Quick answer: compound interest uses A = P × (1 + r/n)n×t. 10,000 at 8% a year, compounded monthly for 10 years, grows to 22,196.40 – that’s 12,196 of interest without adding a single extra deposit. Add 200 a month and the balance after 10 years is about 58,800.
Why compounding matters so much
With simple interest you earn the same amount every year. With compound interest, each year’s interest is added to the balance and earns interest itself. The difference looks small at first and then becomes huge: 10,000 at 8% simple interest gives 18,000 after 10 years; compounded monthly it gives 22,196; after 30 years the gap is 34,000 versus 109,357.
The formula
A = P × (1 + r/n)n×t
- P – the starting amount
- r – the yearly rate as a decimal (8% = 0.08)
- n – times per year interest is added (12 for monthly)
- t – years
Monthly contributions are added month by month in the calculator, using the monthly equivalent of your compounding frequency, so the result matches how a savings account or investment platform actually credits money.
The rule of 72
Divide 72 by the yearly rate to estimate how long money takes to double: at 8% about 9 years, at 6% about 12 years, at 12% about 6 years. The calculator shows the exact doubling time next to your result.
Keep expectations real
Bank deposits pay a fixed rate; stock-market returns move up and down and an average of 8% can include losing years. Inflation also matters – if prices rise 4% a year, an 8% return grows your buying power by only about 4%. Try running the calculator with your rate minus inflation to see the “real” growth.
Borrowing rather than saving? The loan calculator shows how interest works against you on a loan. Muslims calculating yearly zakat on savings can use the zakat calculator.
How to calculate compound interest
- Enter the starting amount. Your lump sum today. Use 0 if you are only making monthly deposits.
- Add a monthly contribution. Optional – the amount you plan to save every month.
- Set rate, years and compounding. Use the yearly rate and how often your bank or fund adds interest.
- Read the growth. See the final balance, your total deposits, the interest earned and a year-by-year table.
Does compounding frequency matter?
10,000 at 8% for 10 years:
| Compounding | Balance after 10 years |
|---|---|
| Yearly | 21,589.25 |
| Monthly | 22,196.40 |
| Daily | 22,253.46 |
Going from yearly to monthly adds about 600; from monthly to daily only another 57. The rate and the number of years matter far more than the frequency – start early and keep contributing.
Common mistakes
- Using a monthly rate as the annual rate. 1% a month is about 12.7% a year compounded.
- Ignoring fees. A 1.5% yearly fund fee on an 8% return leaves about 6.5%.
- Forgetting inflation. Run a second calculation with your rate minus inflation for “real” growth.
- Assuming steady returns. Shares can fall for years; use a lower rate for planning.
Frequently asked questions
What is compound interest?
Interest calculated on both the money you put in and the interest already added. Over time your interest starts earning its own interest.
How often is interest compounded?
It depends on the account: savings accounts often compound daily or monthly, many bonds half-yearly, and some deposits yearly. More frequent compounding gives slightly more.
What is the effective annual rate?
The yearly growth including compounding. 8% compounded monthly is an effective 8.30% a year; the calculator shows it.
How long does it take to double my money?
Roughly 72 divided by the interest rate. At 8% it takes about 9 years.
Does the calculator include tax or inflation?
No. To see real growth, enter your expected return minus inflation, and remember that interest may be taxable where you live.
Are contributions added at the start or end of the month?
At the end of each month, which is the conservative and most common assumption.
How much will 1,000 a month grow to in 20 years?
At 8% compounded monthly, about 589,000 – of which 240,000 are your deposits. Enter your own rate to see the result.
Is compound interest the same as APY?
APY (annual percentage yield) is the effective annual rate including compounding – the “Effective annual rate” shown in the results.