About Compound Interest
Compound Interest projects what a lump sum becomes when the interest is credited back to the balance and earns interest in subsequent periods. The formula is principal times one plus the rate divided by the compounding frequency, raised to the frequency times the number of years, and the output reports the final balance and the interest portion separately.
The compounding frequency field is what distinguishes this from a flat-rate estimate. Setting it to 12 models monthly credit, 4 quarterly, 1 annually; higher frequencies produce a slightly larger total on identical inputs, which is the effect that makes two accounts advertising the same rate pay differently.
No contributions, withdrawals or fees are modelled — the principal sits untouched for the whole term. The result is also subject to the usual floating-point arithmetic of a browser: expect the last two decimal places of a long projection to be approximate rather than exact. Treat the figure as a planning estimate, not a statement.
The single deposit is the shape of the whole model, and several consequences follow from it. Nothing can be added to the balance during the term and nothing can be taken out, so a regular savings plan or a drawdown phase has to be modelled elsewhere. The rate is fixed for the entire period: there is no schedule of changing rates, no stepped or declining rate, and no way to run a term in which the rate is adjusted partway through. Growth is reported only as a nominal figure with no inflation adjustment and no tax or fee column, and the projection ends at the final balance rather than showing a year-by-year table, so the compounding path has to be inferred rather than read off.
How to use Compound Interest
- 1 Enter the starting principal.
- 2 Enter the annual rate as a percentage.
- 3 Enter the number of years the money stays invested.
- 4 Set how many times per year interest is credited — 12 for monthly, 1 for annual.
Frequently asked questions
Does a higher compounding frequency always earn more?
Yes, for the same nominal rate — the effect is real but small. Going from annual to monthly compounding on a 5% rate adds roughly a tenth of a percent per year, while the step from monthly to daily adds about a hundredth.
Can I model regular monthly deposits?
Not here: this calculator compounds a single starting amount and never adds to it. A series of equal deposits at a fixed rate is a different formula, covered by the annuity future value calculator.
Should I enter the term as months or years?
Years. The frequency field handles the sub-yearly detail, so a five-year monthly-compounded projection is 5 years with a frequency of 12. Pre-converting to months would compound the rate far more often than intended.
Why does the projection look slightly optimistic in real terms?
Because it is nominal growth with no inflation adjustment. The same nominal balance buys less at the end of a long term; the Inflation Calculator applies a rate to show that erosion.
Free, online, and private
Free to use
Compound Interest is completely free. There is no account to create, no login, and no hidden charge or usage limit.
Runs online in your browser
It is a free online tool that runs entirely in your browser. Nothing to download or install — open the page and start using it right away.
Your data stays private
All processing happens locally on your device. Your input is never uploaded, stored, or tracked, so it never leaves your browser.
How Compound Interest behaves in detail
These are the specifics of this implementation — the exact ranges, defaults and fixed settings you are working with.
| Parameter | Value |
|---|---|
| Formula | principal × (1 + rate ÷ compounds)^(compounds × years) |
| Compounding input | a plain number of periods per year — default 12, so monthly by default |
| Outputs | two lines: total amount, then interest earned, each fixed to 2 decimal places |
| Zero rate | the bracketed term collapses to 1, so the closing amount equals the principal exactly |
| Defaults | 10,000 principal · 5% annual rate · 10 years · 12 compounds per year |
Compiled from the implementation of this tool and checked against it by itoolnet.
What Compound Interest does not do
Knowing the edges of a tool saves more time than another list of its features. These are the cases where it will not do what you might expect.
- The rate is treated as a nominal annual percentage and divided straight by the compounding count. There is no nominal-to-effective conversion, no continuous-compounding option, and no way to declare that the rate you were quoted is already an effective annual figure.
- Only the single starting principal is modelled. No regular deposit, no withdrawal, and no contribution schedule exists here — the whole calculation is one lump sum raised to a power, so a monthly-savings plan has to be run through a different tool.
- Nothing intermediate is reported. You get the final balance and the interest it contains, but no year-by-year schedule, so comparing two compounding frequencies means re-running the tool and subtracting the two results by hand.
Tested and documented by itoolnet, the team that builds and maintains this site.