Compound Interest Calculator
See what steady saving really becomes. Add a starting amount and a monthly contribution, and watch how much of your future balance is growth versus what you actually put in.
Your money earned more than you contributed — that's compounding doing the heavy lifting.
Put a real plan behind the number
The MoneyMapped Toolkit tracks your savings goals and net worth month by month, so the compound-growth line above becomes something you actually watch climb.
The most important chart in personal finance
The gap between the two lines above is compounding. Early on it is small and the two lines hug each other. Give it enough years and the growth line pulls dramatically away from what you contributed. That is why the single biggest lever most people have is simply starting, and then leaving it alone.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original money and the interest it has already earned. Over time the growth accelerates, because each year you earn a return on a bigger and bigger balance.
How is compound interest calculated?
This calculator compounds monthly: each month your balance grows by the monthly rate (annual return divided by 12), then your contribution is added. Do that for every month over your time horizon and you get the final balance.
What return rate should I use?
For a diversified stock-market portfolio, people often model around 7% after inflation, though returns vary year to year and are never guaranteed. For a high-yield savings account, use around 4%. For cash under the mattress, use 0%.
Why does starting early matter so much?
Because compounding needs time. A dollar invested at 25 has decades to multiply; the same dollar at 45 does not. Try setting the same contribution with different time horizons above to see the gap.
MoneyMapped provides educational tools, not financial advice. Investment returns are not guaranteed and past performance does not predict future results.