Compound Interest Calculator
Project how savings grow with compound interest and regular contributions, with a year-by-year breakdown showing how much came from deposits and how much from growth — or name a target and be told what you would have to put in each month to reach it.
Why the last decade does the heavy lifting
Compound growth is exponential, so the absolute gains arrive late. Investing 500 a month at 7 percent for 30 years produces roughly 610,000, of which about 430,000 is growth. Stop at 20 years and you have around 260,000 — the final ten years add nearly 350,000, more than the first twenty produced in total.
This is the entire argument for starting early, and it is why the same monthly amount started ten years later produces a dramatically smaller result. Time in the market is doing more work than the size of the contribution.
The rule of 72
| Annual rate | Years to double | Actual |
|---|---|---|
| 3% | 24.0 | 23.4 |
| 6% | 12.0 | 11.9 |
| 9% | 8.0 | 8.0 |
| 12% | 6.0 | 6.1 |
This is a projection tool, not financial advice. Real returns vary year to year and can be negative; a fixed percentage is a modelling convenience, not a forecast.
Frequently asked questions
What is the formula for compound interest?
A = P(1 + r/n)^(nt), where P is the starting amount, r is the annual rate as a decimal, n is how many times per year interest compounds, and t is the number of years. Regular contributions are added with a separate future-value-of-an-annuity term, which is what this calculator does.
How much difference does compounding frequency make?
Less than most people assume. On 10,000 at 5 percent for 10 years, annual compounding gives 16,289 and daily gives 16,487 — a difference of about 1.2 percent. The rate and the time horizon dominate; frequency is a rounding detail by comparison.
What is the rule of 72?
Divide 72 by the annual percentage rate to estimate the years needed to double your money: at 6 percent, roughly 12 years. It is accurate to within a few percent for rates between about 4 and 12, and it is the fastest way to sanity-check any growth claim in your head.
Does this account for inflation or tax?
Not directly — turn on the inflation adjustment to see the result in today's purchasing power, which is usually the number that matters. Tax is not modelled at all, and it varies far too much by country and account type for a general calculator to guess at.
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