Goods and Services Tax (GST) replaced a tangle of indirect taxes in India in 2017 with a single, unified system. Whether you run a business or simply want to understand the tax on your bills, this guide explains how GST is structured, how to calculate it both ways, what input tax credit means, and who has to register.
What is GST?
GST is an indirect tax levied on the supply of goods and services. It is a destination-based, multi-stage tax: it is collected at each step of the supply chain but ultimately borne by the final consumer, with businesses claiming credit for the tax they paid on their inputs. This input-tax-credit mechanism is the heart of GST — it prevents tax being charged on tax, which was the main flaw of the older system it replaced.
Why GST was introduced
Before 2017, India had a patchwork of central and state taxes — excise duty, service tax, VAT, octroi and more — that overlapped and cascaded, so tax was effectively paid on tax. GST unified most of these into one tax with a common set of rules across states, making compliance simpler, reducing cascading, and creating a single national market where goods move across state borders without a web of separate levies.
GST rate slabs in India
India's GST is charged at several slab rates depending on the type of good or service: 0%, 5%, 12%, 18% and 28%. Essentials and many food items are taxed at 0% or 5%, standard goods and services typically fall in the 12% or 18% bands, and luxury or 'sin' goods sit at 28%, sometimes with an additional cess. For example, basic groceries may be 0%, packaged food 5–12%, most services 18%, and high-end cars 28% plus cess. Always apply the rate that matches the specific item.
How to calculate GST (adding it)
Adding GST is straightforward: multiply the price by the rate and divide by 100. For example, 18% GST on a 1,000 product is 1,000 × 18 ÷ 100 = 180, making the total 1,180. At 12% the GST would be 120 and the total 1,120. The GST calculator does this instantly and shows the tax and the final amount for any rate. If a payment is late, interest and a daily late fee can apply, which the GST late fee calculator estimates.
Removing GST from a total
Sometimes you have a GST-inclusive total and need to find the original price — for instance, to record the tax component separately. To remove GST, divide the total by (1 + rate ÷ 100). For 18% GST, a 1,180 inclusive amount came from 1,180 ÷ 1.18 = 1,000 before tax, with 180 of GST. This 'reverse GST' calculation is essential for businesses working from inclusive prices, and it is a common source of arithmetic errors when done by hand.
Input tax credit explained
Input tax credit (ITC) is what makes GST a tax on value added rather than on the full price at every stage. A business pays GST on its purchases (inputs) and charges GST on its sales (outputs); it then pays the government only the difference. So a manufacturer who paid 100 of GST on materials and collected 180 on sales remits just 80. This chain of credits ensures the final consumer bears the tax once, on the final price, and is why keeping accurate purchase records matters so much for registered businesses.
CGST, SGST and IGST explained
The total GST rate is the same wherever you buy, but how it is split depends on whether the sale crosses a state border. For a sale within a state, the tax is divided equally into Central GST (CGST) and State GST (SGST) — so 18% becomes 9% CGST plus 9% SGST. For a sale between states, a single Integrated GST (IGST) at the full rate applies, which the central government later shares with the destination state. Either way, the amount you pay is identical; only the accounting differs.
Who must register for GST?
Businesses whose annual turnover crosses the prescribed threshold must register for GST, as must certain categories — such as those making inter-state supplies or selling through e-commerce — regardless of turnover. Registered businesses charge GST, file periodic returns, and can claim input tax credit on their purchases. Smaller businesses may opt for the composition scheme, which offers a lower flat rate with simpler filing but without the ability to claim ITC. The rules and thresholds change from time to time, so always check the current provisions or consult a professional.