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Why 67% of your wealth comes from interest, not your contributions

The Rule of 72, the snowball effect, and the brutal math of lost time. In 6 minutes.

Beginner

7 min

Foundations

Your interest earns its own interest. Every year.

Year 1, you invest $10,000 at 8%. You earn $800. Year 2, your $800 in gains also goes to work. You earn $864. By year 20, your annual interest exceeds your initial contributions. That's compounding: a silent mechanism that turns consistency into wealth, as long as you give it time.

Time vs money: the fight isn't even close

Here's the brutal truth: waiting 10 more years to invest costs you far more than 10 years of contributions. To reach $500,000 at 60 with 8% return: starting at 25 → $220/month. Starting at 35 → $525/month. Starting at 45 → $1,450/month. Each decade of delay multiplies the required effort by ~2.5x.

The mechanism: how $1 becomes $10

Year 1: you invest $10,000 at 8%. Gain = $800. Total: $10,800. Year 2: 8% of $10,800 = $864. Total: $11,664. Year 10: your annual interest exceeds $1,500. Year 20: your annual interest exceeds $3,000. That's more than your initial investment. From here on, money truly works for you.

The Rule of 72: double your capital mentally

Divide 72 by your annual return to know when your capital doubles. At 8%: 72 ÷ 8 = 9 years to double. At 6%: 72 ÷ 6 = 12 years. At 4%: 72 ÷ 4 = 18 years. At 10%: 72 ÷ 10 = 7.2 years. Your capital doubles, then doubles again, then doubles once more. Growth isn't linear, it's exponential.

$200/month: the power of time

Marie starts at 25: $200/month at 8% for 40 years → $698,000 (of which $96,000 contributed, $602,000 in interest). Pierre starts at 35: $400/month at 8% for 30 years → $596,000 (of which $144,000 contributed, $452,000 in interest). Marie invests LESS, starts EARLIER, and ends up $100,000 AHEAD. Time beats money.

⚠️ Waiting 10 years costs $400,000. Proof.

You delay your first contribution by 10 years, telling yourself "I'll start when I have more room". Result: at $200/month and 8%, you miss out on $400,000 of final capital ($698,000 at 25 vs $298,000 at 35). It's not a delay, it's a clean cut. Lost compounding years don't come back, even by tripling contributions later.

Key Takeaways

1Compound interest grows your capital exponentially, not linearly.
2Investing $200/month from age 25 produces more wealth than $1,000/month from 45, purely from time.
3The Rule of 72 gives the doubling time: 72 ÷ rate = years (at 8% → 9 years).
467% of the final capital in a typical FIRE plan comes from compounding, not your contributions.

The best time to start is now

The exact amount doesn't matter at first. What matters is starting. $50/month at 8% for 40 years = $175,000. The second best time after '10 years ago' is today. Set up an automatic transfer, invest in a world ETF, and let time do the work.