How the lump sum calculator works
The curve, not the line
Simple interest adds the same dollars every year; compound interest multiplies. $10,000 at 7% earns $700 in year one and $2,500+ per year by year 30 — same rate, five times the annual earnings, because the earnings themselves earn. The last decade of any projection contributes more than the first two, which is why quitting at year 15 wastes the best part.
The Rule of 72 and its edges
72 ÷ return ≈ doubling time — a 7% return doubles in ~10.3 years, 10% in ~7.2. It is a spectacular heuristic up to ~15% returns and it ignores taxes, fees, and inflation. The calculator shows exact doubling plus the real (inflation-adjusted) trajectory so the heuristic stays useful without lying to you.
Which return to plan on
Historical US equity averages ~10% nominal, ~7% real, with brutal decade-level variance. Planning at 7% nominal (10% real) is a defensible middle for long horizons; anything above 10% is a forecast, not a plan. For horizons under 10 years, use the return of the thing you will actually hold — likely bonds, CD rates, or HYSA.