Calculator Methodology

What MyThaiTaxes calculates

This page documents the calculation performed by the Annual Tax Calculator for a salaried employee (Revenue Code Section 40(1) income) — the base case the worked example below reproduces step by step. Freelancers, sole proprietors, and company directors go through the same underlying steps (assessable income → deductions → allowances → progressive tax → withholding credits) with extra rules layered on for multiple income types, expense methods, and foreign income — those are summarized where relevant but not fully worked here.

Assessable income

Assessable income is gross income before any deduction or allowance. Thai law splits it into eight categories (Section 40(1)–40(8)) by source — employment, liberal professions, contracting, rental, business/sales, dividends, and so on — because each category gets a different expense deduction method. The calculator's salaried flow uses Section 40(1): salary, wages, bonuses, and taxable benefits.

Employment income

Employment income (40(1)) is the sum of annual gross salary plus any taxable cash benefits entered. It is the figure the standard deduction and progressive tax are ultimately calculated against.

Expense deductions

Employment income gets a standard expense deduction of 50% of gross income, capped at 100,000 THB — so anyone earning above 200,000 THB/year hits the cap. This is a flat, no-receipts deduction; there is no option to itemize actual employment expenses instead.

Personal allowances

Allowances are subtracted from income (in addition to the expense deduction), based on family situation:

The 65+ figure is applied by the calculator as a flat 190,000 THB exemption. Its statutory citation ("Section 42(17)") could not be independently confirmed during our sourcing review — treat it as an unverified detail, not the underlying 190,000 THB amount itself, which is corroborated by multiple tax-advisory sources. The "up to 4 parents" cap is not a number stated directly in any source we found; it is derived from the rule that only one sibling may claim each parent and a taxpayer can have at most 2 own + 2 spouse's parents.

Social Security

Employee Social Security Fund (SSO) contributions are deductible, capped at 10,500 THB/year — the ceiling that took effect from 1 January 2026 (Phase 1, through 2028). The prior ceiling was 9,000 THB/year for 2024–2025.

Other deductions

All retirement-fund deductions (pension, provident, RMF, SSF combined) are also subject to a single combined ceiling of 500,000 THB under Thai law. The calculator applies each individual cap above but does not yet enforce this combined ceiling — a known gap, not a hidden feature. The pension fund's individual 500,000 THB cap is also flagged as possibly overstated (a 200,000 THB / 15%-of-income sub-cap may apply instead); it has not been independently re-verified.

Progressive tax

After all deductions and allowances, remaining taxable income is taxed under Thailand's progressive brackets (in effect since 2017):

Each band is taxed only on the portion of income that falls within it (a standard marginal-rate calculation) — income is not pushed entirely into the top bracket it reaches.

Withholding

Tax already withheld by an employer during the year (shown on the year-end withholding certificate, "50 Tawi") is entered separately and is not part of the taxable-income calculation. It is only netted against the final tax owed at the end: if withholding exceeds tax owed, the difference is a refund; if it falls short, the difference is owed at filing.

Foreign income

Foreign-sourced income is out of scope for the salaried worked example below, but in brief: under the remittance rule effective from 1 January 2024 (Revenue Department Order Por. 161/2566, clarified by Por. 162/2566), foreign income remitted into Thailand in the same year it is earned or later is assessable; foreign income earned before 2024 remains under the older rule. Where a Double Tax Agreement (DTA) applies, a foreign tax credit is allowed up to the lesser of the foreign tax actually paid or the Thai tax due on that same income — it cannot exceed the Thai tax on that income. This flow is handled by the freelancer calculator, not the salaried one documented here.

Tax residency

A person present in Thailand for 180 days or more (aggregate, in a calendar year) is a Thai tax resident under Revenue Code Section 41, which affects how foreign income is taxed. The salaried worked example below assumes residency status doesn't change the domestic calculation itself — the 180-day rule mainly matters for cross-border/foreign income cases.

Worked example

Single taxpayer, no children, no dependents, no additional insurance or fund contributions, annual gross salary 800,000 THB, with 40,000 THB already withheld by the employer during the year.

Applying the progressive brackets to 640,000 THB, band by band:

Effective tax rate: 48,500 ÷ 800,000 = 6.06% of gross income. This example matches what the calculator returns for the same inputs — try it to confirm.

Sources

Every constant on this page traces to a source recorded in this repository's tax rules registry (not published as a live URL — the primary sources it cites are):

• Thai Revenue Department — Guide to Personal Income Tax Return 2021 (PND90), rd.go.th — standard deduction, expense deduction rates by income category.
• Thai Revenue Department, English site — rd.go.th/english — Revenue Code Section 41 (residency), Section 40 (income categories), insurance/provident fund caps.
• Revenue Code Amendment Act No. 44 B.E. 2560 (2017) — current progressive tax bracket structure, via PwC and Sherrings Thailand secondary summaries.
• Social Security Office (SSO) contribution ceiling notices, via BDO Thailand and DLA Piper — 9,000 THB (2024–2025) and 10,500 THB (2026 onward, Cabinet-approved 2 Dec 2025, Royal Gazette 12 Dec 2025).
• Revenue Department Orders Por. 161/2566 and Por. 162/2566 — foreign income remittance rule effective 2024-01-01, via Mahanakorn Partners and KPMG.
• Forvis Mazars and Sherrings Thailand tax guides — child/parent/spouse allowance figures and retirement fund caps (independent secondary sources, converged).

Limitations & disclaimer