Future Value = Present Value × (1 + Rate)ⁿ
| Start Age | Monthly Investment | At Age 65 | Total Invested |
|---|---|---|---|
| 25 | $300/month | $1,130,000 | $144,000 |
| 35 | $300/month | $408,000 | $108,000 |
| 45 | $300/month | $137,000 | $72,000 |
| 25 (same total as 35) | $225/month | $848,000 | $108,000 |
Starting 10 years earlier at $300/month produces $1.13M vs. $408K — nearly 3x more money despite only investing $36K more. Time is the most powerful variable in the compound interest formula, and the one you can never get back.
Rule of 72: Years to Double ≈ 72 / Annual Return %
The 8–10% figure is nominal. Real returns — after 2–3% average inflation — run closer to 6% historically (S&P 500, 1926–2023, Ibbotson SBBI). At 6% real, the $1.13M starting-at-25 example shrinks to roughly $700K in today's purchasing power. Volatility matters: the S&P 500's annual standard deviation is ~16%, so one year in six delivers a loss. A Monte Carlo model using the historical return distribution places the 10th-percentile outcome near $450K and the 90th-percentile near $2.1M — a nearly 5x spread around the point estimate. The flat 8% line is the median story; the real story is a band, not a point.
Sit with the ideas.
Twin A invests $10,000 at age 25 at 8% and adds nothing else. Twin B waits until age 35, invests $10,000 at 8%, then adds $100/month for 30 years. At age 65, who has more?