Free tool · Savings
Compound interest calculator
Starting balance plus a monthly contribution, compounded monthly, over any number of years.
Runs in your browser · no account required · amounts use any currency unit
Compounded monthly
Results
In short
What is this?
Compound interest is interest earned on interest already earned. This calculator compounds monthly on a starting balance plus a fixed monthly contribution, so you can see how much of the final figure came from what you put in and how much came from growth.
How this calculator works
Each month, the balance grows by one twelfth of the annual rate, and then your contribution is added. Contributions are treated as made at the end of the month, which is the conservative assumption — contributing at the start of each month produces a slightly higher result.
What the split tells you
The interesting number is not the future value. It is the proportion of it that came from growth rather than from your own contributions.
Over short periods, almost all of the final figure is money you put in. Over long ones, the balance tips — and the point at which it tips is the practical argument for starting earlier rather than contributing more.
What it does not account for
- Tax on interest or gains, which varies entirely by jurisdiction and account type.
- Fees, which compound against you in exactly the same way.
- Inflation — the future value is in nominal terms, so its buying power is lower.
- Variable returns. A fixed rate is a model; real investment returns vary and can be negative.
This is a projection, not a forecast. It is most useful for comparing scenarios against each other rather than for predicting an actual balance.
FAQ
- Is this calculator free?
- Yes. It runs in your browser with no account and nothing stored. Sign up only if you want to track the actual transfers in a private ledger.
- Does it account for inflation?
- No. The result is in nominal terms, so its buying power in future money is lower. Use the inflation calculator alongside it to see the difference.
- Are contributions added at the start or end of the month?
- At the end, which is the conservative assumption. Contributing at the start of each month produces a slightly higher result than shown.
Keep the numbers in a private ledger
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