What is P.N.D.51, and why SMEs need to watch out for P.N.D.51 fines in 2026
P.N.D.51 is the corporate income tax return for the first half of the accounting period (half-year tax). The law requires juristic persons, including limited companies, public limited companies and juristic partnerships, to prepare and file it with the Revenue Department within 2 months from the last day of the first 6 months of the accounting period. If a business neglects it, does not file, files late, or underestimates its net profit beyond the threshold set by law, it faces the burden of P.N.D.51 fines: both a criminal fine and a tax surcharge of up to 20% of the tax underpaid.
Under Section 67 bis of the Revenue Code, the half-year tax is calculated in one of 2 main ways:
- Method 1: Estimate the net profit for the whole year (Section 67 bis (1)) This is the mandatory method for ordinary companies and SMEs. The business estimates its net profit for all 12 months, then takes half (divides by 2) and calculates the tax at the rate the business is entitled to.
- Method 2: Calculate from the actual net profit of the first 6 months (Section 67 bis (2)) This applies only to companies listed on the Stock Exchange of Thailand, financial institutions, commercial banks, or companies approved by the Director-General of the Revenue Department.
For the 2026 accounting period of most businesses whose period follows the calendar year (1 January – 31 December 2026), the filing and payment deadlines are as follows:
- Paper filing: by 31 August 2026 at the area revenue branch office
- Online filing (e-Filing): an extension of 8 more days, ending on 8 September 2026
Why filing the half-year tax matters, and what the tax penalties are
Filing the P.N.D.51 is an advance payment of tax that is credited against the year-end tax (P.N.D.50). If the business does not file, or files incorrectly, the Revenue Code sets 3 main penalties: a criminal fine, a monthly surcharge, and a surcharge for underestimating by more than 20%, as detailed below.
- Criminal fine (Section 35): If the P.N.D.51 is not filed by the deadline, the maximum fine is 2,000 baht, according to the Revenue Department's settlement fine regulations:
- Filed no more than 7 days late: fine of 1,000 baht
- Filed more than 7 days late: fine of 2,000 baht
- Surcharge for late payment of tax (Section 27): Charged at 1.5% per month (a fraction of a month is rounded up to 1 full month) on the tax payable, counted from the day after the filing deadline until the date of full payment (excluding penalties)
- Surcharge for underestimating net profit by more than 20% (Section 67 ter): If the estimated net profit for the whole year falls short of the actual net profit in the year-end financial statements by more than 20% without reasonable cause, the business must pay a surcharge of 20% of the tax underpaid
| Type of offence | Legal basis | Fine and surcharge rate |
|---|---|---|
| P.N.D.51 filed late, no more than 7 days | Section 35 | Criminal fine of 1,000 baht |
| P.N.D.51 filed late, more than 7 days | Section 35 | Criminal fine of 2,000 baht |
| Tax outstanding because of late filing | Section 27 | Surcharge of 1.5% per month (a fraction of a month counts as 1 month) on the outstanding tax |
| Net profit estimated more than 20% below the actual figure without reasonable cause | Section 67 ter | Surcharge of 20% of the half-year tax underpaid |
How to calculate P.N.D.51 fines and the 20% surcharge, with a real example from our accounting specialists in Chiang Mai
Under Section 67 bis (1) of the Revenue Code, an ordinary business must estimate the net profit it expects to earn over the whole accounting year, calculate the tax for the full year from that figure, and divide by two to pay the half-year tax. If the business lacks careful tax planning and the estimate ends up more than 20% below the actual profit, the Revenue Department will calculate the 20% surcharge under Section 67 ter as soon as the P.N.D.50 is filed at year-end.
Corporate income tax rates for SMEs (paid-up registered capital not exceeding 5 million baht and annual revenue not exceeding 30 million baht)
- Net profit 0 – 300,000 baht: tax rate 0% (exempt)
- Net profit 300,001 – 3,000,000 baht: tax rate 15%
- Net profit above 3,000,000 baht: tax rate 20%
Case study: a cafe and bakery in the Nimmanhaemin area, Chiang Mai
Cafe Nimman Co., Ltd. (eligible for the SME tax benefits) has an accounting period of 1 January – 31 December 2026.
- Filing the P.N.D.51 (August 2026):
- The business estimates its net profit for the whole of 2026 at 1,000,000 baht
- Full-year tax on the estimate: (300,000 × 0%) + (700,000 × 15%) = 105,000 baht
- Half-year tax payable (one half): 105,000 ÷ 2 = 52,500 baht
- Actual results at the end of 2026 (year-end financial statements, P.N.D.50):
- Q4 is Chiang Mai's tourist season and sales surge, giving an actual net profit for the year of 2,000,000 baht
- Actual full-year tax: (300,000 × 0%) + (1,700,000 × 15%) = 255,000 baht
- The half-year tax that should actually have been paid (one half): 255,000 ÷ 2 = 127,500 baht
- Checking the 20% shortfall condition (Section 67 ter):
- Actual net profit = 2,000,000 baht
- Safe threshold of 80% of actual profit = 2,000,000 × 80% = 1,600,000 baht (the estimate should not be lower than this)
- The business estimated only 1,000,000 baht, a shortfall of 1,000,000 baht (50% of the actual profit, which exceeds the 20% threshold)
- Calculating the 20% surcharge:
- Actual half-year tax: 127,500 baht
- Half-year tax already paid on the P.N.D.51: 52,500 baht
- Half-year tax underpaid: 127,500 - 52,500 = 75,000 baht
- 20% surcharge payable: 75,000 × 20% = 15,000 baht
The result is that the business must pay an extra 15,000 baht surcharge to the Revenue Department together with its year-end P.N.D.50 filing, and this surcharge cannot be recorded as a tax-deductible expense.
