Inflation and Compound Interest
Compound growth projections have a quiet flaw: they're stated in future dollars, and future dollars buy less. A projection that says "$610,000 in 30 years" is true and still misleading, because at 3% inflation those dollars will buy what about $340,000 buys today. The fix is one adjustment: run the numbers at your real rate of return. Here's how, with worked figures you can reproduce in the free compound interest calculator.
In short: real rate ≈ (1 + nominal) ÷ (1 + inflation) − 1. At a 7% return and 3% inflation that's 3.88%, and entering 3.88% in any compound calculator shows your growth in today's buying power.
Inflation is compound interest in reverse
Prices compound exactly like savings do. At 3% inflation, the cost of living doubles roughly every 24 years (the Rule of 72: 72 ÷ 3), which means a dollar loses half its buying power over the same stretch. Any long-horizon projection that ignores this is quietly mixing two different currencies: today's dollars going in, cheaper future dollars coming out.
The real rate of return
To state growth in today's dollars, discount the return by inflation: real rate = (1 + r) ÷ (1 + i) − 1, where r is your nominal return and i is inflation. At r = 7% and i = 3%: 1.07 ÷ 1.03 − 1 = 3.88%. (The quick approximation r − i = 4% overstates it slightly; the division form is exact.)
The same plan, in both currencies
$500 a month for 30 years, compounded monthly, computed with the same engine as the calculator:
| View | Rate used | Balance after 30 years |
|---|---|---|
| Future dollars | 7% nominal | $609,985 |
| Today's buying power | 3.88% real | $339,908 |
Both rows describe the same account on the same day. The first is what the statement will say; the second is what it will buy. A $10,000 lump sum tells the same story: $81,165 in future dollars, about $32,001 in today's. Neither number is wrong; planning with only the bigger one is.
How to model it in the calculator
The calculator takes whatever rate you give it. Enter your nominal return (say 7%) to see future dollars, then rerun with your real rate (3.88% for 3% inflation) to see today's buying power. The gap between the two runs is inflation's bite. For retirement-length horizons, the real-rate view is the one to size your contributions against.
Frequently asked questions
What real rate of return should I assume?
Historically, diversified US stock returns have run roughly 6.5 to 7 percent after inflation over long horizons, but no rate is guaranteed. The conservative habit is to plan at a real rate a point or two below the historical average and treat anything better as a bonus.
Does inflation affect my contributions too?
Yes, in your favor if your income keeps pace: a fixed $500 monthly contribution gets easier to make over time, and many savers raise contributions with raises. The real-rate method assumes contributions stay constant in today's dollars, which matches that habit reasonably well.
Why not just subtract inflation from my return?
Subtraction is close but slightly generous: 7% minus 3% gives 4%, while the exact real rate is 1.07 divided by 1.03 minus 1, which is 3.88%. Over 30 years that small gap compounds to a difference of thousands of dollars.
Related reading: How compound interest works · The Rule of 72 · APY vs APR · FAQ
Examples assume constant returns and constant inflation for illustration; both vary in reality. Educational only, not investment, tax, or financial advice.
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