Free GST Calculator for Indian Businesses

India's Goods and Services Tax (GST) replaced more than a dozen indirect taxes in 2017. Every registered business charges GST on its taxable supplies and pays GST on its inputs. This calculator gives you the exact CGST, SGST, and IGST breakdown for any transaction in seconds — no signup, no daily limits, no calls home.

It works for five standard GST rates (0%, 5%, 12%, 18%, 28%), handles both exclusive (you enter the base, we add tax) and inclusive (you enter the total, we extract tax), and correctly splits the tax between CGST + SGST for intra-state supplies or shows the full amount as IGST for inter-state supplies.

Use it for invoice estimation, quotation pricing, vendor bill verification, exam preparation, and the daily back-of-the-envelope checks every Indian business owner does.

The GST Council periodically revises the rate slabs. As of 2026, the official slabs are:

• 0% (Nil-rated) — essential commodities like fresh fruit and vegetables, bread, milk, salt, and books. Also zero-rated exports under LUT.
• 5% — basic mass consumption: edible oil, tea, coffee (not instant), sugar, footwear under ₹1,000, hotel rooms below ₹7,500 per night, small restaurants, and small contractors.
• 12% — processed food, mobile phones, business-class air travel, and some construction services.
• 18% — the default slab for most goods and services: capital goods, IT services, professional services, telecom, banking, software, and the vast majority of B2B supplies.
• 28% — luxury and demerit items: cars, motorcycles above 350cc, air conditioners, refrigerators, televisions above 32 inches, tobacco products, aerated drinks, and cement.

A handful of high-revenue items (petroleum products, electricity, alcohol for human consumption) are kept outside GST and continue under the older excise + state VAT regime.

The base formula is straightforward: GST = base × rate / 100. The only thing you need to decide is whether the amount you have is the base (before tax) or the total (with tax already included).

Example 1 — Exclusive, intra-state

You sell a machine in Maharashtra to a buyer in Maharashtra for ₹1,00,000 plus 18% GST.

• Base = ₹1,00,000
• GST = 1,00,000 × 18 / 100 = ₹18,000
• CGST = SGST = 18,000 / 2 = ₹9,000 each
• Total invoice = ₹1,18,000

Example 2 — Inclusive, intra-state

A retailer quotes a price of ₹1,18,000 inclusive of 18% GST in Maharashtra.

• Base = 1,18,000 / (1 + 0.18) = ₹1,00,000
• GST = 1,18,000 − 1,00,000 = ₹18,000
• CGST = SGST = ₹9,000 each

Example 3 — Exclusive, inter-state

A consultant in Karnataka invoices a client in Tamil Nadu for ₹50,000 plus 18% GST.

• Base = ₹50,000
• IGST = 50,000 × 18 / 100 = ₹9,000
• CGST = SGST = ₹0 (inter-state, so no central/state split)
• Total invoice = ₹59,000

Example 4 — Composition scheme rates

Businesses with turnover below ₹1.5 crore can opt for the composition scheme: 1% for traders, 5% for restaurants, 6% for service providers. The calculator does not directly support these because composition dealers cannot pass on GST to customers — they pay the tax out of their own margin. If you are in the composition scheme, use the calculator on the gross amount with rate 0% and compute your liability separately.

Indian GST is destination-based. The place of supply determines whether CGST + SGST or IGST applies, not the location of your office or your buyer's billing address.

• Goods: the place of supply is generally where the goods are delivered or made available to the buyer.
• Services: for B2B, the place of supply is the buyer's location (registered office). For B2C, it is generally the supplier's location, with carve-outs for specific service categories.
• Exports: place of supply is outside India; treated as inter-state, zero-rated.
• Imports: place of supply is the importer's location; IGST is collected at the customs border.

If you are unsure about a particular transaction, the safest reference is Section 10 (goods) and Sections 12–13 (services) of the IGST Act. For most everyday B2B sales the answer is obvious: if your buyer's GSTIN starts with a different state code from yours, it is inter-state.

Every tax invoice must show GST as a separate line. The CGST Rules specify a minimum set of fields:

• Supplier name, address, and GSTIN.
• Buyer name, address, and GSTIN (or the buyer's state for B2C invoices above ₹50,000).
• HSN code for goods (4-digit for turnover up to ₹5 crore, 6-digit above) or SAC code for services.
• Quantity, unit, and rate.
• Taxable value (the base after any discount).
• CGST, SGST, IGST — each shown separately with the applicable rate.
• Total invoice value in figures and in words.

If you want this done automatically with auto-numbered series, validated GSTINs, and e-Invoice JSON generation, ReadyBooks handles all of it on the free plan.

The mechanics are simple, but small errors creep in regularly:

• Mixing inclusive and exclusive on the same invoice. Decide once for the whole invoice and stick to it. If half your line items are inclusive and half are exclusive, the totals will not reconcile during return filing.
• Using a buyer's billing state instead of the place of supply. A buyer in Delhi receiving a service at their Mumbai branch makes it an intra-Maharashtra supply, not a Delhi inter-state supply. Check the place of supply, not the registered address.
• Forgetting to apply the discount before GST. Under Section 15(3)(a) of the CGST Act, a trade discount shown on the face of the invoice reduces the taxable value. Apply the discount first, then compute GST on the post-discount amount.
• Rounding inconsistently. The GST portal rounds at the invoice level to the nearest rupee for reporting, but line-level CGST and SGST are still recorded to the paisa. If your accounting software disagrees with the GST portal on the rupees-of-tax figure by a paisa, that is the source.
• Composition scheme confusion. If you are a composition dealer, you cannot charge GST on the invoice. You pay 1% (traders), 5% (restaurants), or 6% (service providers) on your turnover, but the invoice you issue shows no GST.

This calculator gives you the GST amount in seconds. Turning it into a real, GST-compliant invoice takes a few more fields: invoice number, customer GSTIN, HSN code, place of supply, payment terms, and a digital signature for amounts above ₹100 crore.

ReadyBooks generates fully compliant tax invoices on the free plan: auto-numbered series with custom prefix and FY suffix, validated GSTINs, HSN code library, e-Invoice JSON for B2B sales above the e-Invoicing threshold, and PDF that matches the GSTR-1 schema. Click Save to ReadyBooks above to start — your selected rate becomes the default tax rate for your account.