Model a lump sum and monthly contributions together. The result separates tax, compounding, money-weighted return, and inflation so an advertised rate is not mistaken for spendable value.
Estimated after-tax maturity value
The final comparison is a counterfactual. It is not a loss estimate or a recommendation.
Contributions and gross balance over time
| Month | Contributed | Gross interest | Gross balance |
|---|
Why this calculator shows more than maturity value
You can combine an initial deposit with recurring monthly contributions. Alongside after-tax cash, the tool reports the average holding period and an annualized money-weighted return based on the actual contribution timing.
Inflation is shown separately from tax. This keeps the advertised nominal rate, after-tax return, and present purchasing power from collapsing into one misleading percentage.
Worked example: KRW 500,000 a month for 12 months at 3.5%
Inputs: no lump sum, KRW 500,000 at the start of each month for 12 months, a 3.5% nominal annual rate, 15.4% tax, and 0% inflation. It is tempting to expect 3.5% of the KRW 6,000,000 total, or KRW 210,000. But the first deposit stays 12 months and the last only one, so the average holding period is 6.5 months and gross interest is a little over half of KRW 210,000.
| Item | Simple | Monthly compound |
|---|---|---|
| Gross interest | 113,750 KRW | 114,975 KRW |
| Estimated tax (15.4%) | 17,518 KRW | 17,706 KRW |
| After-tax maturity value | 6,096,232 KRW | 6,097,269 KRW |
| After-tax annualized return | 2.974% | 3.006% |
Over 12 months, monthly compounding adds KRW 1,037 after tax compared with simple interest. The after-tax annualized return annualizes net interest over the time the money was actually held. Figures were recalculated with this page’s calculation engine; real products can differ because of day-count, payment-date, and bonus-rate rules.
Method and limitations
Simple interest is calculated for the number of months each contribution is held. Monthly compounding divides the nominal annual rate by 12. The selected tax rate is applied once to gross interest at maturity.
Real products may use daily accrual, different payment dates, early-withdrawal rates, bonus-rate conditions, fees, and won-rounding rules. Korean comprehensive financial-income taxation is not calculated here. Check the product disclosure before making a financial decision.
With no lump sum and a single end-of-month contribution over one month, the money arrives on the maturity date, so the holding period is zero and an annualized return is undefined. The calculator shows ‘N/A (zero holding period)’. The previous version showed ‘-100%’ for this input (checked October 4, 2026); this version fixes it.
Korean tax basis
The 15.4% standard example combines 14% Korean income tax with local income tax equal to 10% of that income tax. Separate comprehensive taxation may apply when annual financial income exceeds KRW 20 million.
The tax rates and the KRW 20 million threshold are as of October 8, 2026, the date the official sources below were checked, and can change. Foreign-bond tax cautions, including Brazilian government bonds, are collected on the bond calculator page.
Official Korean sources: Income Tax Act Article 129 · Income Tax Act Article 14 · Local Tax Act Article 103-13