GST Calculator India

What the formula says

India's GST runs on slabs rather than a single rate: 0%, 5%, 12%, 18% and 28%, with a compensation cess on some goods above the top slab. Gross = net × (1 + slab rate).

The tax splits by transaction type. An intra-state supply carries CGST and SGST at half the rate each; an inter-state supply carries IGST at the full rate. The total is identical either way. What changes is which government receives it and how the credit flows.

The numbers, worked through

A ₹4,800 net supply at the 18% slab: ₹864 of tax, ₹5,664 gross.

Within the state that is ₹432 CGST and ₹432 SGST. Across state lines it is ₹864 of IGST. The customer pays ₹5,664 in both cases.

The slab matters more than the split. The same product at 5% is ₹5,040 gross and at 28% is ₹6,144: a ₹1,104 spread on one item, which is why classification disputes over which slab applies are among the most common GST litigation in India.

What the number leaves out

Input tax credit depends on the supplier having filed. The system matches invoices between parties through GSTR-2B, and a credit that does not appear there cannot be claimed regardless of the invoice you hold or the payment you made.

Credit also cannot be cross-utilised freely between CGST and SGST, which means a business can hold an unusable credit balance in one head while paying cash in another.

Turning it into a decision

Reconcile purchase records against GSTR-2B every month rather than at filing. A supplier who has not filed can usually be chased in time if the gap is found early and effectively never if it is found at year end.

Then confirm the slab for your specific goods against the rate notifications rather than by analogy. Similar-sounding products sit in different slabs frequently enough that assumption is expensive.

Registration, e-invoicing and the composition scheme

Registration is compulsory above ₹40 lakh of turnover for goods and ₹20 lakh for services, with lower limits in special category states. Inter-state suppliers and ecommerce sellers generally face registration from the first supply regardless of turnover.

E-invoicing, generating an invoice reference number through the government portal before issuing it, applies above a turnover threshold that has been lowered repeatedly since introduction. An invoice that should have carried an IRN and did not is not a valid invoice, which blocks the customer's credit.

The composition scheme offers small taxpayers a flat rate on turnover with minimal filing, at the cost of no input credit and no ability to charge GST to customers. It suits a small business selling to consumers with few taxable inputs, and it is actively unsuitable for anyone selling B2B, their customers get no credit and will buy elsewhere.

A related point: the e-way bill requirement applies to movements of goods above a value threshold and is separate from the invoice itself. Goods moving without a valid e-way bill can be detained regardless of whether the GST position is correct.

It is a logistics compliance step rather than a tax calculation, and it is the one most often missed by businesses new to shipping interstate in India.

Building it into the dispatch process rather than treating it as paperwork avoids consignments being held at checkpoints over a document that takes minutes to generate.