Getting the P.N.D. 51 Half-Year Tax Estimate Right

Filing deadline and the principles of calculation

A company or juristic partnership that is subject to the requirement must file the P.N.D. 51 within 2 months from the last day of the first six months of the accounting period. If the accounting period follows the calendar year, the Revenue Department’s tax calendar for that year should be checked again, because public holidays and the entitlement to file online may make the final day different. An overview of corporate income tax for the whole year, from SME rates to the P.N.D. 50, is in our corporate income tax guide for SMEs (Thai).

1. Estimate full-year revenue from actual figures, orders in hand, and the business assumptions for the second half of the year.
2. Estimate tax-deductible expenses; do not use all accounting expenses without adjusting for Sections 65 bis and 65 ter.
3. Calculate the net profit and the tax for the full year using the applicable rate, SME benefits, BOI benefits, and the net losses carried forward that can actually be used.
4. Calculate the half-year tax in accordance with the return, and credit the tax paid when filing the P.N.D. 50.

The 25% underestimate threshold

Section 67 ter of the Revenue Code uses the term “surcharge” (เงินเพิ่ม) for the case in which net profit is underestimated by more than 25% of the actual net profit without reasonable cause. The statutory rate is 20% of the tax underpaid, not 20% of the entire profit shortfall.

Reasonable cause requires contemporaneous evidence

A second half of the year that is more profitable than expected may constitute reasonable cause, but there should be documents prepared at the time of the estimate, not reasons assembled afterwards when an audit begins. Examples of facts worth analyzing include large orders that arose later, the reversal of provisions, a sale of assets that was not planned on the filing date, or significant expenses that were canceled unexpectedly.

• The actual profit and loss statement up to the latest month, reconciled with the general ledger
• Sales forecasts by customer, project, or product, with the status of quotations and purchase orders
• Cost and expense forecasts that separate the items to be added back for tax
• A record of the assumptions, the preparer, the approver, and the date each data set was used
• A comparison table against the previous year, the budget, and actual results, with explanations of the variances

Forecasting methods better than multiplying by two

What to do when the estimate turns out to be too low

Prepare a new estimate immediately, check whether the return can still be amended and additional tax paid, and assess the surcharge according to the date on which payment will be made. The Revenue Department has guidelines for reducing the surcharge in some cases where the taxpayer corrects the return and pays voluntarily before receiving a warning or a summons, but the entitlement and the rate must be checked against the law and practice in force on the actual filing date.

Official sources (in Thai): Revenue Department — frequently asked questions on the P.N.D. 51, tax calendar, and instructions for the P.N.D. 51 form. Information verified on 27 June 2026 (B.E. 2569).