The GST (Goods and Services Tax) formula calculates the tax amount added to a base price under India's GST system, along with the total tax-inclusive price. It's the standard formula behind every GST-registered invoice, whether for goods, services, or a mix of both.
Formula
GST Amount = Amount × Rate / 100
Total Amount = Amount + GST Amount
| Variable | Meaning |
|---|---|
| Amount | Base price before GST |
| Rate | Applicable GST slab (%), e.g. 5, 12, 18, or 28 |
| GST Amount | Tax charged on the base price |
| Total Amount | Final price including GST |
For intra-state sales, GST splits equally: CGST = SGST = GST Amount ÷ 2
Worked Example
A ₹10,000 service billed at the 18% GST slab:
- GST Amount = 10,000 × 18 / 100 = ₹1,800
- Total Amount = 10,000 + 1,800 = ₹11,800
- CGST = SGST = 1,800 ÷ 2 = ₹900 each
So a customer sees ₹10,000 as the base charge, ₹900 CGST and ₹900 SGST as separate tax line items on the invoice, adding up to a final payable amount of ₹11,800.
Key Things to Know
- The CGST/SGST split only applies to sales within the same state. For an interstate sale, the full ₹1,800 in this example would instead be charged as a single IGST line, not split between two authorities.
- Different products carry different GST slabs, so the same ₹10,000 base amount could produce a very different GST Amount depending on whether it's an essential good (5%), a standard good or service (12–18%), or a luxury item (28%).
- This formula calculates GST on the price exclusive of tax. If you're starting from a GST-inclusive price instead and need to find the base amount, you need the reverse calculation: Base = Total ÷ (1 + Rate/100), not this forward formula.
- Rounding on individual line items can cause small mismatches on invoices with multiple products, since GST is typically calculated and rounded per line item rather than on the invoice total — the GST Calculator rounds to the nearest paisa on each output, matching standard invoicing practice.