The 8th wonder of the world
Albert Einstein supposedly called compound interest "the eighth wonder of the world — he who understands it, earns it; he who doesn't, pays it". The trouble is that our brains think in straight lines, while compound interest grows on a curve. That's why almost everyone gets the next question wrong. Try it.
1. What does it turn into?
You save $200 a month for 30 years, earning 10% a year. In total, you took $72,000 out of your pocket. How much do you end up with?
2. Watch the snowball grow
Move the controls and watch the chart. The light area is money you deposited; the green area is what interest generated on its own. Notice the moment the green passes the light one: that's when your money starts working harder than you do.
Area chart of investment growth over time, separating deposits from interest.
3. Time is your most valuable asset
With compound interest, when you start matters more than how much you save. Here's the price of delaying, keeping $200/month at 10% a year until age 60:
Each year of delay doesn't cost you "one year of deposits" — it costs the biggest interest year at the very end, which now never happens. That's why "start early with little" beats "start late with a lot".
Now run your own numbers
Put your contribution, your time frame and your real rate into the full calculator — with a year-by-year table and initial deposit.
Open the Compound Interest Calculator →See also: Rule of 72 · Savings goal · Lesson: where your loan payment goes