Compound interest calculator
This compound interest calculator shows how savings or an investment grows when you earn interest on both your deposits and the interest already earned.
Why time matters more than the rate
Compounding means interest earns interest. Growth is slow at first and accelerates, so extra years at the end add more than extra years at the start. Starting ten years earlier typically adds more to the final balance than doubling your contributions later.
Using it for debt, too
Compounding works against you on debt such as credit cards. Use this calculator with a card’s APR and no contributions to see how an unpaid balance grows, and use the credit card payoff calculator to see how long repayment takes.
Growth over time
Starting with $10,000 and adding $300 a month at 6%, the balance reaches $172,407 after 20 years, of which $90,407 is interest.
| Years | Balance | You contributed | Interest earned |
|---|---|---|---|
| 5 | $34,524 | $28,000 | $6,524 |
| 10 | $67,604 | $46,000 | $21,604 |
| 20 | $172,407 | $82,000 | $90,407 |
| 30 | $363,087 | $118,000 | $245,087 |
| 40 | $710,009 | $154,000 | $556,009 |
What a different rate does
Even a one-point difference compounds into a large gap over decades.
| Rate | Balance after 20 years |
|---|---|
| 4% | $132,625 |
| 5% | $150,950 |
| 6% | $172,407 |
| 7% | $197,577 |
| 8% | $227,152 |
Frequently asked questions
What is the compound interest formula?
A = P(1 + r/n)^(nt), where P is the principal, r the annual rate, n the compounds per year and t the years. With monthly contributions the calculator adds each deposit as it is made.
Does more frequent compounding matter?
It helps slightly. Monthly versus annual compounding at 6% adds about 0.17 percentage points a year.
Related calculators
Estimates from standard loan formulas, not a loan offer or financial advice. Example rates are assumptions: use your own quote.