Calculator · Windows 10 and 11
Compound interest calculator
Start with an amount, add a deposit every month or year, set the interest rate and how often it is added, and see the balance after any number of years. A chart shows what you paid in against what interest added, a table goes year by year, and an inflation rate turns the result into today's money.
How to calculate compound interest
- 1
Open the Compound interest calculator
In Octoolo's Calculator, choose Compound interest calculator in the Money group. It opens with 10,000 plus 200 a month at 7% for 10 years.
- 2
Enter the money
Type the Starting amount and the Deposit, and choose A month or A year. Use 0 for either if there is none.
- 3
Enter the rate and the time
Set Interest a year, how often Interest added is (Daily, Monthly, Quarterly, Twice a year, Yearly or Continuously) and the Years, up to 100.
- 4
Read the result
The balance, what You put in, the Interest earned and the yearly rate with compounding (APY) show at the top. Hover the chart for any year, or use Copy table to take the Year by year figures to a spreadsheet.
The formula, and the example worked through
With no deposits, a starting amount P at a yearly rate r, added n times a year for t years, grows to P × (1 + r/n)n × t. 10,000 at 5% for 10 years becomes:
| Interest added | After 10 years | APY |
|---|---|---|
| Yearly | 16,288.95 | 5% |
| Monthly | 16,470.09 | 5.116% |
| Daily | 16,486.65 | 5.127% |
| Continuously | 16,487.21 | 5.127% |
More frequent compounding helps, but less than people expect: going from monthly to daily adds about 17 over ten years here. The rate matters far more than the frequency. The APY (annual percentage yield) folds the compounding into one yearly rate, which is why banks quote it: 7% added monthly is an APY of 7.229%.
The example the calculator opens with, 10,000 plus 200 at the end of each month at 7% added monthly, reaches 54,713.58 after 10 years. You put in 34,000; interest added 20,713.58, and in the last year alone it added 3,600.02, more than the 2,400 deposited that year.
Deposits: monthly or yearly, at the start or the end
Regular deposits do most of the work in long-term saving. 300 a month at 7% for 30 years, starting from nothing, grows to 365,991.30, of which 108,000 is what you paid in. The same deposits for 20 years reach 156,278.00: the last ten years add more than the first twenty because the balance they grow on is so much larger.
By default each deposit is made at the end of its month (or year), the way most savings plans and loan illustrations count. Switch on Deposit at the start of each month and every deposit earns one more period of interest: the example then reaches 54,915.51 instead of 54,713.58.
When interest is added at a different pace from your deposits, for example daily interest on monthly deposits, the calculator uses the equivalent rate for each deposit period: (1 + r/365)365/12 − 1 a month. That is how most calculators and banks' own illustrations work it out, though an account that pays nothing on money deposited mid-month will end slightly lower.
Inflation, and what the result is worth
A balance in 10 years is not worth what the same number is worth today. Type an Inflation a year rate and the last tile shows the balance In today's money, the balance divided by (1 + inflation)years; the table gains a column for it. At 3% inflation, the example's 54,713.58 is worth about 40,712.04 in today's money, still well above the 34,000 paid in.
A quick check for any rate is the rule of 72: divide 72 by the yearly rate to find roughly how many years it takes money to double. At 7%, about 10.3 years; the calculator shows 1,000 at 7% added yearly becomes 1,967.15 after 10 years and 3,869.68 after 20.
Real investments do not grow at a steady rate, and fees and taxes come off the top. Treat the result as an illustration of how growth compounds, not as a forecast. For money you borrow rather than save, the loan calculator works out the monthly payment and the interest you pay.
Questions, answered
How do I calculate compound interest with monthly contributions?
Type the starting amount, the deposit with A month, the rate and the years. The calculator adds each deposit and the interest month by month and shows the balance, the total paid in and the interest.
What is the difference between APR and APY?
The rate you type is the yearly rate before compounding (like an APR). The APY tile shows what it becomes with compounding: 7% added monthly is 7.229% a year.
Does it account for inflation?
Yes. Type an inflation rate and the balance is also shown in today's money, in the tiles and in the table.
Can I see the balance for each year?
Yes. The Year by year table lists what was paid in and earned each year and the balance, and Copy table puts it in a spreadsheet.
Which currency does it use?
Any: the amounts are plain numbers with two decimals, so they work for dollars, euros, pounds or anything else.
Is this financial advice?
No. It shows the arithmetic of compound growth at a steady rate. Real returns go up and down, and fees and taxes reduce them.
Compound interest calculator, and 16 more apps.
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