Every Indian business owner has faced this moment — an invoice in front of you and a simple question: should this be CGST + SGST or IGST?
It sounds like it should be straightforward. In most cases, once you know the rule, it is. But getting it wrong means wrong tax deposited, ITC issues for your buyer, and potential compliance notices.
This guide walks through everything — clearly, with real numbers.
What is GST? A Quick Refresher
GST — Goods and Services Tax — replaced 17 different indirect taxes in India when it launched on July 1, 2017. The stated goal was one tax, one nation, one market.
For businesses, it means all sales now attract a single GST instead of a combination of VAT, service tax, excise duty, and state levies. GST is collected at each stage of the supply chain, but each business can offset what it paid on inputs (called Input Tax Credit) against what it collects on outputs.
This chain of credits is what eliminates the "tax on tax" problem that plagued India's old system.
Key fact: GST has two levels — central government (CGST/IGST) and state government (SGST). Which component applies depends on whether the transaction crosses a state border.
The 3 GST Components You Must Understand
CGST — Central GST
CGST is collected by the central government. It applies on intrastate transactions — where both the seller and buyer are registered in the same state.
When you see CGST on an invoice, know that this money goes to Delhi (the central government).
SGST — State GST
SGST is collected by your state government. It always accompanies CGST on intrastate transactions. CGST and SGST are always equal to each other — if GST rate is 18%, then CGST = 9% and SGST = 9%.
IGST — Integrated GST
IGST applies on interstate transactions — when buyer and seller are in different states — and on all imports. The central government collects it and then distributes the state's portion to the destination state.
| Transaction Type | Components | Who Gets It | |-----------------|-----------|-------------| | Intrastate (same state) | CGST + SGST | Centre + State | | Interstate (different states) | IGST only | Centre (distributes to state) | | Import | IGST + Customs | Centre | | Export | Zero-rated | No tax collected |
GST Rate Slabs — FY 2025-26
India has 5 main GST rate slabs:
| Rate | What it Covers | Common Examples | |------|---------------|----------------| | 0% | Essential goods, healthcare, education | Fresh vegetables, books, healthcare services, eggs | | 5% | Basic necessities, mass consumption | Packed food, economy hotel rooms, transport | | 12% | Processed goods, specific services | Computers, processed food, business class air travel | | 18% | Most goods and services — standard rate | Restaurants, electronics, software, professional services | | 28% | Luxury, sin goods, automobiles | Cars, tobacco, aerated drinks, luxury hotels |
In addition to these slabs, certain items like cigarettes and some luxury goods attract a GST Cess on top of the 28% rate.
How to Calculate GST — Step by Step
Method 1: Adding GST to a Base Amount
Use this when your quoted price is exclusive of GST and you need the total invoice value.
GST Amount = (Base Amount × GST Rate) ÷ 100
Total = Base Amount + GST Amount
Example — IT Service, Delhi to Delhi (Intrastate):
Rahul's software company in Delhi bills a Delhi client ₹1,00,000 for development work at 18% GST.
GST = (1,00,000 × 18) ÷ 100 = ₹18,000
Since both in Delhi:
CGST @ 9% = ₹9,000
SGST @ 9% = ₹9,000
Invoice Total = ₹1,18,000
Example — Manufacturing, Gujarat to Maharashtra (Interstate):
A manufacturer in Ahmedabad sells goods worth ₹5,00,000 to a buyer in Mumbai at 12% GST.
GST = (5,00,000 × 12) ÷ 100 = ₹60,000
Since different states (Gujarat → Maharashtra):
IGST @ 12% = ₹60,000
Invoice Total = ₹5,60,000
Method 2: Removing GST from an Inclusive Price (Reverse Calculation)
Use this when you have the total price including GST and want to find the base amount and GST component separately.
Base Amount = (Total × 100) ÷ (100 + GST Rate)
GST Amount = Total − Base Amount
Example — Retailer finding base price:
A shopkeeper is selling a refrigerator for ₹35,400 (this price includes 18% GST). What is the base price and GST amount?
