Introduction: The Dual-GST Tax Architecture
The implementation of the Goods and Services Tax (GST) has fundamentally reshaped India's indirect tax system, replacing a complex maze of excise duties, service taxes, octroi, and value-added tax (VAT) with a unified tax regime. GST is a comprehensive, multi-stage, destination-based tax levied on the manufacture, sale, and consumption of goods and services. India adopts a unique **dual-GST model**, meaning both the Central and State governments simultaneously levy tax on a common transaction base. For business owners, startup founders, and accountants, mastering the math behind CGST, SGST, and IGST, and understanding how to claim the Input Tax Credit (ITC) is vital. Under-calculating your GST leads to steep tax penalties, while over-calculating blocks working capital and damages market competitiveness. This comprehensive guide covers everything you need to manage your indirect taxes seamlessly.
This guide explains the mechanics of dual-GST, details the core tax rates, walks through step-by-step worked mathematical examples for intra-state and inter-state transactions, explains reverse charge mechanism (RCM) rules, outlines input tax credit (ITC) mapping, and provides an invoicing checklist. Track your tax obligations instantly using our direct GST Calculator alongside this guide.
The Core Slabs: How CGST, SGST, and IGST Are Structured
GST operates under a strict territorial division. The tax split depends entirely on whether a transaction occurs within a single state (intra-state) or across state boundaries (inter-state):
- CGST (Central GST): Levied by the Central Government on intra-state transactions.
- SGST (State GST) / UTGST (Union Territory GST): Levied by the State Government or Union Territory administration on intra-state transactions.
- IGST (Integrated GST): Levied by the Central Government on inter-state transactions, exports, and imports. IGST is the sum of CGST and SGST, and the Central Government distributes the state's share to the destination state where the goods or services are actually consumed.
On an intra-state transaction, a 18% GST rate is split evenly: **9% CGST** goes to the Central Treasury, and **9% SGST** goes to the State Treasury. On an inter-state transaction, a flat **18% IGST** is charged by the seller and settled through the central pool.
GST Rate Slabs for FY 2025-26 and FY 2026-27
| GST Slab Rate | Category of Goods / Services | Examples of Items Covered |
|---|---|---|
| 0% (Exempt / Nil Rated) | Essential daily commodities and basic goods | Fresh fruits, vegetables, milk, curd, salt, flour, unbranded foodgrains |
| 5% Slab | Mass consumption items and basic inputs | Tea, coffee, packaged foodstuffs, domestic LPG, spices, life-saving drugs |
| 12% Slab | Standard goods, process products, business services | Butter, cheese, ghee, mobile phones, business class air travel, software contracts |
| 18% Slab (Default Standard) | Most consumer items, electronics, and standard services | Computers, monitors, capital goods, IT services, consulting, hotels, restaurant dining |
| 28% Slab (Luxury & Demerit) | Luxury vehicles, sin goods, high-end entertainment | Automobiles, motorbikes, tobacco products, carbonated soft drinks, online gaming |
Worked Example #1: Intra-State Transaction Math
Let's calculate the exact tax breakdown for an intra-state transaction. **Arohan Electronics** in Bangalore, Karnataka, sells a high-performance computer server to a software startup in Mangalore, Karnataka. The transaction details are:
- **Base Value of Server:** ₹2,00,000
- **Statutory GST Rate:** 18%
- **Transaction Type:** Intra-State (within Karnataka)
Because the buyer and seller are located in the same state, the dual-GST rule applies, splitting the 18% tax base into 9% CGST and 9% SGST:
- Calculate CGST: Base Value × 9% = ₹2,00,000 × 0.09 = ₹18,000
- Calculate SGST: Base Value × 9% = ₹2,00,000 × 0.09 = ₹18,000
- Total GST Amount Collected: ₹18,000 (CGST) + ₹18,000 (SGST) = ₹36,000
- Total Invoice Amount: Base Value + CGST + SGST = ₹2,00,000 + ₹18,000 + ₹18,000 = ₹2,36,000
The Transaction Invoice: The startup pays Arohan Electronics ₹2,36,000. Arohan Electronics files their GSTR-1 returns, paying ₹18,000 directly to the Central Government under CGST, and ₹18,000 to the Karnataka State Government under SGST. The startup, if registered under GST, can claim this ₹36,000 as Input Tax Credit (ITC) to offset their future outward liabilities. Check other business costs using our CTC breakup guide.
Worked Example #2: Inter-State Transaction Math
Now, let's examine the math for an inter-state sale. Aarav IT Consultancy based in Gurgaon, Haryana, provides corporate consulting services to a client headquartered in Mumbai, Maharashtra. The transaction details are:
- **Consulting Base Value:** ₹5,00,000
- **Statutory GST Rate:** 18% (Standard Consulting)
- **Transaction Type:** Inter-State (Haryana to Maharashtra)
Because the transaction crosses state lines, a single Integrated GST (IGST) is charged at the full 18% rate:
- Calculate IGST: Base Value × 18% = ₹5,00,000 × 0.18 = ₹90,000
- Total Invoice Amount: Base Value + IGST = ₹5,00,000 + ₹90,000 = ₹5,90,000
The Result: The client in Mumbai pays ₹5,90,000. Aarav IT Consultancy deposits the entire ₹90,000 IGST with the Central Treasury. Through clearing mechanics, the Central Government allocates Maharashtra's share of the tax to the Maharashtra State Treasury, as Maharashtra is the destination state where the service is consumed. Learn how to map freelance billing alongside this using our freelance hourly billing guide.
Input Tax Credit (ITC) Rules and Offset Order
One of the greatest benefits of the GST regime is the seamless flow of **Input Tax Credit (ITC)**. ITC allows businesses to deduct the GST they pay on business purchases (inputs) from the GST they collect on sales (outputs). However, you must follow strict statutory rules regarding the offset priority. You cannot adjust any credit against any liability. The mandatory offset order is:
- IGST Credit: Must be used first to offset IGST liability. Any remaining IGST credit can be used to offset CGST or SGST liabilities in any order or proportion.
- CGST Credit: Must be used first to offset CGST liability. Any remaining CGST credit can be used to offset IGST liability. **CGST credit CANNOT be used to offset SGST liability!**
- SGST Credit: Must be used first to offset SGST liability. Any remaining SGST credit can be used to offset IGST liability. **SGST credit CANNOT be used to offset CGST liability!**
Reverse Charge Mechanism (RCM) Rules
Under standard GST rules, the supplier collects tax from the buyer and deposits it with the government. However, under the **Reverse Charge Mechanism (RCM)**, the liability to pay tax shifts entirely to the **recipient of the goods or services**. Key instances where RCM applies include:
- Services provided by GTAs (Goods Transport Agencies) to registered businesses.
- Legal service fees paid to advocates or law firms.
- Sponsorship services received by a body corporate.
- Services provided by the Central/State Government to business entities (excluding rent, post, and transport).
- Services provided by an individual director to a company.
RCM tax must be paid in cash through the electronic cash ledger; you cannot use accumulated ITC to pay RCM liabilities. However, once paid in cash, you can claim the paid RCM amount as ITC in the same month!