Compound Interest Calculator
Calculate compound interest growth over time, with the formula, a worked example and a year-by-year view.
Why compounding is powerful
Compound interest is interest earned on your interest as well as your original money, so growth accelerates over time rather than staying flat. It is the single most important idea in personal finance, working for you in savings and investments and against you in debt. This shows how a sum grows from your rate, time and contributions.
The formula
The core compound interest formula is:
A = P (1 + r/n)^(n t)A = final amount P = principal (starting sum)r = annual rate n = times compounded per yeart = number of yearsThe key levers are the rate, and above all the time — because the exponent is time, adding years matters more than almost anything else, which is why starting early beats starting with more.
Worked example
A lump sum left to compound:
Time does the heavy lifting
Watch how the same money grows as the years extend — the later years add far more than the early ones, because compounding builds on an ever-larger base:
| Years at 7% | 10,000 grows to |
|---|---|
| 10 | 19,672 |
| 20 | 38,697 |
| 30 | 76,123 |
| 40 | 149,745 |
This is why the most valuable ingredient in compounding is time, and why regular contributions on top of a lump sum accelerate it further. These figures are illustrative of the maths, not a prediction or financial advice — real returns vary, are not guaranteed, and inflation erodes future sums. For decisions about your own money, a qualified financial adviser is the right source.
Frequently asked questions
What is compound interest?
Interest earned on both your original money and the interest already accumulated, so growth accelerates over time rather than staying flat.
Why does starting early matter so much?
Because time is the exponent in the formula — extra years compound on an ever-larger base, so starting early usually beats starting with more money later.
How is it different from simple interest?
Simple interest is paid only on the original sum; compound interest is paid on the sum plus all prior interest, which is why it grows far faster over time.
Are these figures a prediction?
No — they illustrate the maths. Real returns vary and are not guaranteed, and this is not financial advice. A qualified adviser is the right source for your own decisions.
Is my data uploaded?
No. The calculation happens in your browser.