrangements of the same relationship. Calculators simply make the arithmetic quick.
Nominal vs. real
A bank account might show a nominal interest rate of 4%. If inflation is 3%, your approximate real return is about 1% — the growth in purchasing power, not just the growth in the number on the statement. The precise Fisher relationship is a bit more exact than simple subtraction, but “nominal minus inflation” is the intuition most people need.
Why long horizons matter
At 2% inflation, prices roughly double in about 35 years (Rule of 72: 72 ÷ 2 ≈ 36). At 4%, they roughly double in 18 years. Retirement planning, tuition projections and long rent assumptions all live in this world — which is why inflation inputs appear next to growth rates in serious planning models.
Basket warning: official inflation indexes measure a basket of goods. Your personal inflation can differ if you spend more on categories that rise faster (or slower) than the average.
What the model leaves out
Inflation is not constant; it spikes and fades. Quality improvements, new products and substitution effects complicate comparisons. An educational calculator assumes a smooth rate so you can see the compounding structure clearly.
This article is for general educational purposes only and is not financial advice. Examples use simplified, hypothetical numbers and ignore taxes, fees and personal circumstances. Consider speaking with a qualified professional before making financial decisions. See our full disclaimer.