GST Calculator

Adding GST to a base price is mathematically straightforward. If the base price is ₹100 and GST is 18%, you just calculate 18% of 100 (which is ₹18) and add it to get ₹118.

However, removing GST from an inclusive price is where many people make a critical mathematical error. If you have an invoice for ₹118 (which includes 18% GST), you cannot simply subtract 18% to find the base price.

Let's look at what happens if you subtract 18% from ₹118:

118 - (118 * 0.18) = 118 - 21.24 = ₹96.76

You ended up with ₹96.76, not the original ₹100! Why? Because 18% of 118 is a larger number than 18% of 100. Percentage changes are not symmetrical.

To correctly extract the base price from a GST-inclusive amount, you have to divide the total by 1 + the tax rate:

Base Price = Total Amount / (1 + [GST% / 100])

Using our ₹118 example:

Base Price = 118 / (1 + 0.18) = 118 / 1.18 = ₹100