Compound Interest Calculator
How savings grow with regular contributions and compounding returns.
Your figures
Result
How to use this tool
- Enter your own figures over the defaults — Starting amount, Added each month, Annual return and the rest.
- The answer updates as you type. There is no button, and nothing is sent anywhere.
- Change Annual return on its own to see how much it moves the result — the worked example below shows exactly that.
Compounding rewards time more than amount. Starting ten years earlier usually beats contributing considerably more later — try shifting the years figure and watch what happens.
How it works
This takes 4 inputs — starting amount, added each month, annual return, years — and returns final balance, total contributed, growth and growth as multiple of contributions. The whole calculation happens in this page as you type: there is no submit button, no request, and nothing stored.
Because it recalculates on every keystroke you can use it as a sensitivity test rather than a one-shot answer. Moving annual return alone from 7 to 8 changes final balance from 144,572.72 to 167,072.11 — the worked example below sets that out in full.
A worked example
Worked through with the figures the page already has in it: starting amount of 10,000 and added each month of 200, and the rest as shown, giving final balance of 144,572.72. These numbers come from running the calculator’s own code, not from being typed in here, so they cannot drift away from what you see on screen.
Starting from:
- Starting amount: 10,000
- Added each month: 200
- Annual return: 7 %
- Years: 20 years
| Result | Value |
|---|---|
| Final balance | 144,572.72 |
| Total contributed | 58,000.00 |
| Growth | 86,572.72 |
| Growth as multiple of contributions | 2.49× |
What happens when one input moves
Holding everything else steady and changing Annual return from 7 % to 8 % — one percentage point — moves the answer like this:
| Figure | 7 % | 8 % |
|---|---|---|
| Final balance | 144,572.72 | 167,072.11 |
| Total contributed | 58,000.00 | 58,000.00 |
| Growth | 86,572.72 | 109,072.11 |
| Growth as multiple of contributions | 2.49× | 2.88× |
That is usually the number worth arguing over: it is easier to move than the headline amount, and the difference compounds across the whole term.
Compound Interest Calculator — frequently asked questions
How often does this compound?
Monthly, matching the contribution schedule. Annual compounding produces slightly lower results.
Is the return guaranteed?
No. Market returns vary year to year and can be negative. The figure here is a smooth projection, not a forecast.
Does this account for inflation?
No. To see real purchasing power, enter a return net of expected inflation — for example 4% instead of 7%.
Is this calculator free?
Yes, with no account, no limits and no data collection. It runs entirely in your browser.
Should I rely on this for a financial or medical decision?
No. It is a planning estimate built on standard formulas and stated assumptions. For decisions that matter, speak to a qualified professional who can account for your full circumstances.
How accurate is the Compound Interest figure?
The arithmetic is exact. With the defaults on this page — starting amount 10000, added each month 200, annual return 7 % — it returns final balance of 144,572.72. What limits accuracy is the assumptions, not the maths: change annual return to 8 % and the same formula gives 167,072.11. Treat it as a well-founded estimate, not a quotation.
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Background reading: Loan, Mortgage and Investment Calculators Explained
Nothing is sent anywhere. The figures you type stay in this page and are never transmitted or stored — how that works.
Disclaimer. The formula is exact; the answer is only as good as the figures and assumptions above. Not financial, tax, medical or legal advice. Full disclaimer.