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Compound interest explained: how your money really grows

TallyCrunch

Albert Einstein supposedly called compound interest the eighth wonder of the world. Whether or not he said it, the math is genuinely remarkable: money that earns returns, and then earns returns on those returns, grows in a way that feels almost unfair once you give it enough time.

Simple vs compound interest

Simple interest is earned only on your original amount. Put $1,000 at 10% simple interest and you earn $100 every year — forever $100.

Compound interest is earned on your original amount plus all the interest already added. Put $1,000 at 10% compounded annually and year one earns $100, year two earns $110 (10% of $1,100), year three earns $121, and so on. The growth accelerates because the base keeps getting bigger.

Why time is the real lever

Compounding starts slow and then explodes — most of the growth happens in the later years. That's why when you start matters more than how much you invest.

Consider two savers:

  • Early Emma invests $200/month from age 25 to 35 (10 years), then stops.
  • Late Liam invests $200/month from age 35 to 65 (30 years).

Even though Liam invests three times as much money, Emma often ends up with more at 65 — because her early contributions had decades longer to compound. Starting early is a bigger advantage than investing more.

The Rule of 72

A handy shortcut: divide 72 by your annual return to estimate how many years it takes your money to double.

  • At 6%: 72 ÷ 6 = 12 years to double
  • At 9%: 72 ÷ 9 = 8 years to double

It's an approximation, but it's close enough to do in your head — and it makes the cost of waiting obvious.

What this means for you

  • Start now, even small. Time is the ingredient you can't buy back later.
  • Reinvest your returns so they can compound rather than leaking out.
  • Mind the rate, but don't chase risk — a steady return left alone for decades beats a flashy one you bail on.
  • Be patient. The boring middle years are where compounding quietly does its work.

Want to see it for your own numbers? Explore our finance calculators to model how an amount grows over time at different rates.

The bottom line

Compound interest rewards patience more than size — money left to grow earns returns on its returns, and the earliest dollars do the heaviest lifting. The single most powerful financial move most people can make is simply to start early and leave it alone. The second-best time is today.

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