Use this calculator. Track this in Stroberi.
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What compound interest actually does
Interest on interest. In year one, a 7% return on $10,000 adds $700. In year thirty, that same rate on the accumulated balance adds tens of thousands per year. This is why every financial advisor tells you to start early — the exponential portion of the curve is far out, and you have to be in the market to get there.
Why we compare against a 4% HYSA
A high-yield savings account is roughly risk-free. The gap between your projected balance and the HYSA balance is your risk premium — the reward for taking stock-market volatility instead of cash. If the gap is small, you might be taking risk you’re not being paid for. If it’s large, compound growth is doing its job.
Picking a realistic return rate
The S&P 500 has averaged about 10% nominal, 7% real (after inflation) since 1928. Using 10% in a calculator can flatter the projection; 7% real is the more honest number if you want purchasing power in today’s dollars. For a diversified 60/40 portfolio, 5–6% real is realistic.
What this calculator ignores
Sequence-of-returns risk, taxes on dividends and capital gains, fees (a 1% expense ratio eats ~28% of lifetime gains over 40 years), and the fact that average returns hide big crashes. Use the output as a best-case target, not a promise.