Compound Interest Calculator
Compound interest calculator
Compound interest is what happens when your returns start earning returns of their own. Enter a starting amount, a regular contribution, an assumed rate, and a time frame to see how the total could grow — and how much of it is your own contributions versus growth. This is an illustration, not a guarantee.
Project your growth
Compound interest calculator
These figures are estimates for illustration only. The calculator assumes a steady rate that never changes and a contribution added every period. Real returns rise and fall, can be negative, and are reduced by taxes, fees, and inflation. This is not a prediction and not personalized financial advice.
How this calculator works
Methodology
- Formula
- Future value = P(1 + i)N + PMT × ((1 + i)N − 1) ÷ i, where i is the rate per period (annual rate ÷ periods per year) and N is the number of periods (periods per year × years).
- Assumptions
- The rate is constant, one contribution is added each compounding period, and returns are reinvested. "Your contributions" is the starting amount plus every contribution; "growth" is the rest.
- Data sources
- The formula is the standard future-value-of-a-series calculation. For an official version, see the Investor.gov (SEC) compound interest calculator linked below.
- Limitations
- It ignores taxes, investment fees, and inflation, and it assumes a smooth rate that real markets never deliver. Use it to compare scenarios, not to predict a specific outcome.
Calculations are our own and are estimates. Real investment returns vary and can be negative; taxes, fees, and inflation all reduce what you keep. This is general education, not personalized financial advice.
A worked example
$1,000 to start, $200 a month, 7% for 10 years
Starting with $1,000 and adding $200 every month for ten years, at an assumed 7% compounded monthly, the balance grows to about $36,626.62. Of that, $25,000.00 is money you put in and $11,626.62 is growth. The longer the time frame, the larger that growth slice becomes — which is the whole point of starting early.
Frequently asked questions
What is compound interest?
It is interest earned on both your original money and the interest it has already earned. Over time that interest on interest is what makes savings and investments grow faster the longer you leave them.
What rate of return should I use?
There is no guaranteed rate. Cash savings pay less; long-run stock market returns have historically been higher but vary a lot and can be negative in any given year. Try a few rates to see a range rather than trusting a single number.
Does this calculator account for taxes, fees, or inflation?
No. It shows growth before any taxes, investment fees, or inflation, all of which reduce what you actually keep. Treat the result as a gross illustration, not a spendable figure.
Sources
- Investor.gov (U.S. Securities and Exchange Commission)Official compound interest calculator
- Federal ReserveResearch on household saving and investing