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 β $175/month. Starting at 35 β $400/month. Starting at 45 β $1,000/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
$200/month: the power of time
Marie starts at 25: $200/month at 8% for 40 years β $698,000 (of which $96,000 contributed, $602,000 in interest). Pierre starts at 35: $400/month at 8% for 30 years β $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.
β οΈ 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 $1000/month from 45, purely from time.
- 3The Rule of 72 gives the doubling time: 72 Γ· rate = years (at 8% β 9 years).
- 467% of the final capital in a typical FIRE plan comes from compounding, not your contributions.
Keep going
Frequently asked questions
Compound interest is the mechanism by which interest earned in one year itself produces interest in following years. Over 30 years at 8%, $10,000 grows to ~$100,000 thanks to this snowball effect. The longer the horizon, the more dramatic the gap with linear calculation.
Simple interest applies only to the initial principal: $1,000 at 8% generates $80/year, totaling $1,800 after 10 years. Compound interest applies to principal + accumulated interest: the same sum reaches ~$2,159. The gap explodes over time: over 40 years, compound delivers ~$21,725 versus $4,200 simple.
The formula is Final Capital = Initial Γ (1 + rate)^years. For regular monthly contributions, add the sum of compounded annuities. Our calculator automates everything: enter your capital, contribution, rate and duration to visualize the exponential curve.
Time is the #1 ingredient of compounding. Investing $200/month from age 25 generates more capital at 65 than $1,000/month from age 45, despite a 5Γ smaller financial effort. Each year delayed costs exponentially more.