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 & Employee Insurance Deductions
2026 employee contribution rates:
- National Pension: 2026 employee rate 4.75% (5% in 2027). The KRW 410K–6.59M income band applies Jul 2026–Jun 2027; the income base is truncated below KRW 1,000 and the premium below KRW 10
- Health Insurance: 3.595% (2026 employee premium floor/cap: KRW 10,080–4,591,740)
- Long-term Care: health premium × (0.9448% ÷ 7.19%), displayed as approximately 13.14%. The official ratio is used before truncating below KRW 10, not the rounded display rate
- Employment Insurance: employee 0.9%; employer 1.15–1.75% by size and priority-support status
- Workers' compensation: employer-paid at an industry-specific rate (excluded from employee deductions and this calculator total)
Income tax uses the 100% standard amount from the earned-income withholding table in Income Tax Act Enforcement Decree, Appendix 2, effective for withholding from March 1, 2026. Monthly taxable pay below KRW 10M uses all 588 official bands and eligible-family columns; the appendix formulas cover higher pay, more than 11 family members, and credits for children aged 8–20. Actual withholding can use an 80%, 100%, or 120% election and differs with employer processing and year-end settlement. Local income tax is 10% of the calculated income tax. The calculator does not model age, residence, or employment-status coverage exceptions. Rate references: National Pension, health insurance, premium limits, long-term care, and employment insurance.
2. Severance & Retirement Tax
Compare the deductions, annualization and worked examples with the NTS retirement-income tax calculation guide.
Statutory severance formula:
Severance = Daily avg wage × 30 × (service days / 365) Avg wage = Total pay (last 3 months) / calendar days
Retirement tax uses a two-stage deduction (tenure + annualized pay). 2026 tenure brackets are KRW 1M/yr for ≤5 years, 2M/yr for 5–10, 2.5M/yr for 10–20, and 3M/yr for >20. The statutory severance formula follows the Ministry of Employment and Labor calculation guide.
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 repayment uses P/n principal each month and interest = remaining balance × r. Mixed loans pay interest-only initially and then switch to fixed payments. Prepayment charges depend on the lender, product, and origination date, so check the actual agreement.
The refinancing comparison models the same current debt under each loan’s contract interest rate. Total cost equals interest plus entered switching costs. Financed costs are included in new principal and its interest, without adding the same fee again to total outflow. Nominal sums cover each maturity and exclude time value, floating rates and future prepayments.
4. VAT
The standard VAT rate for general taxpayers is 10%. VAT = supply price × 10%; total = supply + VAT; reverse supply price = total ÷ 1.1. Simplified-taxpayer liability is not simply 10% of sales: industry value-added ratios and credits also apply. See the Korea NTS VAT guide for eligibility and calculation rules.
5. Exchange Rate
Base rates come from the Korea Eximbank official API on publication days. Cash-buying, cash-selling, remittance-send, and remittance-receive values apply the published spreads. Source: the Korea Eximbank exchange-rate Open API.
6. BMI, Korean Age, Ovulation
Adult categories follow the WHO obesity and overweight guidance. Pediatric growth assessment is separate and is not calculated by this tool.
- BMI — weight(kg) / height(m)². This calculator is adult-only and uses WHO international cut-offs (overweight 25, obesity 30). Ages 5–19 require sex- and age-specific WHO BMI-for-age growth references interpreted with a clinician. BMI is a screening measure, not a diagnosis.
- Korean Age — Unified Age Act (2023). Calculated from birthdate vs. today; year-age and traditional Korean age shown alongside.
- Ovulation — Estimates the next period from the entered average cycle and subtracts 14 days for a central ovulation estimate. Actual ovulation can vary each cycle; do not use this result for contraception, pregnancy diagnosis, or medical decisions.
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)
The default after-tax amount assumes standard 15.4% withholding: 14% income tax under Article 129 of the Income Tax Act, plus 1.4% local income tax (10% of income tax). The calculator's 1.4% option is only a comparison scenario for certain existing mutual-finance member deposits where that rate applies; verify the rate for the opening date and product terms. Use 0% only within actual eligibility and limits for tax-free comprehensive savings or another exemption. For the age-based tax-free comprehensive savings category, being 65 or older is not enough by itself: the person must receive the Basic Pension. Check the other statutory eligibility categories separately. An ISA is tax-free only up to its statutory allowance; income above that allowance is taxed separately.
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)
When all or part of an existing jeonse deposit is converted to rent, the statutory cap is the lower of the Bank of Korea base rate + the 2-point decree margin or 10% a year, under Article 7-2 of the Housing Lease Protection Act and its Enforcement Decree. The current cap is 5.00%, reflecting the 3.00% base rate from August 27, 2026. The calculator enforces this only for Jeonse→Monthly; Monthly→Jeonse is labeled as a comparison-only reverse calculation. Applicability, contract validity, and overpayment disputes require case-specific review. Verify the current rate in the Bank of Korea base-rate history.
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
These are pure arithmetic calculations with no statutory rates. The screen separately supports original-plus-discounted price to find the discount rate, original price plus rate to find the discounted price, and cost plus selling price to find margin and markup. Margin is based on selling price; markup is based on cost.
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 to reduce redundant transfers. This greedy approach does not guarantee the mathematically smallest possible transfer count. 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 (an editable example charged on both buy and sell) and a combined 0.20% securities transaction tax for 2026 — KOSPI: 0.05% tax + 0.15% special rural development tax; KOSDAQ: 0.20%. Transaction tax applies to the sell amount regardless of profit or loss. Break-even is the lowest whole-won price with net profit at least zero after fees and tax; it does not guarantee a valid exchange tick or execution. Dividend and capital-gains taxes are excluded. Rate basis: Article 5 of the Securities Transaction Tax Enforcement Decree.
12. Freelancer 3.3% Withholding
Income tax = ⌊Contract × 3 / 100⌋ Local income tax = ⌊Income tax / 10⌋ Net pay = Contract − income tax − local income tax Reverse = smallest whole-won contract whose truncated result meets the target net pay
The 3% business-income withholding rate comes from Article 129 of the Income Tax Act; the 0.3% local income tax is 10% of that amount. The 3.3% is a prepayment, not the final tax. The following May filing reconciles expenses and deductions, so extra tax or a refund may result.
13. Scientific Calculator
Expression parsing and evaluation use open-source math.js and JavaScript IEEE 754 64-bit floating-point arithmetic, with results displayed to 14 significant digits. Expressions follow standard operator precedence. Trigonometric functions support DEG/RAD switching, where 180° = π rad.
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.