How to Calculate GST in India — CGST, SGST, IGST Explained

Goods and Services Tax (GST) is India's unified indirect tax on the supply of goods and services. Whether you are invoicing a client, checking a restaurant bill, or filing returns, you need to know how GST is calculated — and whether CGST, SGST, or IGST applies. This guide walks through the basics with formulas you can use immediately.

What is GST?

GST replaced many central and state indirect taxes (like excise, service tax, and VAT) with one system. Registered businesses charge GST on taxable supplies, collect it from customers, and pass it to the government after claiming input tax credit. The rate depends on the product or service — not every item is taxed at 18%.

CGST + SGST vs IGST

India splits GST into components based on where the seller and buyer are located:

• Intra-state supply (same state): GST is split equally into CGST (Central GST) and SGST (State GST). If the rate is 18%, you pay 9% CGST + 9% SGST.
• Inter-state supply (different states): a single IGST (Integrated GST) applies at the full rate — 18% IGST, not 9% + 9%.

Example: a Mumbai seller ships to a Pune buyer (both Maharashtra) → 18% = 9% CGST + 9% SGST. The same seller ships to a Chennai buyer → 18% IGST.

Standard GST rates

Common GST rate slabs in India:

• 5% — essentials like packaged food items, transport services (some categories)
• 12% — processed foods, business-class air travel, some construction inputs
• 18% — most services, electronics, soaps, capital goods (the most common rate)
• 28% — luxury items, automobiles, aerated drinks, tobacco products (often with cess)

Some items are zero-rated or exempt (fresh vegetables, educational services in many cases). Always confirm the HSN/SAC code for your specific product.

Forward GST formula

To add GST to a base (exclusive) amount:

GST Amount = Base Amount × (Rate ÷ 100)

Total = Base Amount + GST Amount

Example: Base ₹10,000 at 18% → GST = 10,000 × 0.18 = ₹1,800. Total = ₹11,800. For intra-state, CGST = ₹900 and SGST = ₹900.

Reverse GST formula (GST-inclusive price)

When the price already includes GST and you need the base value:

Base Amount = Inclusive Price ÷ (1 + Rate ÷ 100)

GST Amount = Inclusive Price − Base Amount

Example: MRP ₹11,800 inclusive of 18% GST → Base = 11,800 ÷ 1.18 = ₹10,000. GST component = ₹1,800.

Practical tips

• On invoices, show base value, GST rate, CGST/SGST or IGST separately, and gross total — GST law requires clear breakup.
• Input tax credit lets you offset GST paid on purchases against GST collected on sales; net payable is the difference.
• Composition scheme dealers charge GST at a lower effective rate but generally cannot pass full input credit — calculations differ.

For everyday price checks and invoice drafting, you do not need a spreadsheet. A GST calculator handles forward and reverse math, splits CGST/SGST for intra-state, and shows IGST for inter-state — all in your browser.