How to Calculate GST in India (2026 Guide)

GST, or Goods and Services Tax, is charged on almost every product and service sold in India. Whether you're a freelancer invoicing a client, a shop owner pricing a product, or just checking whether a bill was calculated correctly, understanding how GST is worked out saves you from both overpaying and undercharging.

The current GST slabs

GST in India is charged at one of a small number of fixed rates depending on the category of goods or service. The most common slabs you'll run into day to day are 5%, 12%, 18%, and 28%. Essential goods are usually taxed at the lower end, while luxury and sin goods sit at 28%. Most professional services, including freelance and consulting work, are typically taxed at 18%.

Adding GST to a base amount

If you have a base price and need to add GST on top, the formula is straightforward: multiply the base amount by the GST rate, divide by 100, and add that figure back to the base amount.

• GST amount = (Base Amount × GST Rate) / 100
• Final Amount = Base Amount + GST Amount

For example, on a base amount of ₹10,000 at 18% GST: the GST amount works out to ₹1,800, making the final billed amount ₹11,800.

Removing GST from an inclusive amount

This is the calculation most people get wrong. If you already have a GST-inclusive amount and want to know what the original base price was, you cannot simply subtract the percentage — you have to divide it out first.

• Base Amount = Inclusive Amount / (1 + GST Rate / 100)
• GST Amount = Inclusive Amount − Base Amount

For example, if a bill shows ₹11,800 as the final amount at 18% GST, dividing by 1.18 gives a base amount of ₹10,000, meaning ₹1,800 of that bill was GST. Subtracting the percentage directly from ₹11,800 would give you the wrong number, which is the single most common GST calculation mistake.

Why this matters for freelancers and small businesses

If you're quoting a client a round number like ₹50,000 for a project, you need to be clear upfront on whether that figure is inclusive or exclusive of GST — the difference at 18% is ₹9,000, which is a significant gap to discover after the fact. Getting the add/remove calculation right the first time avoids awkward renegotiation later.

Rather than doing this math by hand every time, you can use the GST calculator below to instantly add or remove GST at any of the common slabs.