🧮 Calculators & Daily Life

Compound Interest Calculator

Enter a starting amount, monthly contribution, annual rate and term — your projected future value appears right away. It uses monthly compounding and breaks out your contributions and gains. Everything stays in your browser.

Note: this projection assumes a constant annual return. Real investment returns vary and the future value is not guaranteed.

How to Use the Compound Interest Calculator

Enter a starting amount, monthly contribution, annual rate and term to instantly see the future value, total contributions and investment gains. Useful for planning savings and long-term investing.

  1. Enter the Starting Amount (the lump sum you begin with — leave it at 0 if there is none).
  2. Enter the Monthly Contribution (the amount you add every month).
  3. Enter the Annual Rate (your assumed yearly return, as a percentage).
  4. Enter the Term in years. The result recalculates live as you type, so you can compare scenarios without pressing any button.

Worked example: enter a starting amount of ¥1,000,000, ¥0 monthly, an annual rate of 5% and a term of 10 years, and the future value shows as about ¥1,650,000 (an investment gain of roughly ¥650,000). The "Examples (click to try)" chips above load the same scenario in one click.

When This Helps

  • Retirement and savings plans: see roughly how much regular monthly saving could grow into at a given return.
  • Goal setting: adjust the monthly amount to find out what it takes to reach a target.
  • Understanding compounding: compare how the gain changes with different rates and terms for the same contributions.

How It Is Calculated (Monthly Compounding)

This tool uses monthly compounding. The monthly rate is the annual rate divided by 12, and the number of months is years times 12. The future value is starting amount × (1 + monthly rate)^months + monthly contribution × (((1 + monthly rate)^months − 1) ÷ monthly rate). When the annual rate is 0%, the contribution part becomes monthly contribution × months. Total contributed is starting amount + monthly contribution × months, and the investment gain is the future value minus the total contributed.

Frequently Asked Questions

Is my data sent to a server?
No. All compound interest calculations happen entirely in your browser. The amounts and rates you enter are never transmitted to or stored on any external server. You can use this tool with complete confidence.
How is the compounding calculated?
It uses monthly compounding. The monthly rate is the annual rate divided by 12, and gains are reinvested into the balance each month as contributions are added. The future value is computed as principal × (1 + monthly rate)^months + monthly contribution × (((1 + monthly rate)^months − 1) ÷ monthly rate). When the annual rate is 0%, the contribution part is calculated as monthly contribution × months.
Will my money actually grow like this?
No. This is a simple projection that assumes a constant annual return. Real investment returns vary with the market, and the future value is not guaranteed. Treat the result as a rough estimate only.