Compound Interest Calculator
Find out how your money can grow when interest compounds over time. Add monthly contributions to see how steady saving accelerates the result.
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Projected balance
$0.00
Contributions are grouped per compounding period. Projection only — not a guarantee.
Balance and contributions over time
How your total balance pulls away from your cumulative contributions as compounding kicks in.
- Total balance
- Cumulative contributions
Year-by-year growth schedule
| Year | Contributions (cumulative) | Interest earned (cumulative) | Balance |
|---|
Compare two scenarios
Same initial investment and years as above, compounded monthly. Two rate/contribution combos side by side.
Scenario A
Scenario B
| Scenario A | Scenario B | B − A | |
|---|---|---|---|
| Final balance | — | — | — |
| Total contributions | — | — | — |
| Interest earned | — | — | — |
How it works
Compound interest means each period you earn interest not only on your original deposit, but also on all the interest added before. The standard formula is:
Where P is the starting principal, r the annual rate (as a decimal), n the number of compounding periods per year, and t the number of years.
Monthly contributions are treated as an ordinary annuity — each contribution is assumed to be made at the end of its period:
When compounding is quarterly or annual, your monthly contributions are grouped into one amount per compounding period. If the rate is 0%, the result is simply principal plus all contributions.
The power of starting early
Starting early is the single highest-leverage move in investing, and the numbers are stark. Put $200 a month into an account earning 7% a year. Start at age 25 and invest for 40 years: $96,000 of your own money grows to about $525,000. Start at 35 with the same $200 a month and you reach 65 with about $244,000, from $72,000 contributed. The early starter's first ten years — just $24,000 of extra contributions — turned into roughly $281,000 of extra balance. That decade of compounding can't be bought back at any price, which is why the best day to start was years ago and the second-best day is today.
Does compounding frequency matter?
Less than you'd expect. At 7% over 30 years, $10,000 plus $200 a month grows to about $325,000 with monthly compounding versus about $303,000 compounded annually — a gap of roughly $22,000, or about 7%. That's real money, but compare it with the gap from starting ten years earlier or raising your rate by one point, which run into the hundreds of thousands. Banks advertise compounding frequency because it's free to offer; your attention is better spent on the rate, the time horizon, and the size of your contributions.
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Frequently asked questions
What is compound interest?
Compound interest is interest calculated on both your original principal and on the interest that has already been added. Because each period's interest starts earning interest of its own, your balance grows faster the longer you leave it invested.
Does compounding frequency matter much?
Somewhat. Monthly compounding grows slightly faster than annual compounding at the same rate, but the difference is small. The interest rate and the number of years matter far more than how often interest compounds.
Are monthly contributions really worth it?
Yes. For most savers, regular contributions add more to the final balance than chasing a slightly higher rate. Contributing early and consistently gives compounding more time and more principal to work with.
Is the projected balance guaranteed?
No. This calculator shows a mathematical projection assuming a constant rate. Real investments fluctuate, and fees and taxes also reduce returns. Treat the result as an illustration, not a promise.