Compound Interest Calculator
Einstein reportedly called compound interest the eighth wonder of the world — whether or not he actually said it, the math backs up the reputation. This calculator shows exactly how much of your final balance comes from what you contributed versus what growth alone added on top.
Enter your details to see your projected growth.
How to use this calculator
- Enter your starting amount and planned monthly contribution.
- Enter an expected annual return.
- Click "Calculate compound growth" to see your projected final value, split between contributions and growth.
How the calculation works
Your starting amount grows using compound monthly returns at your entered rate, and your monthly contributions are added throughout, each new deposit then compounding for the remaining months. The key mechanic: each month's return is calculated on the full balance so far — including all prior growth — which is why the curve accelerates rather than growing in a straight line the longer money stays invested.
A worked example
Say you start with $5,000, add $300 a month, and earn an average 7% annual return over 25 years. Your own contributions over that period total $95,000 — but the projected final balance lands north of $255,000. That means growth alone contributed more than your actual deposits did, purely from compounding working in the background year after year.
Frequently asked questions
What return rate is realistic?
Long-term stock market averages have historically been in the 7-10% range before inflation, though any specific year can vary widely and isn't guaranteed.
Does this account for taxes?
No — this shows gross growth. Taxable accounts would owe tax on realized gains, dividends, or interest along the way, reducing the net result.
How is this different from the Investment Calculator?
This tool is a straightforward growth projection. The Investment Calculator also shows an inflation-adjusted "real" value for comparison.
Does the compounding frequency matter?
Yes, slightly — more frequent compounding (monthly versus annually) produces marginally higher growth at the same stated annual rate, since returns are calculated and reinvested more often.
Why does starting a few years earlier make such a big difference?
Because compounding is exponential, not linear — money invested early has more total compounding periods, so a head start of even 5-10 years can meaningfully outgrow a larger amount invested later.