GST Calculator Guide: CGST, SGST, and IGST Explained

Goods and Services Tax (GST) touches almost every invoice, bill, and price tag in India. Whether you run a shop, freelance as a consultant, or manage accounts for a small business, you need to know how CGST, SGST, and IGST split — and how to add or remove tax from a price correctly.

This guide explains GST components with plain examples and points you to the free GST Calculator and GSTIN Validator in Desi Tools.

GST in one paragraph

GST replaced many indirect taxes with a single system. Registered businesses charge GST on taxable supplies, file returns, and pass tax to the government. Rates are commonly 5%, 12%, 18%, and 28% depending on the product or service (with exemptions and special rates for some items).

For daily billing, you care about two things:

1. Which tax applies — intra-state (CGST + SGST) or inter-state (IGST)
2. Whether your price is exclusive or inclusive of GST

CGST, SGST, and IGST

Intra-state supply (within the same state)

When seller and buyer are in the same state, tax is divided equally:

Example: ₹10,000 taxable value at 18% GST intra-state:

• CGST = 9% = ₹900
• SGST = 9% = ₹900
• Total GST = ₹1,800
• Invoice total = ₹11,800

Inter-state supply (across states)

When seller and buyer are in different states, IGST applies at the full rate (no CGST/SGST split on the same invoice line).

Example: Same ₹10,000 at 18% inter-state:

• IGST = ₹1,800
• Invoice total = ₹11,800

The total tax is the same; only the labels and routing between centre and states differ. Your invoice must show the correct type for compliance.

Exclusive vs inclusive pricing

Tax-exclusive (GST added on top)

Common in B2B quotes: you state base price, then add GST.

GST = Base × Rate ÷ 100
Total = Base + GST

₹1,000 + 18% GST:

• GST = ₹180
• Total = ₹1,180

Use the GST Calculator in “exclusive” mode to avoid manual errors.

Tax-inclusive (GST already in the MRP)

Retail shelves often show inclusive prices. To find base and tax:

Base = Inclusive × 100 ÷ (100 + Rate)
GST = Inclusive − Base

₹1,180 inclusive at 18%:

• Base = 1,180 × 100 ÷ 118 = ₹1,000
• GST = ₹180

Getting this wrong skews your margin reports. Shopkeepers use inclusive mode daily.

GST rate slabs (common examples)

Exact classification depends on HSN/SAC codes. Use HSN Code Lookup when you are unsure which rate applies to a product category.

Worked example: Kirana shop bill

A customer buys goods worth ₹2,500 (exclusive) at mixed rates:

• Items at 5%: ₹1,000 → GST ₹50
• Items at 18%: ₹1,500 → GST ₹270
• Total GST = ₹320
• Bill total = ₹2,820

Intra-state invoice shows CGST ₹160 + SGST ₹160 (or line-wise splits if your software supports it).

For a single-rate quick bill, the GST Calculator is faster than a spreadsheet.

Worked example: Online sale to another state

You ship from Maharashtra to Karnataka. Taxable value ₹5,000 at 12%:

• IGST = ₹600
• Total = ₹5,600

E-commerce and courier invoices must show IGST for inter-state B2C/B2B supplies correctly.

GSTIN: what to check

A GST Identification Number (GSTIN) is 15 characters:

• First 2 digits: state code
• Next 10: PAN-based entity id
• Remaining: entity number, Z, checksum

Before trusting a new vendor, validate format with the GSTIN Validator. Offline checks catch typos and invalid checksums. For registration status on the government portal, you still need an online lookup.

Composition scheme vs regular GST

Small taxpayers may opt for the composition scheme (limits and rules apply). They pay tax at a fixed turnover percentage and face restrictions on interstate sales and input tax credit. If you are on composition, your invoices and rates differ from regular GST — confirm with your CA.

This guide focuses on standard rate calculation; scheme-specific rules are outside scope.

Input Tax Credit (ITC) in brief

Registered businesses can often offset GST paid on purchases against GST collected on sales. ITC rules have conditions (valid invoice, goods received, not blocked credits). Calculation of output tax on a bill is still CGST/SGST/IGST as above; ITC is a return-time adjustment.

For product costing with tax and margin together, try Product Price Estimator in Desi Tools.

Common GST calculation mistakes

1. Adding 18% to an already inclusive price — Double taxation on paper.
2. Using CGST+SGST on inter-state invoices — Should be IGST.
3. Wrong rate for the HSN — Rate follows classification, not guesswork.
4. Rounding per line vs per invoice — Match your billing software’s method for reconciliations.

Tools for daily GST work

Desi Tools runs the GST calculator offline on your phone — handy at the counter when Wi‑Fi is weak. Free on Google Play and the App Store, no signup.

Compliance reminder

Calculator output helps with billing and estimates. Filing GSTR-1, GSTR-3B, and e-invoicing must follow current government rules and deadlines. When in doubt, consult a chartered accountant or GST practitioner.

Try it now

Open the GST Calculator and test both modes:

• ₹5,000 exclusive at 18% intra-state
• ₹5,900 inclusive at 18% — extract base and tax

Download Desi Tools for offline GST, GSTIN validation, HSN lookup, and 85+ more India utilities.

Read next: How to create a UPI QR code for your shop — pair GST billing with digital payments.