Compound Interest Calculator — See how savings or monthly investments grow over time
Enter an initial amount, a monthly contribution, an annual return and a number of years to see the final value, total contributions and gain, with a year-by-year table. A rough guide for NISA or iDeCo planning; not investment advice.
See what regular monthly investing could grow into over the years.
Before you rely on thisThis tool only shows what happens if the rate you enter never changes. Real returns move up and down each year and can fall below the amount invested. Taxes and fees are not included. Nothing here recommends a financial product or constitutes investment advice. Check any product carefully and consult a financial institution or adviser if needed.
Compound Interest Calculator: tips and things to know
- Rule of 72: divide 72 by the annual return in percent to get the years it takes money to double.
- The longer the horizon, the larger the compounding effect.
- Real investments carry risk and can fall below what you put in.
How to use the Compound Interest Calculator
Enter the amounts
Type the initial investment and the monthly contribution, or load the example.
Set the rate and term
Type the assumed annual return and the number of years.
Read the result
The final amount, total contributions and gain appear, with a year-by-year table.
Compound versus simple interest
Simple interest is paid only on the original principal: ¥1,000,000 at 3% earns ¥30,000 every year.
Compound interest adds each year's earnings to the principal before calculating the next. Year one earns ¥30,000; year two earns 3% of ¥1,030,000, or ¥30,900. The growth accelerates a little each year.
Over one or two years the difference is tiny; over decades it becomes large.
Why time matters more than the rate
Compounding builds on itself, so growth is fastest in the later years. The same sum at the same rate gives a very different result over 10 years and over 30.
That is why starting early is usually said to matter more than finding a slightly higher return. In practice returns are not smooth: some years fall, and the curve is never as clean as the chart.
Adding monthly contributions
Investing a fixed amount every month keeps growing the principal, which compounds alongside the returns.
Later contributions have less time to grow, so early money does most of the work. For the same total invested, starting sooner tends to end higher.
NISA and iDeCo for residents of Japan
Residents of Japan, including foreign residents, can use NISA, a tax-free investment account with a growth allowance of ¥2.4 million a year and a total lifetime allowance of ¥18 million, in which gains are not taxed. iDeCo is a private pension account with tax-deductible contributions but no withdrawals before 60.
Outside these accounts investment gains are taxed at about 20%. This calculator ignores tax and fees, so a taxable account will end lower than the figure shown. Check the tax treatment in your home country too, since some countries tax their citizens on worldwide income.
Where the calculation differs from reality
The tool assumes exactly the same return every year, which never happens. Averages hide the swings, and the wider the swings the more the real outcome tends to fall short of the smooth calculation.
Use it to compare scenarios at several rates rather than as a forecast.
Compound Interest Calculator FAQ
What is compound interest?
Earnings are added to the principal so that future earnings are calculated on the larger total. The gap over simple interest grows with time.
Why does the example use 5%?
It is a figure often used to illustrate long-term diversified investing. It is not a forecast or a recommendation of any product.
Are taxes included?
No. Gains in a NISA account are tax-free; in an ordinary taxable account in Japan they are taxed at about 20%.