JP

Compound Interest Calculator

Three modes: compound interest, savings simulation, and reverse calculation. Compare with simple interest, Rule of 72, and yearly breakdown table.

Last updated: 2026/03/01 Finance
yen
0.1%50%
%
150
years
Final Amount
-
Total Earnings
-
Total Principal
-
vs Simple Interest
-
Rule of 72
Rule of 72: Divide 72 by the annual interest rate to estimate years to double your investment.
Growth Chart
Yearly Breakdown
Year Start Balance Interest End Balance Cumulative Interest vs Simple
yen
yen
0.1%50%
%
150
years
Final Amount
-
Total Earnings
-
Total Contributions
-
Return Rate
-
Rule of 72
Rule of 72: Divide 72 by the annual interest rate to estimate years to double your investment.
Growth Chart
Yearly Breakdown
Year Annual Contribution Interest End Balance Total Contributed Cumulative Interest
yen
%
years
Required Principal
-
* This tool provides approximate simulations. Actual investment results may vary due to market conditions. Investment decisions are your own responsibility. Taxes and fees are not included.

Formulas Used

Compound Interest: A = P × (1 + r/n)(n×t)

A: Final amount, P: Principal, r: Annual rate (decimal), n: Compounds per year, t: Years

Simple Interest: A = P × (1 + r×t)

Simple interest only earns on the principal, so the gap with compound interest widens over time.

Savings: A = PMT × {(1+r)n - 1} / r + PV × (1+r)n

PMT: Monthly contribution, r: Monthly rate, n: Total months, PV: Initial investment

How to Use

1

Select a Mode

Choose from three tabs: Compound Interest for lump-sum investments, Savings Simulation for regular monthly contributions, or Reverse Calculation to find what you need to reach a goal.

2

Enter Your Values

Input your principal (or monthly amount), annual rate, and investment period. Drag the sliders for real-time results. You can also select compounding frequency (monthly/quarterly/semi-annual/annual).

3

View Results

See your final amount, total earnings, and Rule of 72. The chart visually compares compound vs simple interest, and the yearly table shows detailed breakdowns.

About Compound Interest Calculator

A web tool that simulates compound interest - where interest earns interest on itself. Features three modes: lump-sum compound calculation, monthly savings simulation, and reverse calculation from a target amount. Includes visual comparison with simple interest, Rule of 72, and yearly breakdown tables. All calculations run in your browser with no data sent to any server.

Features

  • Three Calculation ModesLump-sum compound interest, monthly savings simulation, and reverse calculation from a target amount.
  • Compound vs Simple Interest ChartVisually compare compound and simple interest growth on the same chart to understand the power of compounding.
  • Rule of 72Automatically calculates how many years it takes to double your money at the given interest rate.
  • Yearly Breakdown TableView annual balance, interest earned, cumulative interest, and difference from simple interest.

Use Cases

  • Simulate 1 million yen at 5% for 20 years
  • Calculate future value of monthly 30,000 yen savings
  • Reverse-calculate monthly savings needed for retirement
  • NISA and iDeCo investment simulations
  • Educational tool for teaching compound interest

FAQ

What is the difference between compound and simple interest?
Simple interest earns only on the original principal, while compound interest earns on both the principal and accumulated interest. Over long periods, this difference becomes substantial.
What is the Rule of 72?
The Rule of 72 is a quick estimation method: divide 72 by the annual interest rate to find approximately how many years it takes to double your money. For example, at 6% it takes about 12 years.
How does compounding frequency affect results?
More frequent compounding (monthly > quarterly > semi-annual > annual) yields slightly higher returns because interest earns interest sooner. However, the practical difference is small.
Are taxes included in the calculation?
No, this tool provides pre-tax estimates. In Japan, investment gains are taxed at 20.315%. NISA accounts offer tax exemptions within certain limits.
How is the savings simulation calculated?
It assumes monthly contributions invested at the end of each month with monthly compounding. Initial investment also grows with monthly compounding.
How is the "Required Rate" reverse calculation done?
It uses Newton's method (numerical analysis) to iteratively approximate the interest rate needed to reach the target amount.
Can I input values in units of 10,000 yen?
Yes, check the "Input in 10,000 yen" checkbox to enter values in man-yen units. Entering "100" will be treated as 1,000,000 yen.
Does this work on mobile devices?
Yes, the tool is fully responsive and works on smartphones, tablets, and desktops. Charts also support touch interaction.
Can I copy the results?
Yes, click "Copy Result" to copy the calculation conditions and results as text to your clipboard.
Is my data saved anywhere?
No, all calculations happen in your browser in real-time. No data is sent to any server.

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