Money

Savings & Investment Calculator

Enter your initial investment, monthly contribution, annual return rate, and investment period to simulate your future balance. Track the power of compound interest year by year.

Initial Investment
JPY
Monthly Contribution
JPY
Annual Return Rate
%
Investment Period
years
Final Balance
{{ fmt(result.total) }} JPY
Total Principal Invested
{{ fmt(result.invested) }} JPY
Investment Returns
{{ fmt(result.returns) }} JPY
Total Principal Invested: {{ (result.invested / result.total * 100).toFixed(0) }}% Investment Returns: {{ (result.returns / result.total * 100).toFixed(0) }}% (+{{ fmtRate(result.returnsRate) }}%)
Year Balance Cumulative Principal Investment Returns
{{ row.year }} {{ fmt(row.balance) }} {{ fmt(row.invested) }} {{ fmt(row.returns) }}

Tips

  • This calculation includes the compound interest effect. The longer the period, the greater the impact.
  • A 5% annual return is a rough historical average for indices like the S&P 500 and global stock index funds. It does not guarantee future results.
  • The longer your investment period, the greater the benefit of compounding. Starting early is the single most important factor.
  • Using tax-advantaged accounts (such as NISA in Japan, ISA in the UK, or 401(k)/IRA in the US) can make your investment returns and dividends tax-free.
  • Even with zero initial investment, regular contributions alone can build significant wealth. A larger initial investment lets you benefit from compounding sooner.

Frequently Asked Questions

Typical benchmarks: savings accounts 0.1–0.5%, government bonds 1–3%, index fund investing 5–7% (based on historical performance). Future returns are not guaranteed.

Yes, you can use this as a reference. Note that this tool calculates pre-tax figures. In tax-advantaged accounts, investment returns are typically tax-free, so your actual outcome may be even better.

A larger initial investment benefits from compounding immediately, making it highly effective over the long term. The optimal strategy is to invest a lump sum upfront while also making regular monthly contributions.

This tool calculates in nominal terms. To assess real purchasing power, subtract the expected inflation rate (typically 1–2% per year) from your expected return rate and use that as the real return rate.

This tool assumes a fixed monthly contribution. To model a change, calculate each period separately (before and after the change) and combine the results.

Side Note — Why Compound Interest Was Called "the Eighth Wonder of the World"

The quote "compound interest is the greatest invention of mankind" is often attributed to Einstein, though its true origin is uncertain. Regardless, it is cited worldwide as a concise expression of the extraordinary power of compounding.

Investing $300 per month at a 5% annual return for 20 years turns a $72,000 principal into approximately $123,000. Over 30 years, it grows to roughly $250,000 — with investment returns adding more than $120,000 on top.

Compound interest works against you just as powerfully with high-interest debt. Revolving credit at high rates means you are continuously paying compound interest as a borrower. In wealth building, make compounding work for you.

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