How ISA Loss Offsetting Works: A 4x Tax Gap vs. a Regular Account

A Korean ISA (Individual Savings Account) adds up every gain and loss inside the account and taxes only the net figure. The Financial Services Commission describes this as offsetting "across products" and "across periods" — deposit interest, bond income, and fund or ETF results held in one account are computed as a single block. The first 2 million won of net gain is tax-free; only the excess is taxed at 9.9% (local income tax included).

Run the same trades through an ordinary brokerage account and the losses never enter the calculation. Each realized gain is withheld at 15.4% and the losses simply stay losses. The reason the tax bills diverge is not the 5.5 percentage-point rate gap — it is the offsetting structure.

The Tax Rate Comes Last

Reverse the order of operations and you get a different answer. Offsetting comes first, not the rate.

1. Offset — sum all taxable gains and losses in the account. A 5 million won gain against a 2 million won loss leaves a 3 million won net gain.
2. Subtract the exemption — 2 million won for the standard type, up to 4 million won for the low-income type (Korea Investment & Securities ISA guide).
3. Apply the rate — 9.9% on whatever remains. On a 3 million won net gain, that means 99,000 won on the taxable 1 million.

The FSC's mention of offsetting "across periods" means results are not cut off year by year. A loss in year three is netted against gains from years one and two. In a regular account there is no way to claw back tax already paid on last year's gains using this year's losses. That timing offset accounts for roughly half of the ISA's benefit.

The Same Trades in Two Accounts

Assume the four results below occurred in one year, and run them through a regular brokerage account and an ISA. The starting point: capital gains on Korea-listed foreign ETFs, bond ETF income, and REIT dividends are all classified as dividend income and taxed at 15.4% (Sisa Journal e).

That is a 4.04x gap. The 2 million won loss does nothing for the regular account and shaves the ISA's gains by the full amount. The 2 million won exemption then comes off on top, pulling the tax base from 6.5 million won down to 2.5 million.

What cuts the tax bill is not the 9.9% rate — it is offsetting, which drags that 2 million won loss into the calculation.

Why Domestic Stock Losses Do Not Offset

This is where the most common misunderstanding lives. The Korea Investment & Securities guide states plainly that because capital gains on Korea-listed stocks are already tax-exempt, a loss in a domestic equity fund will not offset gains from deposits or other funds held in the same ISA. Domestic listed-stock trading results are not taxable income to begin with, so neither the gains nor the losses enter the offsetting math.

The consequence: hold only Korean large caps, take a 3 million won loss, then collect 1 million won of deposit interest in the same account, and the only income being offset is that 1 million won of interest. The net gain sits under the exemption so the tax is zero — but the 3 million won loss did no work at all. The assets where ISA offsetting actually bites are the ones that carried 15.4% withholding in the first place: Korea-listed foreign ETFs, bonds and bond ETFs, REITs, deposits, and RPs. The same income classification explains why the tax on Korea-listed foreign ETFs turns at the 8.33 million won mark.

How the Gap Widens by Net Gain

Below is what the exemption and the rate each contribute, computed by bracket. The regular account applies a flat 15.4%; the ISA applies (net gain − exemption) × 9.9%.

At a 10 million won net gain, the 946,000 won gap between a regular account and a low-income ISA splits in two: 550,000 won from the rate dropping 5.5 points (10m × 5.5%) and 396,000 won from the 4 million won exemption (4m × 9.9%). The smaller the net gain, the more the exemption dominates; the larger it gets, the more the rate does.

Another line that often goes missing: ISA income is not aggregated into Korea's comprehensive financial income tax (Korea Economic Daily). Once interest and dividends clear 20 million won a year, regular-account income moves into progressive rates, while ISA income stops at the 9.9% separate taxation. Assets that already use their own separate-taxation regime — such as the 9% separate tax on REIT dividends — need their own side-by-side calculation.

What to Watch

• Contribution caps — 20 million won a year, 100 million won total per the FSC. Fail to use the room and you limit the pool of assets available to offset
• The three-year minimum holding period — early termination cancels both the exemption and the 9.9% rate, reverting to ordinary taxation
• The income classification of what you hold — a heavy weighting in Korea-listed stocks means offsetting never engages
• Cumulative net profit or loss at the time you close the account — closing in a year that still carries losses forfeits the offsetting advantage
• Proposals to raise the exemption and contribution caps have been debated in the National Assembly for years. The effective date can only be confirmed from the enacted amendment to the Restriction of Special Taxation Act

References

• Financial Services Commission — ISA policy Q&A
• Korea Investment & Securities — ISA product guide
• Sisa Journal e — Korea-listed foreign equity ETFs and ISA tax savings
• Korea Economic Daily — Using an ISA for dividend stocks and foreign equities