PND 51: what happens if you get the half-year estimate wrong?
If your estimated annual profit comes in more than 25% below the actual figure without reasonable cause, the Revenue Department adds a 20% surcharge on the tax you underpaid. There is a safe harbour, and it is a test on tax rather than on profit: paying at least half of last year’s corporate income tax is treated as reasonable cause.
Last reviewed 26 August 2026
What PND 51 actually is
It is a prepayment, not a separate tax. Halfway through your accounting year you estimate what the company will earn for the whole year, pay tax on half of that estimate now, and the amount is credited against your annual bill when you file PND 50 after year end.
Every Thai company files it, including one that made a loss. The usual exception is a company in its first accounting period where that period is shorter than twelve months, which has no half-year to report on.
Listed companies, banks and financial institutions work differently: they use their actual half-year results rather than an estimate, so none of the estimating risk below applies to them.
How the number is calculated
- Estimate net profit for the full accounting year, not the half year.
- Halve it.
- Apply the corporate income tax rate to that half, taking account of any SME rate bands your company qualifies for.
- Deduct withholding tax already suffered during the half year, and any other available credits.
- The balance is what you pay with the return.
The 25% rule, and the 20% surcharge
When the year closes and the real number is known, the Revenue Department compares it with what you estimated. If your estimate was more than 25% below the actual full-year net profit, and you have no reasonable cause for the gap, a 20% surcharge is applied to the amount of tax you underpaid, not to the profit.
This is where owners get hurt. The surcharge is not a fine for filing late; you can file perfectly on time, pay what you calculated, and still be penalised nine months later because the business did better than you expected. Plenty of profitable years end with a bill nobody budgeted for.
The safe harbour, and the detail almost everyone gets wrong
The distinction matters. It is widely repeated in English that the safe harbour is half of last year’s net profit. The Thai text says กึ่งหนึ่งของภาษีเงินได้นิติบุคคล, one half of the corporate income tax. For a company paying a flat rate the two land in the same place; for an SME on the graduated rates, where the first bands are exempt or reduced, they do not.
Clause 1(2) covers exactly that case. If you estimated net profit at no less than last year’s actual net profit, but the tax still came out below half of last year’s tax because of an exemption or a reduced rate, that is also reasonable cause.
So the practical check is: take the corporate income tax on last year’s PND 50, halve it, and make sure the tax you are about to pay with PND 51 is at least that. If it is not, and your profit estimate is also below last year’s profit, you are outside the safe harbour.
It is a floor, not a target. If you already know this year will be materially better, say a large contract signed in the first half or a new location trading, estimate to what you actually expect. The safe harbour protects an honest estimate that turned out low; it is not a licence to under-declare a year you can see coming.
What happens if you over-estimate instead
There is no surcharge for over-estimating. You will have overpaid, and the excess sits as a credit against the annual liability or comes back as a refund.
But it is not free. The cash is out of the business for the better part of a year, and refunds in Thailand are slow and tend to attract questions. A refund claim is one of the more reliable ways to invite a closer look at your return. Deliberately padding the estimate to stay safe has a real cost, which is why the prior-year floor is usually the better answer than simply guessing high.
What to do this week
- Get your actual first-half figures closed. You cannot estimate a year you have not measured half of.
- Pull the corporate income tax figure from last year’s PND 50 and halve it. The tax you pay with PND 51 should be at least that much.
- Compare the two. If the first half already exceeds half of last year, estimate upward from what you are actually seeing.
- Write down how you arrived at the number and keep it with the return. Reasonable cause is much easier to argue with a contemporaneous note than a reconstruction a year later.
- File and pay by 31 August, or 8 September if you file electronically.
If nobody has told you which of these your accountant handles, that is worth settling before the deadline rather than after it. A quote that says "tax filings" and a quote that lists PND 51 by name are not the same quote, which is exactly what comparing three of them side by side is for.
Related questions
- What happens if you file PP 30 late?
- What is actually included in a Thai bookkeeping fee, and what almost never is?
- Who is legally responsible if my accountant files something wrong?
Sources
- Revenue Code, sections 67 bis and 67 ter (Revenue Department, English)
- Departmental Instruction Paw. 50/2537 on reasonable cause (Revenue Department, Thai)
- Forvis Mazars: half-year corporate tax return filing reminder
This is general information about how a filing works, not accounting or tax advice for your company. Confirm anything that matters against the official notice or with your accountant. Spotted something out of date? Tell us and we will fix it.