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複利計算機

元金が複利と任意の毎月積立でどう増えるかを、単利と比較して確認できます。

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このツールの詳しい解説（使い方、例、よくある質問）は、現在英語のみでご用意しています。

What this tool does

Enter a starting amount, an annual rate, a time period, and how often interest compounds. You can also add a monthly contribution. The page shows the ending balance, the interest, and a simple-interest comparison.

How to use

  1. Enter the starting amount and the annual rate. The sample starts at 10000 and 5%.
  2. Choose years or months, then choose annual, semiannual, quarterly, monthly, or daily compounding.
  3. Leave the monthly contribution at 0, or enter an amount you add each month.
  4. Press Calculate. Reset restores the sample values.

Examples

  • 10,000 at 5% for 10 years, compounded monthly

    With no monthly contribution, the ending balance is the starting amount grown by the monthly rate. The interest line is the ending balance minus 10,000.

  • The same inputs plus 100 a month

    The result splits the ending balance into the starting amount, the contributions, and the interest. Contributions are added at the end of each month.

Main features

  • Principal-only growth and growth with monthly contributions, shown separately.
  • A year-by-year balance.
  • A simple-interest result on the same starting amount.

How it works

With no contributions, the ending amount is P times (1 + r/n) raised to n times t. P is the start, r is the annual rate as a decimal, n is the number of compounding periods in a year, and t is the time in years. Monthly contributions are added after that month’s compounding step. Simple interest on the same starting amount is P times r times t, with no compounding. Neither result is a promised investment return.

Tips

  • Use 0 for the contribution when you only want to see the starting amount grow.
  • Daily compounding uses 365 periods. It is still a formula, not a market forecast.

Limitations

The calculator does not include taxes, fees, or a change in the rate. A zero time period leaves the balance at the starting amount. Results that would overflow are rejected instead of shown as an infinite number.

See the disclaimer for what these tools do not cover.

FAQ

What is compound interest?
Interest that is added to the balance and then earns interest too. A higher compounding frequency applies that step more often.
How is compound interest calculated?
The page uses A = P(1 + r/n)^(nt) when there is no contribution. With a monthly contribution, it grows the balance period by period and adds the contribution at the end of each month.
How does monthly compounding work?
The annual rate is divided by 12. That monthly rate is applied to the balance each month.
Are these numbers sent to a server?
No. The balance, rate, and contribution are compounded in this tab. Those figures are not posted to a server.

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