Is VAT already included?
First identify whether your number is the pre-VAT amount or the VAT-inclusive total, then use the corresponding calculation.
When VAT is excluded
Multiply the base by the entered rate, then add the tax to the base. A base of 1,000 and a sample rate of 7% produces 70 tax and a 1,070 total. The rate is an arithmetic example; verify the rate and VAT treatment that apply to the transaction.
When VAT is included
Divide the total by one plus the decimal rate to recover the base. The calculator rounds the base to two decimal places and derives tax as total minus base so the displayed figures reconcile.
What is outside the tool
It does not determine VAT registration, exemptions, the legally applicable rate, or the monthly net of output tax and input tax.
Primary references
The Revenue Department's Revenue Code sections 80–82 provide context for rates, liable persons and output/input tax. See section 86/4 for tax-invoice particulars. The primary texts are in Thai; this guide is explanatory and is not a tax determination.
Content and sources checked 11 September 2026