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.