Compound Interest Calculator
Enter a starting amount, a monthly deposit and a rate to see what your money grows to — year by year.
Quick answer
Compound interest formula: A = P × (1 + r/n)^(n×t). $10,000 invested at 7% a year, compounded monthly, grows to $40,387 in 20 years. Adding $500 a month on top grows it to $300,851 — of which $170,851 is interest.
Balance after 20 years
$300,851
- Total contributions
- $130,000
- Interest earned
- $170,851
- Interest share of balance
- 56.8%
Contributions are added at the end of each month. Before tax and inflation.
Year-by-year growth
| Year | Contributions | Interest | Balance |
|---|---|---|---|
| 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 |
Growth of $500 a month (no starting amount), compounded monthly
| Years | Deposited | At 5% | At 7% | At 10% |
|---|---|---|---|---|
| 5 years | $30,000 | $34,003 | $35,796 | $38,719 |
| 10 years | $60,000 | $77,641 | $86,542 | $102,422 |
| 15 years | $90,000 | $133,644 | $158,481 | $207,235 |
| 20 years | $120,000 | $205,517 | $260,463 | $379,684 |
| 25 years | $150,000 | $297,755 | $405,036 | $663,417 |
| 30 years | $180,000 | $416,129 | $609,985 | $1,130,244 |
| 40 years | $240,000 | $763,010 | $1,312,407 | $3,162,040 |
How it works
Compound interest means you earn interest on your interest. Each period, the interest earned is added to the balance, so the next period's interest is calculated on a larger amount. Over long periods this snowball effect does most of the work — after 30 years of steady deposits, interest is often more than half of the final balance.
The formula for a single deposit is A = P × (1 + r/n)^(n×t), where P is the starting amount, r the annual rate as a decimal, n the number of times interest compounds per year and t the number of years. With regular monthly contributions the calculator adds each deposit at the end of the month and compounds it along with the rest.
The rate you choose matters more than anything else. High-yield savings accounts pay roughly 3%–5%, while a diversified stock index fund has historically returned around 7%–10% a year before inflation — with ups and downs along the way. Results here are before tax and inflation; to see growth in today's money, subtract the expected inflation rate from your interest rate.
Frequently asked questions
What is the rule of 72?
Divide 72 by the annual interest rate to estimate how many years it takes money to double. At 8% it doubles in about 9 years; at 6% in about 12.
Does compounding frequency make a big difference?
Less than people expect. At 5% a year, daily compounding earns an effective 5.13% versus 5.12% for monthly and 5.00% for yearly. The rate and the time invested matter far more.
What is the difference between simple and compound interest?
Simple interest is paid only on the original amount. Compound interest is paid on the original amount plus all interest already earned, so it grows faster every year.