Base = (35,400 × 100) ÷ (100 + 18) = 35,40,000 ÷ 118 = ₹30,000
GST = 35,400 − 30,000 = ₹5,400
CGST @ 9% = ₹2,700 | SGST @ 9% = ₹2,700
Intrastate vs Interstate — The Golden Rule
This is where most people get confused. The determining factor is simple:
Is the seller's Place of Supply in the same state as the buyer's?
- Same state → CGST + SGST (each at half the rate)
- Different states → IGST (at full rate)
Special Cases to Know
Branch transfers count as interstate: If your company is registered in Haryana and transfers stock to your branch in Delhi, that is interstate — even though it's the same business. IGST applies.
Place of supply for services is usually buyer's location: A software company in Bengaluru serving a client in Mumbai applies IGST (Karnataka → Maharashtra), even if the work is done entirely in Bengaluru.
Export is zero-rated: Goods and services exported from India attract 0% GST. The exporter can claim a refund of GST paid on inputs.
Most Common GST Calculation Mistakes
Understanding these mistakes can save you from costly corrections later.
Mistake 1: Calculating GST on an Inclusive Price Directly
If a product is ₹1,18,000 (inclusive of 18% GST), the tax is NOT ₹1,18,000 × 18% = ₹21,240. The correct amount using the reverse formula is ₹18,000. The error here can be ₹3,240 — significant at scale.
Mistake 2: Wrong Component on Interstate Supplies
A business that charges CGST + SGST on an interstate supply has deposited tax with the wrong authority. The IGST must be paid and CGST/SGST refunded — a compliance headache.
Mistake 3: Forgetting Cess on 28% Items
Some products — luxury cars, tobacco, aerated drinks — attract GST cess on top of 28%. Not charging cess when required leads to underpayment.
Mistake 4: Treating All Business Expenses as GST-able
Salary payments, bank interest, insurance (in most cases) — these don't attract GST. Including these in your output tax calculations inflates your liability.
Calculate GST Instantly — Free Tool
Rather than doing these calculations manually every time, use the SmartGST Calculator. It handles:
- All 5 GST rate slabs
- Automatic CGST/SGST vs IGST determination based on states
- Both "add GST" and "remove GST" modes
- Downloadable PDF of results
- WhatsApp share
No login. No signup. Completely free.
Frequently Asked Questions
Is GST calculated on MRP or selling price?
GST is always on the transaction value — the actual price agreed between seller and buyer. MRP is a ceiling for consumer products but is not the basis for GST.
Can I claim ITC on all GST I pay?
No. Certain categories of GST payments are blocked under Section 17(5) of the CGST Act — including cars, health insurance, food expenses for employees, and construction of immovable property.
What is the GST rate on services?
Most services are taxed at 18% GST. Exceptions include financial services, transport, healthcare, and education — which have different rates or exemptions. Check the HSN/SAC finder for specific services.
Does a small business need to register for GST?
Registration is mandatory if your annual turnover exceeds ₹20 lakh (₹10 lakh for special category states) for service providers, or ₹40 lakh for goods suppliers. Below these thresholds, registration is optional but may be beneficial for ITC.
Related Tools on SmartGST
Once you've calculated your GST, you may also need:
- GST Invoice Generator — Create professional tax invoices with auto CGST/SGST/IGST calculation
- ITC Calculator — Find your net GST payable after claiming Input Tax Credit
- GST Penalty Calculator — Calculate late filing penalty if you've missed a due date
- HSN Code Finder — Find the right HSN code and GST rate for your product
External References
- Official GST Portal — gst.gov.in — For filing returns and checking official rates
- CBIC GST Rate Finder — Official HSN/SAC rate lookup
- GST Council Notifications — For latest rate change notifications
Last updated: February 2026 | Covers GST rules for FY 2025-26
Disclaimer: This article is for informational purposes. GST rates and rules are subject to change via GST Council notifications. Always verify with gst.gov.in or consult a qualified Chartered Accountant for your specific situation.
