💰 Finance & Investing

Compound Interest Calculator

Calculate compound interest on any investment with monthly contributions. Free US compound interest calculator.

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What This Calculator Tells You

Compound interest is interest earned on interest. Each period, the interest you earned is added to your balance, so the next period's interest is calculated on a bigger amount. Over long periods, that snowball effect does most of the work.

This calculator projects the future value of a starting balance plus optional monthly contributions, and splits the result into what you put in and what interest added.

What to Enter

Principal ($) — the amount you start with. Enter 0 if you are starting from scratch.
Annual rate (%) — the yearly interest rate or expected return. For savings accounts, use the APY; for investments, use a conservative long-run estimate.
Years — how long the money grows.
Compounds per year — how often interest is added to the starting balance: 12 for monthly, 365 for daily, 1 for yearly.
Monthly contribution ($) — what you add each month. Contributions are added at the end of each month and compound monthly.

Worked Examples

1. A lump sum left alone

$10,000 earning 5% a year, compounded monthly, for 10 years, with no contributions.

Enter $10,000, 5%, 10 years, 12, $0 → $16,470, of which $6,470 is interest.

2. How much compounding frequency matters

The same $10,000 at 5% for 10 years, but compounded once a year.

Change compounds per year to 1 → $16,289.
Monthly compounding added just $181 more. The rate and the time matter far more than the frequency.

3. Small monthly savings over a career

Starting from $0, saving $300 a month at a 7% average annual return for 30 years.

Enter $0, 7%, 30 years, 12, $300 → $365,991.
You contributed $108,000; compounding added $257,991 — more than twice what you put in.

How It’s Calculated

FV = P × (1 + r/n)^(nt) + PMT × (((1 + r/n)^(nt) − 1) ÷ (r/n)) for periodic contributions.

FV: Future Value
P: Principal amount
r: Interest / discount rate (decimal per period)
n: Number of periods or sample size
PMT: Payment per period

How to Read Your Result

Interest earned is the gap between the future value and what you contributed. Over long periods it often becomes the larger share.
Nominal, not real, dollars. The result does not adjust for inflation. Use the inflation calculator to see what it would be worth in today's money.
Before tax. Interest in a regular taxable account is taxed; in retirement accounts it may be tax-deferred or tax-free.
Returns vary. Investments do not earn a steady rate every year. Treat the result as one scenario and try lower rates too.

Common Mistakes to Avoid

Using an optimistic return. Small differences in the rate compound into very large differences over decades. Test a cautious rate as well.
Confusing APR and APY. APY already includes compounding; if you enter an APY, set compounds per year to 1 to avoid double-counting.
Ignoring fees. A 1% annual fee lowers your effective rate by about a point, which compounds too.
Forgetting inflation. $366,000 in 30 years will buy much less than $366,000 today.

Built and maintained by GenAlpha Tools Editorial. Formula tested against known answers and edge cases · last reviewed October 2026. How we calculate · Report an error

When to Use This Calculator

See how a savings account or CD will grow.
Show the cost of starting to save a few years later.
Compare monthly saving targets for a long-term goal.
Teach how compounding works with real numbers.

Frequently Asked Questions

What is the compound interest formula?

For a lump sum, A = P × (1 + r/n)^(n×t), where P is the principal, r the annual rate, n the number of compounding periods per year and t the number of years. Monthly contributions add the future value of an annuity on top.

Does compounding frequency make a big difference?

Less than most people expect. At 5% over 10 years, monthly compounding beats yearly compounding by about 1.1% of the final balance.

What is the Rule of 72?

Divide 72 by the annual rate to estimate how many years it takes money to double. At 6%, that is about 12 years.

Is the result adjusted for inflation or taxes?

No. It is a pre-tax, nominal figure. Adjust for inflation and your tax situation separately.

Can I use this for debt?

It shows how a balance grows when nothing is paid off, which illustrates how unpaid credit-card debt compounds. For repayment plans, use the loan calculator.

Do you store the values I enter?

No. All calculations happen in your browser. Your data never leaves your device.

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