How Is GST Calculated? Add, Remove and Split GST

GST, or Goods and Services Tax, is the single indirect tax applied to most goods and services sold in India. Whether you are a business issuing an invoice or a buyer checking a bill, the same simple arithmetic decides how much tax is involved. This guide shows you how to add GST to a base price, how to work backwards and remove GST from a tax-inclusive amount, how the tax splits into CGST and SGST, and which rate slab applies.

The GST formula

Adding GST to a price is straightforward multiplication:

GST amount = Base price x (GST rate / 100)

Total price = Base price + GST amount

So at an 18% rate, a base price of 1,000 carries 1,000 x 0.18 = 180 in GST, and the customer pays 1,180 in total. The GST Calculator does this in one step, but the formula is worth knowing so you can sanity-check any invoice.

How to remove GST from an inclusive price

Often a price already includes GST and you need to find the base amount and the tax inside it - for example to fill in an invoice or claim input credit. You cannot simply subtract the percentage; you divide by one plus the rate:

• Base price = Total / (1 + rate)
• GST amount = Total - Base price

If a bill shows 1,180 inclusive of 18% GST, the base price is 1,180 / 1.18 = 1,000, and the GST inside it is 1,180 - 1,000 = 180. A common mistake is to take 18% of 1,180 (which gives 212.40) - that is wrong, because the 18% was charged on the base of 1,000, not on the tax-inclusive total.

CGST and SGST: how the tax splits

For a sale within a single state (an intra-state supply), GST is split equally into two halves:

• CGST (Central GST) - collected by the central government.
• SGST (State GST) - collected by the state government.

On the 180 of GST above, CGST is 90 and SGST is 90. For a sale between two states (an inter-state supply), the whole amount is instead charged as a single IGST (Integrated GST) of 180. The total tax is identical either way - only the labelling and which government receives it change.

The GST rate slabs

India uses a small set of standard GST rates, and the correct one depends on the product or service:

• 0% - essential items such as fresh produce and unbranded staples.
• 5% - common household goods, packaged food and economy transport.
• 12% - processed food, business-class air travel and some electronics.
• 18% - the most common slab, covering most services, electronics and restaurant bills.
• 28% - luxury and 'sin' goods such as cars, tobacco and aerated drinks.

Because the slab varies by item, always confirm the correct rate for what you are selling or buying before applying the formula. When in doubt, the HSN or SAC code on an invoice maps to a specific rate.

A worked invoice example

Suppose a shop in the same state sells a gadget with a base price of 1,000 at the 18% slab. The invoice reads: base 1,000, CGST at 9% = 90, SGST at 9% = 90, total payable 1,180. The two 9% halves add up to the single 18% rate, which is why an intra-state bill always shows two 9% lines rather than one 18% line. If the same gadget were shipped to a buyer in another state, the invoice would instead show IGST at 18% = 180 and the same 1,180 total.

Why removing GST matters for businesses

Registered businesses care about the base-and-tax split because they can claim input tax credit - the GST they paid on purchases offsets the GST they collect on sales. To do that accurately they must separate the tax portion from every inclusive price, which is exactly the 'remove GST' calculation above. Getting this right keeps returns accurate and avoids paying tax twice on the same value.

The arithmetic is simple once you have seen it, but for quick day-to-day work - adding GST to a quote or extracting the tax from an inclusive bill - the fastest route is to enter the figures in the GST Calculator and read the breakup instantly.