GST tools and guides

GST registration is mandatory once your aggregate annual turnover crosses the threshold: ₹40 lakh for a business supplying goods in most states (₹20 lakh in the reduced-threshold states and ₹10 lakh in the four special-category states — Manipur, Mizoram, Nagaland, Tripura) and ₹20 lakh for services (₹10 lakh in those four special-category states). Registration is also compulsory — regardless of turnover — if you sell across state lines, sell through an e-commerce operator like Amazon or Flipkart, or fall under the reverse-charge mechanism. Voluntary registration is allowed below the threshold and is common among small businesses that want to claim input tax credit or that sell to larger GST-registered customers who insist on a proper tax invoice.

Once registered, you receive a 15-character GSTIN — the Goods and Services Tax Identification Number — which encodes your state, your PAN, and a registration sequence number. That identifier sits on every invoice you raise and every invoice you receive, and verifying it is the first line of defence against fake invoices and input-credit fraud.

A GSTIN is not a random string — it is structured, and that structure is what makes offline validation possible. The first two digits are the state code (07 for Delhi, 27 for Maharashtra, 29 for Karnataka, and so on, following the same census state codes used elsewhere in Indian administration). The next ten characters are the PAN of the business. The thirteenth character is the entity number for that PAN within the state (how many registrations the same PAN holds in that state). The fourteenth is the letter Z by default. The fifteenth is a checksum character, computed from the first fourteen using a mod-36 algorithm.

Because the final digit is a deterministic checksum of the preceding fourteen, you can catch most typos and many fabricated numbers without ever calling a government server — a mistyped digit will almost always fail the checksum. Our GSTIN Validator does exactly this: it checks the length, the state code, the embedded PAN pattern, and recomputes the checksum, all inside your browser, then points you to the official portal for live active/cancelled status.

GST is a destination-based tax with a dual structure. On a sale within the same state (intra-state), the tax splits into two equal halves: CGST (Central GST) and SGST (State GST). A product taxed at 18% within Maharashtra carries 9% CGST and 9% SGST. On a sale between states (inter-state), a single IGST (Integrated GST) applies at the full rate — 18% IGST on that same product if it ships from Maharashtra to Karnataka.

The split matters for both the buyer and the seller because input tax credit flows along these heads, and because the place of supply determines which state actually collects the revenue. Our GST Calculator handles both cases: enter an amount, choose whether the figure is GST-inclusive or exclusive, pick the rate, and it shows the base, the tax, and the correct CGST/SGST or IGST breakdown — the same arithmetic you would otherwise redo by hand on every invoice.

GST rates in India fall into a small number of slabs. Under the GST 2.0 reform effective 22 September 2025, the structure is 0% (exempt — most unprocessed food, fresh produce), 5% (essentials and mass-consumption items), 18% (the standard slab covering the bulk of goods and services), and 40% (sin and luxury goods such as tobacco, aerated drinks, and high-end vehicles). This reform merged the earlier 12% and 28% slabs into the 5/18/40 structure. Which slab applies depends on the HSN code for goods or the SAC code for services — a classification system that maps every product and service to a rate.

Rates do change: the GST Council meets periodically and revises slabs, so a rate that was right last year may have moved. That is why our rate tools cite the source notification and date, and why any GST calculation should be checked against the current CBIC schedule for anything high-value or unusual. For everyday invoicing in the common 5% and 18% bands, the post-reform slabs are the figures to use.

The GST tools in this hub are built to work as a set. When you receive an invoice from a new supplier, run their GSTIN through the Validator to confirm it is well-formed before you accept it for input credit. When you raise your own invoice, use the GST Calculator to split the tax correctly into CGST/SGST for a local customer or IGST for an out-of-state one. Each tool runs entirely in your browser — nothing you type is uploaded — and each links across to the others so you are never more than a click from the next step.

Everything here is free and needs no login. The tools are informational and are not a substitute for advice from a qualified chartered accountant; for filing, assessment, or anything contentious, confirm against the official GST portal and the latest CBIC notifications, which we link from each tool.