Note
Withholding Tax on Stage Payments
When the main contractor settles a billing stage, the net cash reaching your account is the certified value minus 3% withholding tax (WHT). That deduction is not a shortfall or a dispute. It is a tax prepayment taken at source by the payer and remitted directly to the Revenue Department on your behalf.
The rate is set by law and cannot be waived by contract. The deduction runs on every stage from the first to the last. Unlike a mobilization advance recovery, which closes when the balance reaches zero, WHT has no end point. Every supplier commitment, factory order, and labor payment across the project competes with incoming cash that is consistently 3% below the gross invoice values.
What Is Withholding Tax and Why Does It Apply to Subcontract Work?
Withholding tax is a collection mechanism in Thai revenue law. When a juristic person pays for hire of work (จ้างทำของ), it is required to deduct a percentage from the payment before transfer and remit that sum to the Revenue Department on the payee’s behalf.
For subcontract installation, fit-out, and civil work, the applicable category is hire of work. The standard rate is 3% of the invoice amount calculated before VAT. On each certified stage payment, the paying party deducts 3% from the pre-VAT invoice value, remits that amount to the Revenue Department under your tax identification number, and transfers the balance to your account.
The deducted amount is not lost. It becomes a credit applied against your corporate income tax for the same calendar year. The gap it creates is in your working capital, not in the tax system.
How Does the Deduction Affect Your Cash Flow Per Stage?
A worked example makes the arithmetic concrete. Consider a subcontract for installation across twelve zones, structured as eight billing stages with broadly equal certified values. Each stage is certified at 500,000 baht excluding VAT. The WHT deduction per settlement is 15,000 baht.
The invoice total including 7% VAT reads 535,000 baht. The actual cash transferred to your account is 535,000 minus 15,000, which is 520,000 baht. Across eight stages, the total WHT withheld over the project is 120,000 baht.
That 120,000 baht is not forfeited. It becomes a credit that reduces your annual tax bill or generates a refund if credits exceed liability. But it is not available as working capital in the window between the first stage settlement and the annual tax settlement. For a contractor who plans cash against gross invoice values, the gap appears on the same day the first payment arrives.
What Is the WHT Certificate and Why Does It Matter?
When the main contractor deducts withholding tax from a payment, it is required by law to issue a withholding tax certificate (หนังสือรับรองการหักภาษี ณ ที่จ่าย). This document records the payer’s tax identification number, your tax identification number, the amount deducted, and the month in which that amount was remitted to the Revenue Department.
The certificate is the evidence used to claim the tax credit when filing your annual return. Without it, or with one that contains an incorrect tax identification number, the credit cannot be applied as issued. The most common error in practice is an incorrect entity name or identification number. The remedy is to return the certificate and request a corrected one. If the payer has already remitted under the wrong identification, correction requires a formal amendment at the Revenue Department.
Requesting a WHT certificate for each stage payment is not a year-end task. The practical approach is to request it immediately after each settlement, alongside your receipt or payment confirmation.
How Do You Build WHT Into Your Stage Billing Plan?
Three inputs are needed, all available at contract signing: the expected certified value of each billing stage from the BOQ, the 3% WHT rate, and the billing schedule.
The WHT deduction per stage is a multiplication. The net cash per stage is the certified value minus 3%. The total WHT credit for the project is the sum of all deductions.
That figure belongs on the timeline alongside the billing pins. When the project also carries a mobilization advance, both deductions appear on the same line against each stage, and the net receipt is the certified value minus both. When planning a material order or factory production run against incoming cash, the relevant figure is the net receipt after all deductions, not the gross certified value.
What Happens When a Certificate Is Issued Late or Batch-Delivered?
Some main contractors issue certificates in a single batch at year-end. This is within the rules, but it limits your ability to verify the running WHT total before that batch arrives. A contractor who relies on certificates alone to track WHT cannot confirm the accumulated figure until December.
The practical solution is a running record maintained from the first payment. Each row records the stage number, the certified value excluding VAT, the WHT deduction at 3%, the net cash received, and the cumulative WHT total. When the annual certificate batch arrives, the figures reconcile exactly. Any discrepancy is visible before the filing deadline rather than after.
Can the withholding tax rate be negotiated or waived?
For a Thai juristic subcontractor receiving payment for hire of work, the 3% rate is set by statute and cannot be waived by contract. Restructuring the contract to price materials and labor under separate arrangements may affect the rate on the materials portion, but this involves a genuine change in contract structure and invoicing. That decision is one for a qualified Thai tax adviser, not a clause to add without professional review.
What if my WHT credits exceed my annual tax liability?
Excess credits are refundable after filing a tax return that shows the excess. The Revenue Department reviews the return before releasing the refund, and that review typically takes several months. Counting the refund as cash available on the day of filing is not a reliable assumption. Building the review period into year-end cash projections is more accurate than treating it as an immediate recovery.
How do I track accumulated WHT without waiting for the annual certificates?
A running log in your existing stage payment record is enough. Each row records the pre-VAT certified value, the 3% WHT deduction, the net cash received, and the running total. This adds two columns to a record you already maintain. When the year-end certificate batch arrives, the figures should match exactly. Any discrepancy indicates an error in either the certificates or your record, and finding it before the filing deadline is significantly easier than finding it after.
If the WHT deductions across your active projects have not been mapped against your billing stages, a 45-minute call is a practical starting point. We open your stage payment records together, calculate the net receipts per stage, and place the figures on your timeline alongside the billing pins.