Compound Interest Calculator
See exactly how your money grows over time with compound interest — the effect of earning returns on both your original deposit and the interest it has already earned. Enter a starting amount, an optional monthly contribution, your interest rate, and a time frame, and this calculator shows your future balance, how much of it is contributions versus interest, and a year-by-year breakdown. Everything updates instantly in your browser.
Future balance
$19,318
How to Use
Enter your initial amount, an optional monthly contribution, the annual interest rate, and the number of years, then choose how often the interest compounds — daily, monthly, quarterly, semi-annually, or annually. The results update as you type: a future balance, the total you contributed, the total interest earned, and a year-by-year growth table so you can watch the balance accelerate over time.
Why This Tool Is Useful
Compound interest is the engine behind long-term saving and investing, but its effect is hard to picture in your head because growth accelerates rather than moving in a straight line. This calculator makes it concrete — you can compare scenarios in seconds, see how much a small monthly contribution adds over decades, and understand why starting early matters so much. It is useful for planning retirement savings, a high-yield savings account, a brokerage account, or any goal where money grows over time.
How Compound Interest Works
Compound interest means you earn interest not only on your original deposit (the principal) but also on the interest that has already been added to your balance. Each compounding period, interest is calculated on the new, larger balance — so your money grows by an increasing amount every period. This 'interest on interest' effect is what makes long-term growth curve upward instead of rising in a straight line.
The standard formula is A = P(1 + r/n)^(nt), where A is the final amount, P is the principal, r is the annual interest rate as a decimal, n is the number of times interest compounds per year, and t is the number of years. When you also add regular contributions, each deposit starts compounding from the moment it is added, which this calculator accounts for month by month.
The Power of Monthly Contributions
A modest amount added every month often ends up contributing more to your final balance than your initial deposit — especially over long periods. Because each contribution has its own runway to compound, money you add early works harder than money you add later.
For example, $100 a month at 7% for 30 years means $36,000 of your own deposits, but the balance can grow to well over $100,000 — with the majority coming from interest rather than the contributions themselves. Adjust the monthly contribution above and watch how much the 'total interest earned' figure changes.
How Compounding Frequency Affects Growth
Interest can compound annually, semi-annually, quarterly, monthly, or daily. The more often it compounds, the more often interest is added to your balance and starts earning its own interest — so daily compounding produces a slightly higher result than annual compounding at the same nominal rate.
In practice the difference between monthly and daily compounding is small; the rate and the time horizon matter far more than the frequency. This is also why banks quote an APY (annual percentage yield), which folds the compounding frequency into a single effective annual rate so you can compare accounts fairly.
The Rule of 72 — Estimate Doubling Time
The Rule of 72 is a quick mental shortcut: divide 72 by your annual rate of return to estimate how many years it takes for your money to double. At 8% a year, money doubles in roughly 72 ÷ 8 = 9 years; at 6%, about 12 years.
It is an approximation, not an exact figure, but it is remarkably close for typical rates and helps you sanity-check long-term plans without a calculator. The table below shows doubling times for common rates.
Compound vs Simple Interest
Simple interest is calculated only on the original principal, using A = P(1 + rt), so it grows in a straight line. Compound interest is calculated on the principal plus accumulated interest, so it grows faster and faster over time.
Over a year or two the difference is small, but over decades it is enormous — which is why compound interest is so powerful for long-term goals, and why high-interest debt (which also compounds) is so dangerous.
Tips to Make Compound Interest Work for You
A few habits dramatically change your end result:
- Start as early as you can — time is the most powerful factor, more than the rate or the amount.
- Contribute regularly; automatic monthly deposits keep the compounding fed.
- Reinvest interest and dividends instead of withdrawing them.
- Avoid interrupting the growth — early withdrawals reset the compounding.
- Remember the result is before tax and inflation; real spending power will be lower.
Rule of 72: Years to Double Your Money
| Annual Return | Years to Double (72 ÷ rate) |
|---|---|
| 2% | 36 years |
| 4% | 18 years |
| 6% | 12 years |
| 8% | 9 years |
| 10% | 7.2 years |
| 12% | 6 years |