How compound interest works
Compound interest is the engine behind long-term wealth building. Unlike simple interest, which is calculated only on your original deposit, compound interest is calculated on your principal plus all the interest you have already earned. Each compounding period, the interest you earned last period starts earning interest of its own. Over years and decades, that snowball effect is what turns modest, consistent saving into a substantial balance.
The compound interest formula
For a one-time deposit with no further contributions, the future value is:
A = P (1 + r/n)nt
- A — the final amount (future balance)
- P — the principal (your initial investment)
- r — the annual interest rate as a decimal (7% = 0.07)
- n — compounding periods per year (12 for monthly)
- t — the number of years
When you add regular contributions, the calculator also applies the future value of a series formula to each deposit, so every contribution compounds for the remaining time it stays invested. That is why the tool above asks for both a contribution amount and a frequency.
A worked example
Suppose you start with $10,000, add $500 every month, and earn a 7% annual return compounded monthly for 20 years. You will have personally contributed $130,000 ($10,000 up front plus $120,000 in monthly deposits), yet your ending balance is roughly $300,000. The extra ~$170,000 is pure compound interest — money your money earned. Try changing the rate or years in the calculator to see how dramatically the interest portion grows with time.
Why time matters more than rate
Because compounding builds on itself, the number of years you stay invested usually matters more than chasing a slightly higher rate. An investor who starts at 25 and stops contributing at 35 can end up with more than someone who starts at 35 and contributes until 65 — simply because the early money had more time to compound. The practical takeaway: start early, contribute consistently, and let time do the heavy lifting.
Tips for using this calculator
- Use a realistic long-term return. Historically, a diversified stock portfolio has averaged roughly 7% after inflation, while savings accounts and CDs pay far less.
- Match the compounding frequency to your account. Most savings accounts compound daily or monthly; bonds often compound semi-annually.
- Remember the result is nominal — it does not subtract taxes or inflation, which reduce your real return.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original principal and on the interest you have already accumulated. Because each period’s interest is added to the balance, future interest is calculated on a larger amount — so your money grows faster over time than it would with simple interest.
What is the compound interest formula?
The core formula is A = P(1 + r/n)^(nt), where P is the principal, r is the annual interest rate (as a decimal), n is the number of times interest compounds per year, and t is the number of years. When you also add regular contributions, a future-value-of-a-series term is added on top.
How does compounding frequency affect growth?
The more often interest compounds, the more you earn, because interest starts earning its own interest sooner. Daily compounding produces slightly more than monthly, which produces more than annual — though the difference shrinks as rates get lower.
Does this calculator account for taxes or inflation?
No. The results show nominal growth before taxes and inflation. Real (inflation-adjusted) returns and any tax on interest will be lower. Use the result as an estimate, not as financial advice.
What is the difference between APR and APY?
APR is the simple annual rate without compounding. APY (annual percentage yield) includes the effect of compounding, so it reflects what you actually earn in a year. For the same APR, more frequent compounding produces a higher APY.
Disclaimer: This calculator provides estimates for educational purposes only and does not constitute financial, investment, or tax advice. Actual returns vary and are not guaranteed.