How to Calculate GST for Your Shop — Free Calculator, Invoice & HSN Tools (2026)

If you run a shop in India, you already know GST isn't optional — but the mechanics of it (which rate, which code, CGST or IGST) trip up even experienced shopkeepers. This guide walks through the actual calculation, and links to free tools so you never have to do the math by hand.

Step 1: Find the right HSN code and GST rate

Every product you sell has an HSN (Harmonized System of Nomenclature) code, and that code determines your GST rate — 0%, 5%, 12%, 18%, or 28%. Get this wrong and you either overcharge customers or underpay tax (which shows up in a mismatch during GSTR filing).

Common mistakes: assuming all groceries are 5%, or that all electronics are 18% — rates vary by exact product category. Rice is 5%, but milk is 0%. Mobile phones are 18%, but air conditioners are 28%.

Use our free HSN Code Finder — search any product name and get the exact code and rate instantly, no guessing.

Step 2: Know when it's CGST+SGST vs IGST

This is the part that confuses most first-time GST filers:

• Same state sale (intra-state): Split the GST into CGST (Central) and SGST (State) — each is exactly half the total GST rate.
• Different state sale (inter-state): Charge the full amount as IGST (Integrated GST) instead — no split.

Example: on an 18% GST sale within your own state, that's 9% CGST + 9% SGST. Sell the same item to a customer in another state, and it's 18% IGST, full stop.

Step 3: Calculate the actual amount

Two common scenarios:

Price is GST-exclusive (you know the base price, need to add GST): GST amount = base price × GST rate. Total = base price + GST amount.

Price is GST-inclusive (customer sees a fixed MRP, you need to back out the GST): base price = total ÷ (1 + GST rate). GST amount = total − base price.

Doing this manually for every sale is exactly why shopkeepers make errors at month-end. Our free GST Calculator does both directions instantly, with the CGST/SGST/IGST split built in.

Step 4: Put it on a proper invoice

A GST-compliant invoice needs, at minimum: your GSTIN, the buyer's GSTIN (for B2B sales), a unique invoice number, the date, HSN codes per item, and the GST breakdown. Missing any of these is a common reason invoices get rejected during audits or bounce back at GSTR-1 reconciliation.

Our free Invoice Generator builds this automatically — add line items with their own GST rates, and it computes the totals and formats a print-ready invoice.

Bonus: check where your cash is actually going

GST compliance is one half of running a healthy shop finance — the other half is knowing where your money is stuck. Two quick self-checks worth doing this month:

• Working Capital Calculator — estimates how much cash is trapped in slow-moving stock and unpaid customer credit (udhaar).
• Shrinkage & Cashier Fraud Risk Quiz — 7 quick questions to check if your shop has billing or stock-control gaps that quietly leak money.

Doing this for every sale, every day

These free tools solve the one-off calculation. If you're doing this dozens of times a day, that's exactly what Astra Atlas automates — correct HSN codes, GST rates, and invoice formatting applied automatically on every bill, with GSTR-1/3B-ready exports at month-end.