Compound Interest Calculator with Monthly Deposits
Enter your principal, annual rate, period, and monthly deposit to instantly see your final balance and total interest, with a year-by-year growth chart included.
Result
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| Total invested | ¥ {{ fmt(result.invested) }} |
| Total interest | ¥ {{ fmt(result.interest) }} |
What Is Compound Interest — and How It Differs from Simple Interest
Compound interest means the interest earned in one period is folded back into the principal, and the next period's interest is calculated on that larger total. Because interest earns interest, the final balance grows further ahead of simple interest — where interest is always charged on the original principal alone — the longer the money is invested. This tool takes a principal, an annual rate and an investment period, and also accepts a monthly contribution, so it can work out the final balance, the total you put in and the total interest for a plan that keeps contributing while compounding.
The calculation is a simulation that assumes a fixed annual rate holds throughout. Real returns on shares and funds move up and down from year to year rather than staying constant, and taxes, fees and inflation are not included either. Treat the figures shown as a guide rather than a promise of future performance.
How to Use the Compound Interest Calculator
- Enter the principal Enter any money you already have available to invest. If you are starting from contributions alone, you can leave this at zero and the calculation still works.
- Enter the annual rate Enter the annual return you want to assume, as a percentage. What is realistic differs by product — deposits, funds and shares all sit in different ranges.
- Enter the investment period Enter how many years you will keep investing. The longer the period, the larger the compounding effect.
- Enter the monthly contribution (optional) Fill this in if you plan to add a fixed amount each month. Leave it blank if you are not making contributions.
- Check the result and the chart The final balance, total contributions and total interest appear automatically, and a stacked chart shows the year-by-year progression.
Tips for getting more out of it
- Rule of 72: divide 72 by the annual rate to estimate the years needed to double your money. At 6%, that's about 12 years.
- Monthly deposits use monthly compounding; the principal uses annual compounding.
- In real-world investing, account for taxes (≈20% on gains), fees, and inflation.
- Try the long-term average return of a broad stock index (historically around 7–8% annually) as a reference rate.
When a Compound Interest Calculation Comes in Handy
Simulating a regular investment plan
For a fixed monthly amount through a scheme such as NISA or iDeCo, you can see roughly what it becomes after a given number of years. For scheme-specific detail, try the NISA calculator as well.
Setting a savings target
Against a goal like «save a certain amount in N years», it helps you gauge the principal or monthly contribution you need. To look at compounding on plain deposits, the savings calculator is also useful.
Running the same maths on debt
Compounding works identically against you on borrowings. Set a high annual rate and you can feel how fast high-interest debt such as revolving credit swells.
Comparing the value of starting early
Compare 10, 20 and 30 years and you will see the total interest accelerate the earlier you begin. If you also want to weigh the effect of rising prices, the inflation calculator is a useful companion.
Compound Interest Terms Explained
- Compound Interest
- A method where interest arising in each period is added to the principal and the next period's interest applies to that total. Because interest earns interest, balances grow faster than under simple interest.
- Simple Interest
- A method where interest applies only to the original principal for the whole period. Since interest is never folded in, long-run growth is gentler than with compounding.
- Annual Rate
- One year's interest expressed as a percentage of the principal. This tool uses that figure for both its annual and its monthly compounding calculations.
- Effective Rate
- The rate that reflects how much you actually gained over a year once the compounding frequency — yearly, monthly and so on — is taken into account. For the same annual rate, more frequent compounding gives a slightly higher effective rate.
- Compounding Frequency
- How many times a year interest is folded into the principal. This tool compounds the principal once a year and the monthly contributions once a month.
- Rule of 72
- A rule of thumb that estimates the years for a principal to roughly double under compounding as 72 divided by the annual rate in per cent. It is not exact, but it is handy for a quick sense of scale.
FAQ
Note — Is Compound Interest "Humanity's Greatest Invention"?
The phrase "Compound interest is the eighth wonder of the world" is widely attributed to Albert Einstein. However, it has never been found in any of his writings or recorded lectures, and many researchers consider it a misattribution. That said, the power of compound interest is real.
Invest ¥1,000,000 at 5% annually: after 10 years it grows to ≈¥1,630,000; after 20 years ≈¥2,650,000; after 30 years ≈¥4,320,000. Time is the real asset — that's the essence of compounding. The same logic works against you with high-interest debt: a revolving credit card balance at 15–18% nearly doubles in 4–5 years.
One reason Warren Buffett built extraordinary wealth is that he started investing in his teens. Because compound growth is exponential with time, starting just 10 years earlier can multiply the final outcome several times over.