Indian GST Calculator Guide 2026 — HSN, Rates, ITC

India's Goods and Services Tax replaced a confusing patchwork of central and state levies in 2017 and has been refined steadily since. For small businesses and freelancers in 2026 it is the single most important tax to understand correctly — get the rate or the HSN code wrong on an invoice and you can lose customers, miss input credit, or invite a notice.

This guide explains how GST works at a working level: enough detail to file correctly, no more. It is not a substitute for a chartered accountant on complex matters, but it covers the cases most small businesses encounter day to day.

What GST replaced

Before 2017, indirect taxation in India was layered. Central excise on manufacturing, service tax on services, VAT on goods (different in every state), entry tax, octroi, luxury tax. A single sale could attract three or four overlapping taxes with no way for businesses downstream to claim credit for taxes paid upstream. GST collapsed all of this into a single tax with input credit flowing through the supply chain.

The four-tax structure

GST is collected as one of four taxes depending on the transaction:

• CGST (Central GST): collected by the central government on intra-state transactions.
• SGST (State GST): collected by the state government on intra-state transactions, at the same rate as CGST.
• IGST (Integrated GST): collected on inter-state transactions and imports, at the combined rate.
• UTGST (Union Territory GST): the SGST equivalent for transactions in union territories without a legislature.

For a typical 18% GST rate, an intra-state sale shows 9% CGST + 9% SGST on the invoice. An inter-state sale shows 18% IGST. The customer pays the same total either way; the split matters only for tax allocation between governments.

The slabs in 2026

GST has multiple rate slabs:

• 0%: essential goods (unprocessed food grains, fresh vegetables, healthcare services, education).
• 5%: items of mass consumption (packaged food, footwear under ₹1000, transport services, small restaurant meals).
• 12%: standard processed goods, business-class travel, mid-range hotels.
• 18%: most services, electronics, household appliances, processed food. The default for most businesses.
• 28%: luxury items, sin goods, high-end vehicles. Often combined with a cess for tobacco, soft drinks, large cars.

Special rates exist for specific items: 0.25% on rough diamonds, 3% on gold and silver, 1.5% on cut diamonds. These don't apply to most businesses.

HSN codes: the classification system

The Harmonized System of Nomenclature is an internationally standardised classification of goods. India uses it (with some local extensions) to identify what GST rate applies to what product. Every taxable good has an HSN code, between 4 and 8 digits long.

For services, India uses SAC (Services Accounting Code) instead. SAC codes start with 99 and are 6 digits long. The same logic applies — every service has a code and a corresponding GST rate.

How many digits you need depends on your turnover:

• Turnover up to ₹5 crore: 4-digit HSN codes are sufficient.
• Turnover above ₹5 crore: 6-digit codes required.
• For exports and imports: 8-digit codes required regardless of turnover.

Input Tax Credit (ITC)

ITC is what makes GST work as a value-added tax instead of a cascading tax. When you buy raw materials or services for your business, you pay GST on them. When you sell your finished product or service, you collect GST from your customer. You only owe the government the difference.

Worked example: a furniture maker buys ₹1,00,000 of timber with 12% GST (₹12,000). They sell finished furniture for ₹2,00,000 with 18% GST (₹36,000). The customer pays ₹2,36,000. The furniture maker owes the government ₹36,000 minus ₹12,000 ITC = ₹24,000. The government effectively only gets tax on the ₹1,00,000 of value the furniture maker added.

For ITC to flow correctly, several conditions must be met. The supplier must have actually filed their GST return (the system cross-checks). The invoice must include valid GSTIN, HSN, and tax breakdown. The goods or services must be used for business (personal expenses don't qualify). And the recipient must claim the credit within the deadline (currently the September return after the financial year of the invoice).

The GSTIN format

Every registered business gets a 15-digit GST Identification Number. The format is: 2 digits state code, 10 digits PAN, 1 digit entity code, 1 default digit (Z), 1 check digit. So a business in Karnataka (state code 29) with PAN ABCDE1234F gets a GSTIN like 29ABCDE1234F1Z5.

Always verify GSTIN on customer invoices using the GST portal's public verification tool. Invalid GSTINs are a common cause of ITC denial during audits.

Calculation: forward and reverse

GST is a percentage of the base amount. Forward calculation: base amount × (1 + GST rate) = total. So an item priced at ₹1,000 plus 18% GST is ₹1,180.

Reverse calculation (when you have the total and want the base): base = total / (1 + GST rate). A total of ₹1,180 inclusive of 18% GST has a base of ₹1,180 / 1.18 = ₹1,000.

These look trivial but are the source of more invoice errors than any other single thing. A common mistake: applying 18% to an inclusive amount and then claiming that as ITC. The taxable value on an inclusive ₹1,180 is ₹1,000, not ₹1,180. The GST is ₹180, not ₹212.40.

Reverse charge

Normally the supplier collects GST and pays it to the government. Under reverse charge, the recipient pays directly. This applies to specific categories: services from unregistered suppliers above a threshold, certain goods like cashew or raw cotton, services from goods transport agencies in specific cases, and imports.

If you receive a service liable to reverse charge, you pay GST yourself and report it as both an output (you're the deemed supplier) and an input (you can claim ITC, subject to the usual conditions). The cash flow is neutral but the paperwork is non-trivial; talk to your CA the first time you encounter it.

E-invoicing

E-invoicing is mandatory in 2026 for businesses above a turnover threshold (currently ₹5 crore, lowered from ₹10 crore in 2023). Below that, regular GST invoices are still acceptable. E-invoices require generation through the Invoice Registration Portal (IRP), which assigns an Invoice Reference Number (IRN) and a QR code. The QR code carries the IRN and key invoice details — anyone scanning it can verify the invoice is registered.

For B2C transactions above ₹500 (and some other categories), a dynamic QR code on the invoice is required even for businesses not subject to mandatory e-invoicing. The QR code lets the customer pay by UPI directly from the invoice.

Composition scheme

Small businesses with turnover up to ₹1.5 crore (₹75 lakh in some special-category states) can opt for the composition scheme. Instead of charging GST at standard rates and claiming ITC, they pay a flat percentage of turnover (1% for traders, 5% for restaurants, 6% for service providers under the composition scheme, with a separate threshold). The trade-off: simpler compliance but no ITC, and inability to charge GST to customers (so customers can't claim ITC either).

For B2B businesses, this trade-off rarely makes sense — your customers will prefer suppliers from whom they can claim ITC. For B2C businesses with low margins and few input costs, composition can save real money.

Common pitfalls

• Wrong rate applied. The same product can attract different rates based on packaging or marketing claims. Verify before using a new HSN.
• Place of supply errors. For services, the rules for inter-state vs intra-state are nuanced. Get this wrong and your customer can't claim ITC.
• Missing the supplier-side filing. If your supplier doesn't file their return, your ITC is denied. Track this monthly via GSTR-2B.
• Late filing. Penalties are ₹100/day per Act (₹200/day combined for CGST + SGST), capped but painful. File on time.

Tooling

For day-to-day calculation, a good calculator is enough. Toolkiya's GST calculatorhandles forward and reverse calculation, slab selection, and inter-state vs intra-state breakdown. It runs in your browser — your invoice values never go to a server, which matters when you're running totals on confidential customer pricing.

For filing returns and claiming ITC, you need either accounting software (Tally, Zoho Books, ClearTax) or a CA. The GST portal supports manual filing but it is tedious for any business with more than a handful of invoices per month.