Compound Interest
Einstein probably didn't say it. But it's still true: compound growth is the most powerful force in personal finance — and it rewards the patient.
See it in action
The formula
r = annual rate ÷ 12 (monthly periods) | n = years × 12 (total months)
The Rule of 72
Divide 72 by your annual return rate to get the approximate years to double your money. No calculator needed.
| Return Rate | Years to Double |
|---|---|
| 4% | 18 years |
| 6% | 12 years |
| 7% | 10.3 years |
| 8% | 9 years |
| 10% | 7.2 years |
Time beats returns
A 22-year-old who invests $200/month at 7% until age 65 accumulates roughly $1,034,000. A 32-year-old who invests $400/month — double the amount — at the same rate accumulates roughly $986,000. Starting ten years earlier beats saving twice as much. That's the Coast FIRE insight in a nutshell.
Why Coast FIRE rewards early savers
Every dollar you save in your 20s or 30s that sits in the market for 30 years is worth 3–4x a dollar saved in your 40s. Hitting your Coast FIRE number early matters more than squeezing extra savings later.
This calculator uses constant annual returns. Market returns vary year to year. For educational illustration only, not financial advice.