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Compound Interest Calculator

See how your savings or investments grow over time with compound interest and regular monthly contributions.

Free, no signup Runs in your browser — nothing stored Reviewed October 2026
Your savings plan
$
$
%
years
Compounding frequency

Balance after 20 years

$300,851

Total contributions

$130,000

Interest earned

$170,851

57% of your final balance

At 7%, money doubles roughly every 10.3 years (Rule of 72).

Growth over time
Year-by-year breakdown
YearContributedInterestBalance
1$16,000$919$16,919
2$22,000$2,339$24,339
3$28,000$4,294$32,294
4$34,000$6,825$40,825
5$40,000$9,973$49,973
6$46,000$13,782$59,782
7$52,000$18,299$70,299
8$58,000$23,578$81,578
9$64,000$29,671$93,671
10$70,000$36,639$106,639
11$76,000$44,544$120,544
12$82,000$53,455$135,455
13$88,000$63,443$151,443
14$94,000$74,587$168,587
15$100,000$86,971$186,971
16$106,000$100,683$206,683
17$112,000$115,820$227,820
18$118,000$132,486$250,486
19$124,000$150,790$274,790
20$130,000$170,851$300,851

How compound interest works

With simple interest, you earn the same dollar amount every year. With compound interest, each year's interest is added to your balance and starts earning interest of its own. That's why long time horizons matter so much: in the later years, most of your growth comes from interest rather than your own deposits.

A = P × (1 + r/n)n×t

A is the ending balance, P the starting amount, r the annual rate, n the number of compounding periods per year, and t the number of years.

What $500 a month becomes

YearsYou contributeAt 5%At 7%At 10%
10$60,000$77,641$86,542$102,422
20$120,000$205,517$260,463$379,684
30$180,000$416,129$609,985$1,130,244
40$240,000$763,010$1,312,407$3,162,040

Monthly contributions with monthly compounding, no starting balance.

Three ways to make compounding work harder

  • Start early. Ten extra years can more than double your ending balance — see why starting early makes you rich.
  • Automate contributions. Regular investing (dollar-cost averaging) keeps you consistent through market ups and downs.
  • Keep fees low. A 1% annual fee can consume roughly a quarter of your growth over 30 years. Low-cost index funds keep more money compounding for you.

Compounding also works against you with debt: credit card balances at 22% APR double in about 3.3 years. If you carry high-interest debt, the Debt Payoff Calculator shows the fastest way out.

Frequently Asked Questions

Compound interest is interest earned on both your original money and on the interest it has already earned. Because each year's growth builds on the last, your balance grows faster over time instead of in a straight line.
For a single deposit, A = P × (1 + r/n)^(n×t), where P is the starting amount, r is the annual rate, n is how many times per year interest compounds, and t is the number of years. With regular contributions, each deposit compounds from the date it's added — this calculator handles that month by month.
At 5% compounded annually, $10,000 grows to about $16,289 in 10 years. At 7% it grows to about $19,672, and at 10% to about $25,937.
A little. Monthly or daily compounding earns slightly more than annual compounding at the same stated rate, but the difference is small compared with the impact of your rate, how much you contribute, and how many years you stay invested.
High-yield savings accounts recently paid around 4–5% APY. The US stock market (S&P 500) has averaged roughly 10% a year before inflation and about 7% after inflation over the long run, with large swings from year to year.
Divide 72 by your annual return to estimate how many years it takes to double your money. At 6% your money doubles in about 12 years; at 9%, in about 8 years.

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Disclaimer: This calculator provides estimates for educational purposes only and is not financial, tax, or legal advice. Results depend on the numbers you enter and the assumptions described above. For decisions about your specific situation, consult a qualified professional.