The government issues Government Regulation Number 58 of 2023 and also the Minister of Finance Regulation PMK 168 of 2023 concerning Rates for withholding Income Tax Article 21 on income in connection with the work, services or activities of Individual Taxpayers; to simplify the calculation of Income Tax Article 21.
To calculate Article 21 Income Tax, apart from using the rate of Article 17 paragraph (1) letter a of the Income Tax Law, the average effective rate, both daily and monthly, can also be used. Effective monthly rates are grouped into categories:

Based on non-taxable income status (M/O) and total monthly gross income of IDR 10,000,000 (ten million rupiah), Article 21 Income Tax deductions on income received or earned by Mr. R for the tax period January 2024 to November 2024 are carried out by using the effective Category A rate of 2% (two percent).
The amount of Income Tax Article 21 per month withheld by PT ABC on Mr R’s income for the tax period January to November 2024 is IDR 10,000,000 x 2% = IDR 200,000.
The amount of Income Tax Article 21 withheld by PT ABC on Mr. R’s income for the December 2024 tax period is as follows:

Income Tax Article 21 for the year
= Rate of Article 17 paragraph (1) letter a of the Income Tax Law
x Annual Taxable Income
= 5% x IDR 54.300.000 = IDR 2.715.000
Income Tax Article 21 in December 2024
= A n n u a l I n c o m e Tax Article 21 – t h e a m o u n t o f I n c o m e Tax from
January 2024 to November 2024 that has been deducted
= IDR 2.715.000 – (IDR 200.000 x 11) = IDR 515.000

PDF: Calculation Of Income Tax Article 21 Using The Average Effective Rate