Thai tax is not an annual event — it is twelve monthly deadlines with an audit trail attached.

Three filings define the month for an operating company: the VAT return covering output tax on sales and input tax on purchases, the withholding tax returns for payments made to service providers and staff, and the social security remittance for employees. None of them can be skipped for a quiet month; a VAT-registered company with no sales still files a nil return, and a month with no withholdable payment still needs the position documented.

The Revenue Department's first move in any examination is reconciliation, not interpretation. Revenue in the VAT returns is compared with revenue in the audited financial statements, and with the amounts customers reported withholding from the company. Expenses claimed are compared with the withholding tax the company remitted on those same payments. Where the three sets of numbers do not meet, the difference becomes the assessment, and the company then carries the burden of explaining it.

This is why bookkeeping discipline is a tax strategy in Thailand. A company that issues sequentially numbered tax invoices, obtains a compliant tax invoice for every input VAT claim, issues withholding certificates at the time of payment, and keeps supporting documents for every expense will usually settle an examination on the documents alone.

VAT registration is compulsory once revenue from taxable supplies crosses the statutory threshold, and voluntary registration is available earlier — often desirable for a business whose customers are themselves VAT registered, and unhelpful for one selling to consumers. Some supplies are exempt and some are zero-rated, and the two are not the same: a zero-rated exporter still registers and still reclaims input VAT, while an exempt business cannot reclaim it at all.

Input VAT is credited only against a tax invoice that satisfies the formal requirements, including the buyer's registered name, address and tax identification number. An invoice addressed to a director personally, or bearing an old company address, is not creditable. Certain input VAT is non-creditable by rule regardless of the paperwork — entertainment being the familiar example — and passenger vehicles have their own restrictions.

Cross-border services bought from abroad create a self-assessment obligation: the Thai buyer remits VAT on the imported service, then claims it as input VAT in the following period. Companies paying overseas software, advertising and platform fees frequently miss this, and the exposure accumulates quietly until an examination finds the payments in the bank statements.

Thailand collects tax at source on a wide range of domestic payments — services, professional fees, rent, transport, advertising, commissions — each at its prescribed rate, and on salaries under the personal income tax rules. The obligation sits with the payer. The company that failed to withhold owes the tax itself, along with surcharge, and cannot usually recover it from the supplier who has already been paid in full.

The certificate matters as much as the remittance. A supplier who receives no withholding certificate cannot credit the tax against its own liability, which turns a compliance slip into a commercial dispute. Issuing certificates at the moment of payment, rather than in a year-end catch-up exercise, is the practice that prevents this.

Payments to non-residents follow different rules and rates, and a double tax agreement may reduce or eliminate the Thai tax — but only where the treaty conditions are satisfied and the required documentation, including residence certification, is held. Applying a treaty rate without that file is a common finding in examinations of companies that pay foreign parents for services, royalties or interest.

Corporate income tax is assessed on net profit computed under Thai tax rules, which differ from accounting profit: some provisions are not deductible until realised, entertainment is capped, donations are limited, and expenses without adequate supporting documents are added back. Reduced rates apply to companies meeting the small-enterprise criteria on paid-up capital and revenue, and promoted activities may carry exemptions with their own reporting conditions.

The half-year return is an estimate of full-year profit, and the estimate is not a free guess. Understating projected profit beyond the tolerance the law allows attracts a penalty on the shortfall of the half-year payment, so the estimate deserves a proper forecast rather than a copy of last year.

Late filing carries a fixed statutory fine and a monthly surcharge on unpaid tax, capped by reference to the tax itself. The practical damage is wider than the money: a company with a history of late filings loses the benefit of the doubt on judgement calls, and a request for a VAT refund from a persistently late filer invites a full examination rather than a desk review.