Compound interest calculator
With compound interest you earn interest on your interest. The formula for a single deposit is A = P × (1 + r/n)n×t, where P is the starting amount, r the annual rate, n the number of compounding periods per year and t the number of years. This calculator also adds a monthly contribution and shows the balance year by year.
How compound interest is calculated
Each period, interest = balance × (annual rate ÷ periods per year), and that interest is added to the balance. Monthly contributions are added at the end of each month. More frequent compounding earns slightly more: 5% compounded monthly is an effective 5.12% a year, and daily about 5.13%.
Example
$10,000 at 5% compounded monthly for 10 years with no contributions grows to $10,000 × (1 + 0.05/12)120 ≈ $16,470.
How much difference does compounding frequency make?
$10,000 at 5% for 10 years, no additions:
| Compounding | Balance after 10 years | Effective annual rate |
|---|---|---|
| Yearly | $16,289 | 5.000% |
| Quarterly | $16,436 | 5.095% |
| Monthly | $16,470 | 5.116% |
| Daily | $16,487 | 5.127% |
The rate and the time matter far more than how often interest is compounded.
How much will $200 a month grow?
Saving $200 at the end of every month, compounded monthly, starting from $0:
| Annual rate | 10 years | 20 years | 30 years |
|---|---|---|---|
| 3% | $27,948 | $65,660 | $116,547 |
| 5% | $31,056 | $82,207 | $166,452 |
| 7% | $34,617 | $104,185 | $243,994 |
| 10% | $40,969 | $151,874 | $452,098 |
Over 30 years you contribute $72,000; everything above that is compound growth.
How much will $10,000 grow?
A one-time $10,000 deposit, compounded monthly, no additions:
| Annual rate | 10 years | 20 years | 30 years |
|---|---|---|---|
| 3% | $13,494 | $18,208 | $24,568 |
| 5% | $16,470 | $27,126 | $44,677 |
| 7% | $20,097 | $40,387 | $81,165 |
| 10% | $27,070 | $73,281 | $198,374 |
How to use this calculator
Enter what you have now, what you will add each month, the yearly interest rate and how long you will save. For a savings account or CD, use its APY and choose daily or monthly compounding. For stock investments, use a long-term average return you are comfortable with and remember that real returns vary year to year. The table under the results shows your balance, contributions and interest for every year.
Rule of 72: how fast money doubles
Divide 72 by the annual rate to estimate the years to double: about 24 years at 3%, 14.4 years at 5%, 10.3 years at 7% and 7.2 years at 10%.
Common questions
What is the compound interest formula?
A = P(1 + r/n)^(nt): P = starting amount, r = annual rate as a decimal, n = compounding periods per year, t = years.
How much will $10,000 grow in 10 years?
At 5% compounded monthly with no additions, about $16,470. At 7%, about $20,097.
What is the rule of 72?
Divide 72 by the annual rate to estimate the years to double: at 6%, about 12 years.
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Last reviewed October 31, 2026. This calculator gives estimates for education and planning only. Results depend entirely on the numbers you enter and the assumptions stated on this page; they are not investment, tax or trading advice, and real results will differ.