₩20M of financial income is a door, not a cliff — why crossing the line by ₩1M costs ₩264,000 in tax

₩20M a year in interest and dividends is the famous line where aggregate taxation of financial income begins. Cross it and a tax bomb goes off, people say, so some break deposits and sell dividend stocks every December. But run the numbers and something is odd. For someone whose other income has a ₩50M tax base, crossing the line by ₩1M with ₩21M of financial income adds ₩264,000 in tax. Small for a bomb. The real weight of this line lies somewhere other than tax.

Financial income means interest plus dividends: deposit interest, bond interest and stock dividends all count. One thing up front: the threshold is counted per person. Not per household or per couple, but on the financial income in your own name.

Step one, the world up to ₩20M. Ordinary interest and dividends, such as deposit interest and stock dividends, are already taxed when you receive them: 14% income tax plus local income tax, 15.4% (Income Tax Act Art. 129 para. 1). Interest on private loans between individuals (non-business lending) is an exception at 25%. If the annual total is ₩20M or less, that is the end. It is called separate taxation (Art. 14 para. 3). Nothing to file in May, nothing combined with other income. So ₩20M is not where tax begins. Tax was being withheld from the first won. ₩20M is where the method of calculation changes.

Step two, above the line. The bomb theory's first misconception is that the whole amount meets high rates once you cross ₩20M. It is true that all financial income then enters aggregate income. But a separate article sets the tax (Art. 62): the larger of ₩20M at 14% plus the excess combined with other income under progressive rates, and all financial income at 14% plus the tax on other income. With ₩21M of financial income, only ₩1M meets progressive rates.

The difference from the 15.4% that would have been withheld anyway is ₩110,000 per ₩1M. It is not a cliff where the whole amount jumps the moment you cross; it is a slope where only the excess switches to your own bracket's rate. The more other income you have, the steeper the slope, but the structure is a slope, not a cliff.

Step three, comparative taxation. There is a hope in the other direction too: if financial income is your only income, might the combined rate be low enough to give back some of the 14% withheld? Run it and it does come out that way. With no other income and ₩30M of financial income, the aggregate calculation gives ₩3.4M of income tax, while ₩4.2M was withheld. But the law sets the tax at the larger of the two (Art. 62, where the comparison is with all financial income taxed at the withholding rate). However small the combined figure, it never drops below the withheld amount. This person's tax is ₩4.2M, or ₩4.62M with local tax. The upward door exists; the downward door is locked. There is no reason in this structure to opt into aggregate taxation hoping for a refund.

Step four, the real weight. Two things outside tax hang on this line. First, health-insurance dependent status. This is not about someone like Case A with large other income; it is about someone covered as a dependent of an employee, paying no premiums. When combined annual interest, dividend, wage, pension, business and other income exceeds ₩20M, that status ends and the person becomes a regional subscriber (financial income of ₩10M or less is left out of the sum, and for a married couple both spouses must meet the test). From then on premiums attach to assets as well as income. For a former dependent, the new premiums can weigh more than the added tax. That is what the "bomb" mostly refers to. Two systems use the same ₩20M figure under different rules. Second, an aggregate-income filing duty arises: you must file in May together with other income.

There is also a timing trap. When maturities cluster, interest clusters in one year. Receive three years of interest at once at maturity and it all counts as that year's financial income, because the threshold is by year received. That is why merely staggering deposit maturities changes how the threshold is managed.

Now count your own number. Bank deposit interest, savings-bank and brokerage cash-balance interest, domestic and foreign stock dividends, bond interest and fund distributions all go in one basket. Hometax shows last year's financial-income data, and each institution's app shows annual income. Far below ₩20M, this is not your concern yet. Near the threshold, it is time to look at maturity placement and dividend years, and if you are a dependent, check the status rules before the tax. Well above it, the May filing duty comes first; miss it and penalties apply. Within one household, each person has their own ₩20M.

Two things to do now. Look up this year's total financial income and hold it against ₩20M, and if you are near the line, put maturities and dividends on a calendar by year. Tax-exempt and special separately-taxed products, the separate-taxation option for dividends from high-dividend listed companies received from 2026 (Restriction of Special Taxation Act Art. 104-27: on application, kept out of aggregate income and taxed at its own rates, 14% up to ₩20M), foreign tax on overseas dividends, and the detailed dependent rules are each separate. For large amounts, work it through with a tax adviser. The ₩20M line was a door, not a cliff.