Compound Interest Calculator
Result
| Final Amount | ||
|---|---|---|
| Interest Earned |
Share on Social Media:
Compound Interest Calculator — Watch Money Grow on Its Own Growth
Compound interest is interest earned on interest. Unlike simple interest, which pays only on the original principal, compounding reinvests every interest payment so the next calculation is made on a larger base. Over long periods this snowball effect dominates: it is the mechanism behind retirement accounts, index-fund growth, and — on the negative side — credit card debt. This free calculator models it precisely, including regular monthly contributions, with no signup required.
The Formula Behind the Numbers
A = P (1 + r/n)nt — where P is principal, r the annual rate, n compounding periods per year, and t years. With recurring monthly deposits d, each deposit compounds from the month it enters, which the calculator handles by summing the future value of every contribution.
Example: $10,000 at 7% compounded monthly for 30 years grows to about $81,165 with no additional deposits. Add just $200 per month and the final balance jumps to roughly $325,000 — of which only $82,000 is money you actually deposited. The remaining ~$243,000 is pure compounding.
Why Starting Early Beats Saving More
Time is the exponent in the formula, and exponents are ruthless. An investor who saves $300/month from age 25 to 35 and then stops (total deposits: $36,000) typically ends up with more at 65 than one who saves $300/month from 35 to 65 (total deposits: $108,000), assuming ~7% returns. The first decade of compounding is worth more than the following three decades of contributions.
Compounding Frequency Compared
| Frequency | $10,000 at 5% for 10 years |
|---|---|
| Annually | $16,289 |
| Quarterly | $16,436 |
| Monthly | $16,470 |
| Daily | $16,487 |
Frequency matters, but far less than rate and time. When comparing savings accounts or CDs, look at APY — it already folds compounding frequency into a single comparable number.
Common Uses for This Calculator
- Retirement projections — model 401(k) or IRA growth with monthly contributions.
- Savings goals — find the monthly deposit needed to reach a target amount by a target date.
- CD and high-yield savings comparison — compare offers with different compounding schedules.
- Debt awareness — see how card balances balloon when compounding works against you at 20%+ APR.
Free, Instant, and Private
All math runs locally in your browser the moment you type — nothing is uploaded or stored, there is no account, and no limit on how many scenarios you test.