The 4 most common P.N.D.51 mistakes SMEs make
To keep cash flow from draining away in fines and surcharges, business owners should check the 4 key weak points that occur most often, as follows.
1. Estimating profit too low because the high season was left out
Service, retail, restaurant or tourism businesses in economic areas such as Chiang Mai often have uneven income. The first half of the year (January - June) is the low season, when sales may slow down. If the owner simply doubles the results of the first 6 months as the full-year estimate without factoring in the sales trend of Q4 (high season), the estimated net profit will fall below the actual figure by more than the 20% threshold.
2. Assuming that a loss-making company, or one in its first year, does not need to file
The law requires every juristic person with a normal 12-month accounting period to file a P.N.D.51, even if the first half of the year shows a loss or the full-year net profit is expected to be 0 baht. The return must still be filed showing 0 baht. Failing to file is immediately subject to a criminal fine of up to 2,000 baht. (The only exception is a newly registered juristic person in its first year whose first accounting period is no longer than 6 months, which has no duty to file a P.N.D.51 for that first period.)
3. Overlooking the "reasonable cause" condition (safe harbor rule)
The Revenue Department has set out what counts as "reasonable cause" in Revenue Department Instruction No. Tor.Por. 81/2542. It provides that if a juristic person prepares its net profit estimate and pays half-year income tax of not less than half of the corporate income tax filed (P.N.D.50) for the previous accounting period, the business is immediately exempt from the 20% surcharge under Section 67 ter, even if the actual year-end financial statements show net profit that has jumped beyond the 20% threshold. (This applies only to businesses that had a full 12-month accounting period in the previous year.)
4. Mixing up the deadlines for paper and online filing
Time management is a key factor. A paper return filed at the area revenue office must be filed by 31 August 2026 only, while online filing through the e-Filing system is extended to 8 September 2026. If the owner delivers the accounting documents late and the e-Filing system closes at 22:00 on the due date, the business becomes a late filer, must pay a criminal fine of 2,000 baht, and, if there is tax payable, the monthly surcharge of 1.5% per month starts counting immediately from 1 September 2026.
Frequently asked questions about P.N.D.51 fines
If the company made a loss throughout the first 6 months, does it still have to file a P.N.D.51?
Yes, the P.N.D.51 must still be filed as required by law, entering an estimated net profit of 0 baht or the net loss figure. In this case there is no tax to pay, but failing to file by the deadline carries a criminal fine under Section 35 of up to 2,000 baht.
If I find out later that my net profit estimate was more than 20% too low, how can I fix it?
You can file an additional P.N.D.51 to pay the shortfall in half-year tax before the year-end P.N.D.50 filing deadline. You will have to pay a surcharge under Section 27 at 1.5% per month (counted from the day after the P.N.D.51 deadline until the date of the additional filing). This additional filing reduces or removes the 20% surcharge under Section 67 ter for the amount that has been paid additionally.
Can the 20% surcharge under Section 67 ter be reduced or waived?
The 20% surcharge is a statutory tax provision that cannot be reduced at the officer's discretion, unless the business can prove "reasonable cause" under Revenue Department Instruction No. Tor.Por. 81/2542, for example having paid half-year tax of not less than half of the tax paid in the previous year, or an unusual economic event in the second half of the year that could not have been foreseen.
I filed the P.N.D.51 10 days late with 50,000 baht of tax payable. What is the total fine and surcharge?
Three amounts must be paid together: (1) the tax payable of 50,000 baht, (2) a criminal fine of 2,000 baht because the filing was more than 7 days late, and (3) a surcharge of 1.5% per month (10 days is a fraction of a month, counted as 1 full month) = 50,000 × 1.5% = 750 baht. The total amount payable is 52,750 baht.
Summary: plan your half-year tax correctly and stay safe from the 20% surcharge
Managing the half-year corporate income tax (P.N.D.51) correctly and on time not only keeps the business compliant with the law, but also protects the company's profits from being wasted on P.N.D.51 fines and the 20% surcharge for a wrong estimate. Having an accurate accounting partner who works on a fast online accounting system and continuously monitors the numbers is therefore central to business stability.
If you are a business owner looking for a professional team for accounting services in Chiang Mai that truly understands the local business context, handles your tax work end to end with digital documents and no hassle with paper, you can see the details of our full-service accounting right away.
Get advice on business accounting and tax easily via LINE or by phone:
Contact YMTD Accounting Partner Chiang Mai
Tel: 081-542-3019
LINE: @premwittawat
