GST and VAT Calculation: Formulas for Tax-Inclusive and Tax-Exclusive Pricing

Tax-Inclusive vs. Tax-Exclusive Pricing#

• Tax-Exclusive Price (Net Price): The base price of the item before any tax is added. B2B wholesale transactions are typically quoted exclusive of tax.

• Tax-Inclusive Price (Gross Price): The final checkout price that includes all applicable GST/VAT. Consumer retail prices (such as on supermarket shelves) are legally required to be displayed inclusive of tax in most jurisdictions.

Formula 1: Adding GST to Net Price (Tax Exclusive)#

To calculate the tax amount and total gross price from a net amount:

GST Amount  = Net Price * (Tax Rate / 100)
Gross Price = Net Price + GST Amount
            = Net Price * (1 + Tax Rate / 100)

Example ($500 base price at 18% GST):
  GST Amount  = 500 * 0.18 = $90.00
  Gross Total = 500 + 90   = $590.00

Formula 2: Extracting GST from Gross Price (Tax Inclusive)#

When you receive a total invoice of $590 and need to determine the original pre-tax base price and the tax paid:

Net Price   = Gross Price / (1 + Tax Rate / 100)
GST Amount  = Gross Price - Net Price
            = Gross Price * [Tax Rate / (100 + Tax Rate)]

Example ($590 total receipt at 18% GST):
  Net Price  = 590 / (1 + 0.18) = 590 / 1.18 = $500.00
  GST Amount = 590 - 500 = $90.00

The Common Reverse Tax Trap ($118 - 18% ≠ $100)#

A very frequent accounting mistake is attempting to extract 18% tax from $118 by simply calculating $118 \times 0.18 = $21.24$, and subtracting: $118 - 21.24 = $96.76$.

This is mathematically incorrect because the 18% tax was originally added to the base price of $100 ($18 tax), not the final $118 total. Always divide by `1 + (Rate / 100)` to find the true base.

Input Tax Credit (ITC) Basics for Invoicing#

In a GST system, businesses claim Input Tax Credits on the tax paid for business purchases (laptops, software subscriptions, office rent) against the tax collected from client sales, paying only the net difference to the government.