Calculation Methodology
This page discloses the core formulas and data sources used by each Calc Tani tool, so users can verify results independently. All numbers reflect 2026 regulations unless noted.
1. Salary & Four Major Insurances
2026 employee contribution rates:
- National Pension: 4.5% (capped at KRW 5.9M/month)
- Health Insurance: 3.545%
- Long-term Care: 12.95% of health premium
- Employment Insurance: 0.9%
Income tax follows the NTS "Simplified Tax Table (2026)"; local income tax is 10% of income tax. Sources: NTS Hometax, National Health Insurance Service, National Pension Service.
2. Severance & Retirement Tax
Statutory severance formula:
Severance = Daily avg wage × 30 × (service days / 365) Avg wage = Total pay (last 3 months) / calendar days
Retirement tax: two-stage deduction (tenure + annualized). 2026 tenure brackets: KRW 1M/yr for ≤5, 2M/yr for 5–10, 2.5M/yr for 10–20, 3M/yr for >20. Sources: Labor Standards Act Art. 34, Employee Retirement Benefit Security Act, Income Tax Act Enforcement Decree.
3. Loan Amortization
Fixed-payment formula for equal principal and interest:
M = P × [r(1+r)^n] / [(1+r)^n − 1] (P: principal, r: monthly rate, n: months)
Equal-principal: P/n principal per month, interest = balance × r. Mixed loans pay interest-only initially. Prepayment penalties vary by bank (typically 0.7–1.4% of balance, sliding over 3 years).
4. VAT
Standard rate 10%. Supply price × 10% = VAT; total = supply + VAT. Reverse: supply = total / 1.1. Simplified taxpayers apply 1.5–4% by industry (2024 reform, unchanged through 2026).
5. Exchange Rate
Base rates come from the Korea Eximbank official API (posted 11am KST on business days). Cash-buying, cash-selling, remittance-send, and remittance-receive apply standard spreads. Source: Korea Eximbank exchange rate feed.
6. BMI, Korean Age, Ovulation
- BMI — weight(kg) / height(m)². Interpretation follows WHO Asia-Pacific (23/25) with Western (25/30) shown side-by-side.
- Korean Age — Unified Age Act (2023). Calculated from birthdate vs. today; year-age and traditional Korean age shown alongside.
- Ovulation — Standard 28-day cycle: next period − 14 days, adjusted by user cycle length. Informational, not medical advice.
7. Limitations & Disclaimer
Calc Tani results are for general reference. For tax, legal, or medical decisions, consult official authorities (NTS) or licensed professionals (CPA, tax attorney, physician). Calculators do not substitute for medical diagnosis.