Thai Tax is dead-simple.
Attached a calculation sheet that my accountant uses for Personal Income Tax for me and the wife.
I filled it in with no deductions or savings, just basic 12 month salary of 100k/month as an example.
To use it: Empty Cells B3 to B5, Cells D14 to D17 and Cells E21 to E28.
Let's start with assumption that your salary is 50,000 Baht per month.
Step 1: Fill in Cell B3, basic income p/month (50,000)
Step 2: Fill in B4 all the way to B17 if applicable (more later).
Step 3: you see a number appearing in Cell E31. In this example, if you put 50,000 in Step 1, the amount should be 540,000..The dark blue cells E21-E28 should be empty.
Now, start with Cell E21, your first tax bracket. Put 150,000 there.
You will see that cell E30 shows 150,000, Cell E31 now shows 390,000
Go to Cell E22, next tax bracket, fill in 150,000. E31 now shows 240,000.
Continue with the next cell until E31 shows zero.
Step 4: Cell H29 shows your tax to be paid. (there's a number below it which is a quick USD calculation. Forgot to delete that.).
Deductions: Cell D9, prefilled and already included in Cell E30 calcultation
Cell D10, expense allowance. I get 100k allowance deductions. Company expenses basically. Check for yourself.
Cell D11, each child (under 18) gets a deduction of 30,000 THB, but only for you or for your wife, not both.
Cell D14 to D17: putting money in various savings can basically reduce your tax by up to 600,000 THB combined. Basic idea is, Not more than 30% of income can go into savings for tax-deduction and the total deductions combined can not be more than 600,000 THB.
All these deductions will show up automatically in cell E31 when you fill them out.
You can choose monthly tax payments if you want (assuming you're employed). Or pay at tax-submission. Column K to do a quick calculation for that.
That's basically it! Can't explain it any more simple.
TaxCalculationThailand.xlsx