"What's 18% GST on ₹1,000?" sounds like a simple question, but it has two different correct answers depending on whether that ₹1,000 already includes GST or not — and the two calculations aren't mirror images of each other, which is where most GST math mistakes come from.
Exclusive: GST added on top
If a price is GST-exclusive, the amount you have is the base price before tax, and GST gets added on top:
GST amount = base × rate / 100Total = base + GST amount
₹1,000 at 18% exclusive: GST is ₹180, total is ₹1,180. Straightforward — this is the calculation most people picture when they think of "GST math."
Inclusive: GST already baked in
If a price is GST-inclusive — say, a shelf price or an MRP that legally must include all taxes — the number you have is the total, and you need to work backward to find the base amount and how much of that total was actually tax:
Base amount = total / (1 + rate / 100)GST amount = total − base amount
₹1,180 at 18% inclusive: base amount is ₹1,180 / 1.18 = ₹1,000, GST is ₹180. Same numbers as the exclusive example above — which is exactly why it's confusing. The two calculations arrive at consistent results when run on the correct matching pair of numbers, but if you accidentally apply the exclusive formula (multiply by 18%) to a number that's actually inclusive, you'll overstate the tax amount and get a base figure that's wrong.
The mistake to watch for
The error that actually happens in practice: taking an inclusive total and multiplying it by the rate directly (treating it as if it were exclusive), instead of dividing by (1 + rate/100) first. On ₹1,180 inclusive, wrongly multiplying by 18% gives ₹212.40 as the "GST amount" — noticeably different from the correct ₹180. Always know which kind of number you're starting with before picking a formula.
Using the tool
Our GST Calculator handles both directions — enter an amount, pick whether it's inclusive or exclusive of GST, and the rate, and it returns the base amount, GST amount, and total, calculated with the correct formula for whichever direction you selected.
Try it here: GST Calculator.