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: 4.75% (capped at KRW 6.37M/month, Jul 2025–Jun 2026)
- Health Insurance: 3.595%
- Long-term Care: 13.14% 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. Deposit & Savings Interest
Both simple and monthly-compound interest are supported:
Simple interest = Principal × annual rate × years Monthly compound maturity = Principal × (1 + rate/12)^months Savings (simple) = Monthly deposit × n × (n+1)/2 × (rate/12) (n: months)
After-tax amounts reflect the 15.4% interest income tax withheld at source — 14% income tax (withholding rate under Article 129 of the Income Tax Act) plus 1.4% local income tax (Local Tax Act; 10% of the income tax) — under standard taxation. Tax-exempt and tax-favored products use different rates. Sources: Income Tax Act, Local Tax Act, NTS Hometax.
8. Jeonse–Monthly Rent Conversion
Monthly rent = (Jeonse amount − monthly-rent deposit) × conversion rate ÷ 12 Jeonse amount = Monthly-rent deposit + (monthly rent × 12 ÷ conversion rate)
The legal cap on the conversion rate is the Bank of Korea base rate plus a presidential-decree margin (currently 2%), per Article 7-2 of the Housing Lease Protection Act and its Enforcement Decree. The default is approximately 4.5% as of 2026 and can be edited as the base rate changes. Rent converted above the cap can be reclaimed by the tenant. Sources: Korea Law Information Center, Bank of Korea base rate announcements.
9. Discount & Margin
Discount rate(%) = (Original − discounted) / original × 100 Discounted price = Original × (1 − rate/100) Margin(%) = (Selling price − cost) / selling price × 100 Markup(%) = (Selling price − cost) / cost × 100
Pure arithmetic with no statutory rates involved. Margin is based on selling price while markup is based on cost. Reverse calculation: original price = discounted price / (1 − rate/100).
10. Dutch Pay Settlement
Three split modes are supported: equal, weighted, and per-item. Equal split divides the total (including tip) by headcount; weighted split assigns share = total × (weight / sum of weights); item mode divides each item's price by the number of people sharing it. When each person's actual payment is entered, the tool computes the difference (amount paid − share) and matches creditors with debtors from largest to smallest, keeping the number of transfers to a minimum. All computation runs in the browser; entered amounts and names are never sent to a server.
11. Stock Return
Net profit = (Sell − buy) × qty − total commission − transaction tax Transaction tax = Sell price × qty × tax rate Return(%) = Net profit / total purchase × 100
Defaults are 0.015% commission (online average, charged on both buy and sell) and 0.18% securities transaction tax (2026; special rural development tax separate) — both editable. The transaction tax applies only to the sell amount, regardless of profit or loss. The break-even sell price where net profit equals zero after fees and tax is computed automatically. Dividend and capital gains taxes are not included. Basis: Securities Transaction Tax Act and its Enforcement Decree, Ministry of Economy and Finance.
12. Freelancer 3.3% Withholding
Withholding = Contract × 3.3% (3% income tax + 0.3% local tax) Net pay = Contract × 96.7% Contract = Net pay ÷ 0.967 (reverse)
The 3% withholding rate on business income comes from Article 129 of the Income Tax Act; the 0.3% local income tax (10% of income tax) from the Local Tax Act. The 3.3% is a prepayment, not the final tax — it is reconciled in the May comprehensive income tax filing with expenses and deductions applied, and refunds may result. Sources: Income Tax Act, Local Tax Act, NTS Hometax.
13. Scientific Calculator
Expression parsing and evaluation use the open-source math.js library with JavaScript IEEE 754 64-bit floating-point arithmetic, displaying results at 14 significant digits. Expressions follow standard operator precedence (parentheses → exponents → multiplication/division → addition/subtraction). Trigonometric functions support DEG/RAD switching, with 180° = π rad. References: math.js documentation, IEEE 754 standard.
14. 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.