Compound Interest Calculator with Inflation

What this calculator does

This inflation-adjusted compound interest calculator shows two numbers most online calculators ignore: the real (inflation-adjusted) value of your portfolio, and the real CAGR — the annualized growth in purchasing power. It's the difference between what your statement says and what your money can actually buy.

Why inflation matters more than you think

At 3% annual inflation, today's $1 has the purchasing power of just $0.74 in 10 years and $0.55 in 20 years. Over a 30-year retirement horizon, ignoring inflation can make a portfolio look about 2.4x larger than it really is in spending terms. A "$1,000,000 retirement" in nominal 2026 dollars might feel more like $410,000 by 2056.

The real-return formula

We use the standard Fisher equation:
Real return ≈ (1 + nominal) / (1 + inflation) − 1

For example, a 7% nominal return at 3% inflation gives: (1.07 / 1.03) − 1 = 3.88% real return. Quick mental math — just subtract — gives 4%, which is close enough for planning.

A worked example

Say you invest $10,000 today and add $500/month for 30 years at 8% return. The nominal balance compounds to about $847,000. But at 3% inflation, the real, today's-dollars purchasing power is closer to $349,000 — still excellent, but a very different number to plan retirement around.

How to use the result

• For retirement planning, always use the real figure. It tells you what your portfolio can buy in today's groceries, healthcare, and travel.
• For mortgage payoff, use the nominal figure. Debt is fixed in nominal dollars.
• For short-term goals (<3 years), the difference is small — nominal is fine.
• For 20+ year horizons, the difference is enormous — always think real.

Investments that historically beat inflation

Over long periods, US stocks have returned roughly 6.5–7% real (after inflation), real estate ~3–4% real, and TIPS / I-bonds match inflation by design. Cash and short-term Treasuries have averaged near zero real return. This is the single strongest argument for long-horizon equity allocation.

Common mistakes

1. Quoting "I'll have $2M at retirement" without specifying nominal vs real
2. Using 2% inflation forever — recent years showed how wrong that can be
3. Holding too much cash "for safety" — guaranteed real loss
4. Comparing two portfolios' nominal returns across different time periods
5. Forgetting that pensions and Social Security have varying inflation protection