Tax Audit Assessment and Penalty Calculator

What is the tax audit assessment and penalty calculator?

When a Korean tax audit finds omitted sales or under-reported income, the bill is never just the tax you should have paid.
On top of the additional principal tax, you also owe a filing penalty for not reporting properly and a late-payment penalty for not paying on time.
This calculator adds all three together under the 2026 Framework Act on National Taxes so you can see the total assessment and the effective penalty burden relative to the principal tax.

It also shows how much you save by filing a voluntary amended return before the audit begins.
Under Article 48 the filing penalty can be reduced by up to 90%, so the calculator compares the ‘caught by audit’ case with the ‘voluntary amendment’ case side by side.

Korea-based, 2026 rules. This calculator reflects the Framework Act on National Taxes Articles 47-2 (non-filing penalty), 47-3 (under-reporting penalty), 47-4 (late-payment penalty) and 48 (penalty reduction), and Enforcement Decree Article 27-4 (interest rate, in force from 2026-02-27). It is a reference estimate for Korean national taxes and does not replace professional filing advice.

How the assessment is built

1. Additional principal tax

The additional principal tax is the tax you should have paid on the omitted tax base.
If you hid sales or overstated expenses to cut your tax, that shortfall is the additional principal tax.
Enter the extra principal tax the audit assesses (income tax, corporate tax, VAT, etc.).

2. Filing penalty (non-filing or under-reporting)

If you never filed, a non-filing penalty applies (Article 47-2); if you filed but reported too little, an under-reporting penalty applies (Article 47-3).
The rate depends on whether there was a fraudulent act such as a double set of books or false evidence.

Penalty rates (on the principal tax)

• Non-filing: 20% general / 40% fraud / 60% offshore fraud
• Under-reporting: 10% general / 40% fraud / 60% offshore fraud
• Double-entry / corporate non-filing: the greater of (principal tax × rate) and (revenue × 0.07%, or 0.14% for fraud)

💡 A simple under-report is 10%, but a fraudulent one is 40% — four times higher.
So whether the act counts as ‘fraud’ is the single biggest driver of the assessment.

3. Late-payment penalty

The late-payment penalty is interest-like and accrues for the days the tax went unpaid after the statutory due date (Article 47-4).
For 2026 the rate is 0.022% per day (22 per 100,000, about 8.030% per year).

Late-payment formula

unpaid principal × days overdue × 0.022%

The days count from the day after the statutory due date to the assessment notice (or voluntary payment) date.
Unlike the filing penalty, the late-payment penalty is not reduced by a voluntary amendment, so paying the principal sooner always helps.

Voluntary amendment reduction (Article 48)

If you correct the return yourself before you are told an audit or reassessment is coming, the filing penalty is reduced.
But a return filed when you already know a reassessment is coming — as in an audit — is excluded from the reduction.
The reduction applies only to the filing penalty; the late-payment penalty is never reduced.

Reduction by elapsed time

After the statutory deadline	Amended (under-report)	Late filing (non-filing)
Within 1 month	90%	50%
1–3 months	75%	30%
3–6 months	50%	20%
6 months–1 year	30%	-
1–1.5 years	20%	-
1.5–2 years	10%	-

For example, if the under-reported principal is KRW 10 million and the under-reporting penalty is KRW 1 million, filing a voluntary amendment within 1 month cuts KRW 900,000 and leaves only KRW 100,000.
The sooner you file, the higher the reduction, so acting before the audit notice is a real advantage.

What changed in 2026 · after the notice

From 2026, Article 47-4 was amended so that if you still fail to pay after the assessment notice, a new 0.67% per month (67 per 10,000) late-payment penalty applies (Enforcement Decree Article 27-4, added 2026-02-27).
On top of that, failing to pay by the designated deadline adds a one-time 3% of the unpaid amount (Article 47-4(1)3).

This calculator’s main result is the assessment at the notice date (principal + filing penalty + the day-based late-payment penalty before the notice).
Because the 3% plus 0.67% per month is added if you keep the tax unpaid after the notice, pay as soon as you receive the notice.
A fraudulent act also extends the assessment statute of limitations to 10 years and can lead to criminal prosecution for tax evasion, so take special care.

Frequently asked questions

Q. How do I know the additional principal tax?

A. The pre-assessment notice or the decision/correction notice from the tax office shows the extra principal tax due.
Before any notice, you can estimate it by multiplying the omitted income by your marginal tax rate.

Q. How is fraud distinguished from a simple under-report?

A. A double set of books, false evidence or contracts, hiding assets, or deliberately omitting sales are fraudulent acts.
A simple calculation error or a missed deduction is treated as an ordinary under-report at 10% (20% for non-filing).

Q. Is local income tax added too?

A. When income or corporate tax is assessed, local income tax of about 10% of it is usually charged as well.
This calculator covers the national principal and penalties, so treat local income tax as an additional amount.

Q. Can I still get the reduction after an audit notice?

A. A return filed when you already know an audit or reassessment is coming is excluded from the Article 48 reduction.
To qualify, you must file the voluntary amendment or late return before the audit notice.

Q. Will the result exactly match the official notice?

A. This is an estimating tool based on the main penalty formulas.
The actual notice can differ with tax-specific rules, the mix of fraudulent and ordinary amounts, local income tax, and other penalties, so have a tax professional review it.

Tips to reduce the assessment

Amend before the notice: a voluntary amended return filed before the audit notice can cut the filing penalty by up to 90%.
Pay the principal early: the late-payment penalty is not reduced, so pay the principal as soon as possible to limit the interest-like charge.
Rebut fraud: proving it was a simple mistake applies 10% (20% for non-filing) instead of 40%, greatly lowering the bill.
Organize evidence: documents supporting expenses or input VAT can shrink the additional principal tax itself.
Consider an appeal: if the facts or the law are disputable, review a pre-assessment review, objection, or tax tribunal appeal.

Estimate your tax audit assessment now

Add the principal and penalties in one place, and compare how much a voluntary amendment saves.

Calculated on 2026 Framework Act on National Taxes rules; confirm with a tax professional before you actually file or pay.

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