Compound interest explained
Compound growth means returns can earn additional returns. Time, rate, contribution size and fees all influence the final value.
Start with the idea
For one starting deposit, the standard formula is FV = P(1 + r/n)^(nt). P is the starting amount, r the annual rate, n the compounding periods per year and t the years.
Regular contributions require an annuity calculation in addition to the starting deposit. A calculator helps combine both parts and show contributed money separately from estimated growth.
Worked example
Invest $10,000 at 5% compounded monthly for 10 years with no extra deposits. The projected value is about $16,470 before tax and fees.
Add $200 at the end of every month and the projection becomes much larger because both new contributions and prior returns participate in later growth.
Quick reference
| Item | Meaning |
|---|---|
| Higher rate | Faster projected growth and usually more uncertainty |
| More time | More compounding periods |
| Regular contributions | More principal working over time |
| Fees and taxes | Reduce the amount that remains invested |
Common mistakes
- Assuming a constant return is guaranteed
- Ignoring fees, tax and inflation
- Confusing annual rate with monthly rate
- Comparing projections with different contribution timing
Practical steps
- Enter a realistic starting balance.
- Use a cautious rate rather than the best historical year.
- Add contributions you can sustain.
- Compare values before and after fees and inflation.
- Review the plan regularly.
Run your own numbers
Use the related Fynzo tool to test different inputs and compare results.
Open the Compound Interest Calculator →Sources and review note
This article was reviewed for language, calculation examples and source links by Yasser Chahir, Fynzo editor. Last reviewed: July 31, 2026. It is general educational information, not medical, financial, tax or legal advice.