Show discount including tax, calculate discount excluding tax

Question

I want to display the discount amount including tax on cart overview page (totals summary).

• Magento 2.1.8

Relevant settings:

• Catalog prices: Excluding tax
• Apply Customer Tax: After discount
• Apply Discount On Prices: Excluding tax

Current situation in total summary within cart:

I cannot set Apply Discount On Prices to Including Tax, because Magento then calculates wrong totals. So in short I need to calculate discount amounts excluding tax but displaying including tax.

Answer 1

Please check the warning message in the Magento Back office in Configuration -> Sales -> Tax -> Calculation Settings -> Apply Discount On Prices:

To apply the discount on prices including tax and apply the tax after discount, set Catalog Prices to “Including Tax”.

To do what you want, you need to customize the Magento Tax module