Compound Interest Calculator
See how savings or investments grow over time with compound interest and regular contributions. Everything runs locally in your browser.
New to this? Read the Compound Interest guide →
- Future balance
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- Total contributions
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- Total interest
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Balance over time
Contributions Total balance
Year-by-year breakdown
| Year | Contributions | Interest | Balance |
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Estimates only, assuming a constant rate and contributions made at each month's end. Not financial advice.
How to use the compound interest calculator
- Enter a starting amount, an optional regular contribution, an annual rate and a time horizon.
- Read the projected balance, the total interest earned, and the growth chart and year-by-year breakdown.
- Try moving the start date earlier and watch how much the final number jumps. That gap is the cost of waiting.
What compound interest is
Compound interest is interest earned on your interest. Instead of growing by the same amount each year, your balance grows by a percentage of an ever-larger total, so it accelerates over time. A one-off deposit grows by the formula A = P (1 + r/n)n·t, where P is the principal, r the annual rate, n how often it compounds, and t the number of years.
Why time is the biggest lever
Because time sits in the exponent, it matters more than the amount you save. A modest sum left to compound for 40 years can easily beat a much larger sum compounding for 20. The four levers, in rough order of impact, are time, rate of return, regular contributions and how often interest compounds.
Where you'll use it
Projecting savings and investment growth, comparing what starting earlier is worth, or seeing how a regular contribution adds up. For a worked example and the two-savers story, read how compound interest works.
Where you'd use this
Seeing what regular saving turns into over a long horizon — and why the same monthly amount started ten years earlier ends up so far ahead.
For example: $300 a month at 7% for 30 years grows to about $340,000, of which $108,000 is your contributions and the rest is growth. Start at 40 instead of 30 and the same habit produces roughly $156,000.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original principal and the interest already added, so your balance grows faster over time than with simple interest.
How do contributions and frequency affect the result?
Regular contributions and more frequent compounding both increase the final balance. The calculator lets you set the rate, contribution amount and compounding frequency to compare scenarios.
Is this financial advice?
No. It is an estimate for planning and education only, using the figures you enter — actual returns vary. Everything is calculated in your browser.