When this calculation actually comes up
- Deciding between a fixed deposit and something with a higher return, and wanting to see the gap first.
- Planning a monthly contribution and wanting a sense of the order of magnitude after twenty years.
- Working out roughly how many years the current saving rate needs to reach a target amount.
- Explaining to family why starting earlier beats contributing more later.
- Checking the numbers a bank or a salesperson handed you.
What this tool does
Fill in a principal, an annual interest rate, a number of years and a compounding frequency on the left, and the right side immediately shows the final balance, total contributed and total interest. To simulate a recurring investment, add an amount in "Additional contribution per period" — it gets added to the balance at the end of every compounding period, on the same schedule as the compounding frequency. Every field recomputes as you type, with no button to press.
The compound interest formula
Without contributions, compound interest follows this formula:
Balance = Principal × (1 + Annual rate / Compounds per year) ^ (Compounds per year × Years)With contributions, this tool switches to a period-by-period simulation: each period applies the formula above to the current balance, then adds that period's contribution. The next period's interest is calculated on the principal plus every contribution made so far — that compounding on top of contributions is exactly what separates compound interest from simple interest.
How compounding frequency changes the result
A higher compounding frequency produces a larger final balance for the same annual rate, because interest gets folded back into the balance more often, and that extra interest then earns interest of its own. The difference grows with longer time horizons and higher rates, and stays small over short periods or low rates. There is usually no need to agonize over which frequency to pick — use whatever frequency is written into the actual loan or savings agreement.
Reading the year-by-year schedule
- Year is how many years the simulation has run at the end of that row.
- Contributed to date is the principal plus every contribution made up to that year.
- Interest to date is the ending balance minus contributed to date — the part compounding actually earned you.
- Ending balance is the total at the end of that year, matching the height of the growth line for that year.
The table always shows one row per year, even when compounding is monthly or daily, so a long time horizon never turns into a table with hundreds of rows.
What the growth chart shows
The chart stacks two layers: contributed principal on the bottom, interest on top. The combined height at any point is the ending balance. Hover over a point on the line to see the exact balance for that year.
Why there is no currency symbol
This tool does not detect your location and has no currency field, and guessing the wrong symbol is more misleading than showing none. Amounts are formatted with thousands separators only — the actual currency is whatever you had in mind when you entered the principal and contribution, as long as both use the same one.
Does anything you enter leave the browser
No. All the math runs in your browser, and the numbers you type are never sent anywhere. That is true of every tool on this site — see the privacy policy for the details.
Frequently asked questions
- Can this simulate a recurring investment, like monthly contributions?
- Yes. Set the compounding frequency to monthly and fill in a contribution amount, and each period in the simulation represents one month.
- Can the contribution amount change partway through the years?
- No, the contribution stays fixed for the entire time horizon. To model a contribution that changes over time, run this tool once per stage and chain the results together yourself.
- Can the annual rate be negative, for an inflation-adjusted real return?
- No, the rate field only accepts values of 0 or higher. Adjust the final balance for inflation separately, outside this tool.
- Can I export the year-by-year schedule?
- Yes, the download-CSV button in the result panel saves the full schedule as a CSV file, assembled in your own browser.
- Is there a limit on principal, rate or years?
- Yes: principal is capped at 100,000,000, the annual rate at 50%, and years at 50. Values outside those ranges are clamped to the limit.