iDeCo / NISA Accumulation Simulator (Japan)
Simulate long-term compound growth of iDeCo / NISA / taxable brokerage accounts including Japanese tax advantages.
NISA つみたて枠 月10万円、iDeCo 月5,000〜68,000円等（拠出上限は職業により異なります）。
全世界株式 過去平均 3〜7% 目安。将来を保証するものではありません。
積立額・年利・年数を入力して「シミュレーション」を押してください。
本ツールは月次複利での概算です。実際の運用成績、退職所得控除、社会保険料の変動、拠出上限は考慮していません。将来の元本を保証するものではありません。
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Overview
Simulate long-term monthly contributions to iDeCo, NISA, and taxable brokerage accounts side by side. The tool applies monthly compounding, models Japan's tax advantages (iDeCo income deduction and NISA tax-free growth), and shows the estimated net take-home value at withdrawal.
How to use
- Enter the monthly contribution amount in yen.
- Enter the expected annual return in percent (e.g. 5% for a global equity index).
- Enter how many years you will keep contributing.
- Pick your marginal income tax bracket (used only for the iDeCo income deduction calculation).
- Click Simulate to compare iDeCo / NISA / taxable outcomes.
Examples
-
¥30,000 / month for 20 years at 5%
Input: Monthly ¥30,000, 5% annual return, 20 years, 20% tax bracket.
Output: Ending balance ≈ ¥12.3M. NISA nets the full balance; taxable loses ~¥1M to 20.315% capital gains; iDeCo adds ~¥1.8M in income-tax savings across the period.
Notes
- Uses monthly compounding at the start of each month.
- Assumes returns are realised at withdrawal for the taxable account and 20.315% capital-gains tax is applied to gains.
- iDeCo income deduction estimate assumes marginal income tax rate + 10% flat resident tax.
- Contribution limits (iDeCo caps by occupation, NISA 1.8M lifetime cap) are not enforced; adjust inputs to stay within your allowance.
FAQ
- Which account is best?
- For most salaried workers, iDeCo tends to win on total net value thanks to the income deduction, but funds are locked until age 60. NISA is a strong runner-up with full liquidity.
- Are returns realistic?
- Historical global equity averages are 3–7% real. Adjust inputs based on your own assumptions; the tool never guarantees results.
- Where is my data stored?
- Nowhere. The simulation runs entirely in your browser.
Embed this tool
Paste this iframe into any HTML page to embed iDeCo / NISA Accumulation Simulator (Japan) on your site:
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