Compound Interest Calculator
Compound growth with monthly contributions
This free Compound Interest Calculator shows the most powerful idea in personal finance: growth that builds on itself. Enter a starting amount, a yearly rate, how often interest compounds, and an optional monthly contribution, and the calculator simulates your money month by month to reveal the future value, exactly how much of it you paid in yourself, and how much the interest contributed. A year-by-year table lets you watch the interest column overtake the contribution column, which usually happens somewhere in the second half of a long savings plan and surprises almost everyone the first time they see it.
Compounding frequency matters more than most people expect: the same yearly rate grows noticeably faster when interest is added monthly rather than yearly, because each addition starts earning sooner. Regular contributions matter even more, since every monthly deposit gets its own compounding runway. The results here are an illustration with a fixed rate, and real investments rise and fall, charge fees and may be taxed, so treat the figures as a planning guide rather than a promise. For comparison, the Simple Interest Calculator on this site shows the same money without compounding, and the contrast is striking. Calculations run locally in your browser; nothing is uploaded or stored.
How to use
- Enter your starting amount, even if it is zero.
- Type the expected yearly interest rate as a percentage.
- Choose the number of years and how often interest compounds.
- Add a monthly contribution if you plan to save regularly.
- Click Calculate growth and study the totals and yearly table.
Frequently asked questions
What is the compound interest formula?
Without contributions it is A equals P times (1 + r/n) to the power of n times t: principal, yearly rate, compounds per year and years. This calculator simulates month by month instead, which lets it fold monthly contributions into exactly the same mathematics.
How big a difference does monthly compounding make?
On ten thousand at five percent over twenty years, yearly compounding reaches about twenty six thousand five hundred, while monthly compounding reaches roughly twenty seven thousand one hundred. The gap grows with higher rates and longer periods, which is why frequency is worth checking.
Do contributions or interest matter more?
Early on, contributions dominate because the balance is small. Later, interest takes over as it is earned on a much larger pile. Over very long horizons the interest share often ends up bigger than everything you paid in, which is the strongest argument for starting early.
Is this how investment returns work in real life?
Only approximately. Real markets do not pay a smooth fixed rate; they jump around, deduct fees and sometimes fall for years. A steady-rate projection is useful for setting savings targets, but it cannot predict an actual portfolio's path.