Compound Interest Calculator
The eighth wonder of the world, visualized. Einstein allegedly agreed.
Future Value after 20 years
$40,387
Total Contributions
$10,000
Interest Earned 🎉
$30,387
Growth Over Time
- Contributions
- Total Value
About This Calculator
How It Works
Compound interest is what happens when your interest earns interest. Each period, your earnings get added to the principal, and the next period's interest is calculated on that larger number. This calculator lets you set an initial lump sum, optional monthly contributions, an annual rate, a time horizon, and a compounding frequency — then projects how your balance grows year by year. The visual chart makes the exponential curve immediately obvious in a way that a spreadsheet number never quite does.
The Formula
A = P(1 + r/n)^(nt) where P = principal, r = annual rate, n = compounds per year, t = time in years
When to Use This
This is most useful for long-term planning: retirement savings, investment accounts, education funds, or just understanding why starting 10 years earlier matters so much more than investing twice as much later. The flip side — compound interest working against you — is exactly what makes credit card debt so dangerous. Running both scenarios through the same formula is genuinely eye-opening.
Limitations
This calculator assumes a constant annual rate of return, which real investments don't deliver — markets go up and down. It also doesn't account for taxes on gains, inflation eroding purchasing power, or investment fees (which can quietly eat 1-2% of returns per year). The numbers are optimistic projections, not guarantees. For retirement planning specifically, pair this with a conversation with a financial advisor.
Example
$10,000 at 7% compounded monthly for 20 years becomes $40,387. You added $30,387 in interest without lifting a finger. Time is the secret ingredient.
Did You Know?
Einstein may or may not have called compound interest the eighth wonder of the world. The quote is disputed. The math is not. Start early. Really.
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The Chart Is Convincing — Now Here's How to Actually Build It
You've just seen what compound interest can do over 10, 20, or 30 years. That exponential curve is real — but only if the money is actually invested, in the right places, starting as early as possible.
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