🏦 Compound Interest, Savings After-Tax (Japan 20.315%, NISA, Korea 15.4%) & Loan Repayment Calculator

Calculate after-tax interest on monthly savings or lump-sum deposits using Japan (20.315% / NISA 0%) or Korea (15.4% / 9.5% / 0%) tax rates, or switch to Loan Repayment to compare Equal Total Payment vs Equal Principal.

✅ Official Rule Verification Date: 2026-11-04 (2026/2027) 🏛️ Official Government Sources: National Tax Service of Korea (국세청) · National Tax Agency of Japan (国税庁)

🔒 100% Client-Side Privacy: Your salary and income inputs are calculated strictly inside your browser and never sent to any server.

📌 Savings, Compound & Loan Calculator — 핵심 요약 (Key Takeaways)

Interest Tax Rates: Japan withholds 20.315% on bank interest and capital gains (0% in NISA); South Korea withholds 15.4% (14% income + 1.4% local tax).
Supports both Monthly Installment Savings (적금 / 積立) and Lump-Sum Deposits (예금 / 定期預金) with compound or simple interest.
Includes a full Loan Amortization mode comparing Equal Principal & Interest (원리금균등 / 元利均等) vs Equal Principal (원금균등 / 元金均等).

📖 How the Formula Works & Official Terminology (with Local Terms)

Savings & Compound Mode: Computes gross interest, statutory withholding tax by selected country rule, and final net maturity payout.
Loan Repayment Mode: Computes first month payment, final month payment, total interest paid, and total repayment across all 3 amortization methods.

📊 Sample Salary & Tax Reference Table by Income Bracket

Plan (4.0% APR, 12 Months)	Total Principal	Gross Interest	🇯🇵 Japan After-Tax (20.315%)	🇰🇷 Korea After-Tax (15.4%)	Tax-Free / NISA
Monthly Savings (100k × 12 mo)	1,200,000	26,000	1,220,718 (Tax 5,282)	1,221,996 (Tax 4,004)	1,226,000
Lump-Sum Deposit (1.2M × 1 yr)	1,200,000	48,000	1,238,249 (Tax 9,751)	1,240,608 (Tax 7,392)	1,248,000
Loan 1.2M (Equal P&I, 12 mo)	Principal 1.2M	Interest 26,160	Monthly Payment: 102,180	—	—
Loan 1.2M (Equal Principal, 12m)	Principal 1.2M	Interest 26,000	Month 1: 104,000 → Mo 12: 100,333	—	—

❓ Frequently Asked Questions (FAQ)

Q. Why does a 12-month installment savings plan earn roughly half the interest of a 12-month lump-sum deposit at the same APR?

Because only your Month 1 deposit stays in the bank for all 12 months; Month 6 stays for 7 months, and Month 12 stays for just 1 month (average deposit duration is 6.5/12 of a year).

Q. Which loan repayment method minimizes total interest?

Equal Principal Repayment (원금균등 / 元金均等) pays down the principal faster in early months, resulting in the lowest total interest cost over the life of the loan.

🏛️ Official Government Citations & Revision Schedule

National Tax Service of Korea (국세청) — Wage Withholding Table, Progressive Tax Brackets, Foreigner 19% Flat Rate & Dividend Tax
Official Rule Verification Date: 2026-11-04 | 매년 1~2월 세법 시행령 개정 및 3월 간이세액표 반영 / 5월 종합소득세 (1·2·3·5월 점검)
National Tax Agency of Japan (国税庁) — 2026/2027 Withholding Tax Tables & Computer Formula (源泉徴収税額表)
Official Rule Verification Date: 2026-11-04 | 매년 9월 차년도 표 공개 → 1월 급여부터 적용 (1월·9월 중점 점검)

→ 📅 Tax Revision Calendar & Sources

⚠️ ⚠️ Important: Figures May Differ — Always Verify with Your Tax Office or Relevant Agency

Calculations, schedules, and guides on HonsMoney are approximate estimates and may differ from your actual tax or insurance amounts due to individual circumstances, municipal rules, visa status, or law revisions. This site does not provide tax or legal advice. Always verify final figures and deadlines directly with your local Tax Office (税務署 / NTS), Municipal Office (役所), Pension Service (年金事務所 / NPS), or your employer’s HR/payroll department.

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