GST Calculator: How to Work Out Inclusive vs Exclusive Tax

GST (Goods and Services Tax) confusion almost always comes down to one question: is the number you're looking at before tax or after tax? Getting that backwards leads to either overcharging a customer or under-quoting a price — both problems.

GST-exclusive vs. GST-inclusive

A GST-exclusive amount is the base price before tax is added — you calculate GST on top of it. A GST-inclusive amount already has tax baked in — the listed price is what the customer pays, and the tax portion needs to be extracted rather than added. Retail prices are usually inclusive; B2B invoices are often exclusive with GST itemized separately.

How to calculate GST

• Adding GST to an exclusive amount: multiply the base price by the GST rate (e.g., 18%) and add it to get the final price.
• Extracting GST from an inclusive amount: divide the inclusive price by (1 + GST rate) to get the base price, then subtract that from the inclusive price to get the tax portion.

How to calculate it without doing the math by hand

• Open the GST Calculator.
• Enter your amount and select the applicable rate (5%, 12%, 18%, or 28%).
• Choose inclusive or exclusive and get both the tax amount and final price instantly.

Why this matters for invoicing

Getting inclusive/exclusive backwards on an invoice either shortchanges the tax you're supposed to collect or overcharges the customer — both are the kind of mistake that's obvious only after a customer or accountant flags it. Double-check which type of amount you're starting with before applying the rate.