Albert Einstein supposedly called compound interest the eighth wonder of the world. Whether or not he said it, the point stands: compounding is the quiet force that turns modest, consistent saving into serious money — and understanding it changes how you think about time and money.
Here's how compound interest works, with plain examples, and why starting early beats saving more later.
Simple vs. compound interest
Simple interest is paid only on your original amount. Put in $1,000 at 5% and you earn $50 every year — forever the same $50.
Compound interest is paid on your original amount *plus all the interest you've already earned*. Year one you earn $50; year two you earn 5% of $1,050 = $52.50; year three, 5% of $1,102.50 — and so on. Your interest earns interest. That snowball is the whole idea.
The formula
A = P(1 + r/n)^(nt), where P is your starting amount, r is the annual rate (as a decimal), n is how many times a year it compounds, and t is the number of years.
It looks intimidating, but the takeaway is simple: the exponent means growth accelerates over time — the longer it runs, the steeper the curve.
Why starting early beats saving more
Because compounding rewards *time* more than *amount*. Consider two savers earning 7% a year:
- Early Ava invests $200/month from age 25 to 35 (10 years, $24,000 total), then stops and never adds another dollar.
- Later Leo waits, then invests $200/month from age 35 all the way to 65 (30 years, $72,000 total).
Despite putting in three times less money, Ava often ends up with *more* at 65 — because her money had an extra decade to compound. Those early years are the most valuable ones you'll ever have.
Project your own growth
See it for your own numbers with the free Compound Interest Calculator — enter a starting amount, rate, time and contributions to watch the curve.
Compound Interest CalculatorSee how savings grow with compounding.Related money tools
- Savings Goal Calculator — how much to save monthly to hit a target.
- Simple Interest Calculator — for loans and basic interest.
- Loan Calculator — see the flip side: how interest compounds against you on debt.
- ROI Calculator — measure the return on an investment.
Compounding cuts both ways. On savings it's your best friend; on high-interest debt like credit cards, it's working against you — which is why paying those down fast matters so much.