GST Calculator: CGST, SGST and IGST Explained with Real Invoice Examples

GST arithmetic is simple once you know whether your price is before tax or already includes tax. The difficult part is deciding the correct rate and tax treatment for the specific supply. A calculator can add or reverse the tax cleanly; it cannot classify an item, decide the place of supply, or confirm whether a special notification, exemption, cess, or reverse-charge rule applies.

That distinction matters. A correct formula applied to the wrong rate still produces the wrong invoice. Use the GST Calculator for the arithmetic, then verify the classification and current rule against CBIC or qualified tax advice before issuing an invoice or filing a return.

What GST means on an invoice

Goods and Services Tax is an indirect-tax framework for many supplies of goods and services in India. A registered supplier usually charges tax on a taxable supply, collects it as part of the invoice, and reports it under the applicable GST rules. The customer pays the invoice total, which includes the taxable value plus the GST charged where applicable.

For a typical intra-state supply, the total GST is split into Central GST and State GST. For example, an 18% rate is commonly shown as 9% CGST plus 9% SGST. For a typical inter-state supply, the total rate is usually charged as IGST, so the same 18% appears as 18% IGST. The supplier's location, recipient's location, and place-of-supply rules determine which treatment applies.

The invoice should make the tax treatment understandable: taxable value, rate, tax components, and total payable amount should be clear. That is better for the customer, for internal accounting, and for any later reconciliation.

GST-exclusive pricing: add tax to the base price

GST-exclusive pricing starts with the amount before tax. The basic formula is:

GST = taxable value × GST rate / 100

Invoice total = taxable value + GST

Suppose a consultant quotes a service at ₹50,000 before GST and the applicable rate is 18%. The GST is ₹9,000, so the total invoice is ₹59,000.

For an intra-state supply, the invoice could show:

| Invoice line | Amount | | --- | ---: | | Taxable value | ₹50,000 | | CGST at 9% | ₹4,500 | | SGST at 9% | ₹4,500 | | Total payable | ₹59,000 |

For an inter-state supply at the same rate, the taxable value and total stay the same, but the tax line changes to ₹9,000 IGST. The calculation does not decide whether a supply is intra-state or inter-state; it only shows the arithmetic after you know the treatment.

GST-inclusive pricing: reverse tax correctly

An inclusive price already contains GST. A frequent invoice error is to subtract 18% directly from the total. That is not correct because the 18% was calculated on the base price, not on the final inclusive price.

Use the reverse-GST formula instead:

Taxable value = inclusive amount / (1 + GST rate / 100)

GST = inclusive amount − taxable value

For an inclusive amount of ₹1,18,000 at 18%:

• Taxable value = ₹1,18,000 ÷ 1.18 = ₹1,00,000
• GST = ₹1,18,000 − ₹1,00,000 = ₹18,000

If this is an intra-state transaction, the ₹18,000 is generally split into ₹9,000 CGST and ₹9,000 SGST. If it is inter-state, it is generally listed as ₹18,000 IGST. Use the GST Calculator when you need to test both inclusive and exclusive quotes without doing the reverse calculation manually.

CGST vs SGST vs IGST in plain language

The same total rate can be displayed in two different ways.

CGST and SGST

CGST is the central component and SGST is the state component. When a taxable supply is treated as intra-state, an 18% total rate commonly becomes 9% CGST plus 9% SGST. The customer still pays 18% total tax; the split changes where the tax is accounted for.

IGST

IGST is commonly used for inter-state supplies. At an 18% applicable rate, the invoice shows 18% IGST rather than two 9% components. Place-of-supply rules can be detailed, especially for services, e-commerce, imports, exports, and special cases. Do not select IGST just because the customer has a different billing address without verifying the rule that applies to the transaction.

Common GST rate examples, with an important warning

The calculator's selector uses common 5%, 12%, 18%, and 28% rates. CBIC rate schedules are detailed and classification-specific; an item with a familiar description can have a different rate because of packaging, composition, use, notification changes, or a specific HSN/SAC entry.

| Rate example | Commonly seen context | What to verify first | | --- | --- | --- | | 5% | Some specified goods and services | Exact HSN/SAC, exemptions, and conditions | | 12% | Selected goods or services in notified entries | Current CBIC schedule and description | | 18% | Many standard service and goods examples | Whether a special lower, higher, or exempt treatment applies | | 28% | Selected luxury or demerit goods | Compensation cess and any product-specific notification |

Treat this table as a navigation aid, not a classification chart. The official CBIC schedules and notifications are the authority. If your business sells a recurring set of products or services, document the HSN/SAC logic and have a professional review it rather than relying on a broad internet category.

Who pays GST and what input tax credit changes

On a regular taxable invoice, the supplier charges GST to the customer and then handles the reporting and payment obligations, subject to the rules that apply. Input tax credit, or ITC, may allow an eligible registered business to offset GST paid on qualifying inputs against GST due on outward taxable supplies.

ITC is not simply "GST paid equals GST refunded." Eligibility depends on valid tax documents, supplier reporting, the nature of the purchase, blocked-credit rules, business use, payment and return conditions, and other requirements. The GST Calculator estimates output-tax arithmetic only. It does not calculate ITC eligibility, reconcile a return, or decide a credit claim.

For pricing, this distinction can change how you think about margins. A business selling to GST-registered customers may quote differently from a business selling to end consumers who care about the inclusive final price. Use the Invoice Generator Calculator to model a line-item total, then use the Pricing Strategy Calculator to test whether the pre-tax price still supports the required margin.

Registration thresholds and exceptions

CBIC's GST FAQs describe a common aggregate-turnover threshold of ₹20 lakh, but there are exceptions, state-specific rules, compulsory-registration cases, and changes that can affect whether a business must register. Casual taxable persons, certain inter-state supplies, e-commerce scenarios, reverse-charge obligations, and specific types of suppliers can be treated differently.

Do not use a generic threshold as a decision to ignore registration. Check the current GST portal guidance against your business model, states of operation, customer type, and aggregate turnover. Registration, composition levy, and voluntary registration can all have consequences beyond the calculator's scope.

Five invoice checks before you send a quote

1. Confirm whether your quoted amount is GST-exclusive or GST-inclusive.
2. Verify the HSN or SAC classification and current rate from an official source.
3. Determine whether the supply is intra-state or inter-state under the applicable place-of-supply rules.
4. Separate the taxable value, CGST/SGST or IGST, and total amount on the invoice.
5. Check whether discounts, freight, insurance, packing, cess, reverse charge, or ITC issues change the treatment.

This checklist can prevent the most common math mistakes, but it is not a substitute for accounting records or tax advice. Use it to prepare better questions and retain the documents that support the answer.

Calculate it yourself

• Add or reverse tax with the GST Calculator.
• Build a simple line-item total with the Invoice Generator Calculator.
• Test pre-tax margins with the Pricing Strategy Calculator.

The best GST calculation is transparent about what it knows and what it does not. Get the rate and treatment right first, use the formula second, and make the invoice show the logic clearly enough that both sides can review it.