See how your money grows over time. Add an initial amount, regular contributions, an expected rate, and watch compound interest do the heavy lifting.
Compound interest rewards time above all else. Because interest earns interest, the earliest contributions have the longest runway to grow — often outweighing much larger sums added later.
FV = P(1+r/n)nt + PMT × [((1+r/n)nt−1)/(r/n)]
P = principal · r = rate · n = compounds/yr · t = years · PMT = contribution
Compound interest is interest earned on both your original money and on the interest it has already earned. That feedback loop is what turns steady, ordinary savings into large sums over time. Albert Einstein is often quoted calling it the eighth wonder of the world — and while the attribution is doubtful, the math behind the awe is very real. This calculator shows how an initial amount, regular contributions, a rate of return, and time combine to grow your money.
The key insight is that compounding rewards time more than any other input. Money invested early has more years to compound, and those extra years matter far more than a slightly higher contribution later.
For a lump sum with no contributions, the formula is:
A = P (1 + r/n)nt
Where P is the starting principal, r is the annual rate, n is how many times per year interest compounds, and t is the number of years. When you add regular contributions, each deposit compounds for the remaining time until the end, and the calculator sums them all.
Start with $10,000, add $300 a month, and earn a 7% annual return compounded monthly:
| After | Balance | You contributed |
|---|---|---|
| 10 years | $71,900 | $46,000 |
| 20 years | $196,000 | $82,000 |
| 30 years | $447,000 | $118,000 |
After 30 years you contributed about $118,000, but the balance is roughly $447,000. The remaining ~$329,000 is pure growth — interest earning interest. Notice how the gap widens dramatically in the later years: that is compounding accelerating.
A handy mental shortcut: divide 72 by your annual return to estimate how many years it takes your money to double. At 7%, money doubles roughly every 10 years. At 9%, every 8 years. It is an approximation, but a remarkably good one for typical rates, and it makes the power of a higher return tangible.
Even a one-point difference in return compounds into a huge gap over decades. This is why fees matter so much — a 1% annual fee quietly removes a large share of your final balance.
More frequent compounding (monthly or daily vs annual) helps a little, but the effect is small compared to the rate and the time horizon. Do not obsess over it.
Regular contributions matter more than large sporadic ones, because each contribution starts compounding immediately. Automating deposits is one of the most effective wealth-building habits.
Estimates are for educational purposes only and are not investment advice. Actual returns vary and are not guaranteed.