How GST works: slabs, forward vs reverse calculation, and input credit
Goods and Services Tax (GST) replaced a web of cascading indirect taxes with a unified, destination-based taxation system. While the framework simplified national trade, everyday invoicing calculations — especially reverse tax extraction and interstate splits — remain a frequent source of costly bookkeeping errors.
This guide explains the four main tax slabs, how to calculate tax on both net and gross totals correctly, and how the Input Tax Credit (ITC) mechanism prevents products from being taxed repeatedly across the supply chain.
The forward versus reverse GST formula
Adding GST to a base price is straightforward: multiply the base amount by the tax rate. But extracting the base price from a tax-inclusive retail total requires a division formula that many people get wrong.
The common mistake is subtracting the tax percentage from the final figure. If an item costs ₹1,180 with 18% GST, deducting 18% from ₹1,180 subtracts ₹212.40, arriving at ₹967.60. That is incorrect — the true base price is ₹1,000, and the tax was ₹180.
| Calculation Type | Formula | Example (18% on ₹1,000 base) |
|---|---|---|
| Forward (add tax) | Gross = Base × (1 + Rate ÷ 100) | ₹1,000 × 1.18 = ₹1,180 |
| Tax Amount (forward) | GST = Base × (Rate ÷ 100) | ₹1,000 × 0.18 = ₹180 |
| Reverse (extract base) | Base = Gross ÷ (1 + Rate ÷ 100) | ₹1,180 ÷ 1.18 = ₹1,000 |
| Tax Amount (reverse) | GST = Gross − (Gross ÷ (1 + Rate ÷ 100)) | ₹1,180 − ₹1,000 = ₹180 |
Always divide by (1 + rate) when reverse-calculating from a retail total. Never multiply the gross by the tax percentage.
Intrastate vs Interstate: CGST, SGST, and IGST
GST is split between central and state governments depending on where the buyer and seller are located:
- Intrastate supply (same state): The total GST is divided equally into Central GST (CGST) and State GST (SGST). For an 18% slab, 9% goes to CGST and 9% to SGST.
- Interstate supply (different states or union territories): The full rate is levied as Integrated GST (IGST), collected by the central government and apportioned to the consuming destination state.
- Union Territory GST (UTGST): Replaces SGST in union territories without their own legislature (such as Andaman and Nicobar Islands, Ladakh, and Chandigarh).
How Input Tax Credit (ITC) eliminates double taxation
Under previous tax regimes, a manufacturer paid tax on raw materials, and the wholesaler paid tax on the finished product's full value — taxing the tax that had already been paid. GST eliminates this via Input Tax Credit.
A business only pays tax on the value it adds. When filing returns, you subtract the GST you paid on business purchases (Input Tax Credit) from the GST you collected from your customers (Output Tax).
- Manufacturer buys raw material for ₹10,000 + 18% GST (₹1,800). Total paid: ₹11,800. Input credit accumulated: ₹1,800.
- Manufacturer processes goods and sells to wholesaler for ₹15,000 + 18% GST (₹2,700). Output tax collected: ₹2,700.
- Manufacturer pays the government: ₹2,700 output tax − ₹1,800 input credit = ₹900 net tax. The government collects tax only on the ₹5,000 value added.
- Wholesaler and retailer repeat this process, ensuring the consumer pays tax on the final retail value without hidden compounding.
Frequently asked questions
- What are the standard GST slabs in India?
- The standard tiers are 0% (essential foods, books, fresh produce), 5% (packaged food, basic medicines, transport), 12% (processed foods, business software, apparel above threshold), 18% (most services, electronics, capital goods), and 28% (luxury items, automobiles, aerated drinks).
- Can an individual consumer claim Input Tax Credit?
- No. Input Tax Credit is strictly available to registered businesses for goods and services used in the furtherance of business. End consumers absorb the final tax.
- What is Reverse Charge Mechanism (RCM)?
- Under normal GST, the supplier collects tax and deposits it with the government. Under RCM, the recipient of the goods or services is directly liable to pay the tax to the government (commonly applied to goods transport agencies and specific legal services).
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