GST calculations trip people up less because of the math — which is simple once you know the formula — and more because of a common misunderstanding about which tax components actually apply to a given sale. This guide covers the formula, the CGST/SGST/IGST distinction that causes the most confusion, and how to find the correct rate for a specific product.
The GST Calculation Formula
There are two directions to calculate GST, depending on whether your starting amount already includes tax:
- Exclusive (adding GST to a base price): GST Amount = (Original Cost × GST%) ÷ 100. Total = Original + GST.
- Inclusive (extracting GST from a tax-included total): Original Amount = (Total × 100) ÷ (100 + GST%). GST = Total − Original Amount.
CGST + SGST vs. IGST: Never Both
This is the single most common point of confusion with GST. A sale is either intra-state (buyer and seller in the same state), in which case the GST splits into CGST and SGST, each at half the total rate — or it's inter-state (buyer and seller in different states), in which case IGST applies alone at the full rate. These two paths are mutually exclusive. A single transaction never has both CGST+SGST and IGST applied to it.
| Situation | What Applies |
|---|---|
| Supplier's state = buyer's state (place of supply) | CGST + SGST, each half the total GST rate |
| Supplier's state ≠ buyer's state | IGST alone, at the full GST rate |
Example: an 18% GST sale within Maharashtra, with both supplier and buyer registered there, charges 9% CGST + 9% SGST. The identical sale from Maharashtra to a buyer in Karnataka charges 18% IGST instead — not both sets of tax stacked together.
Common GST Rate Slabs
India uses four primary GST rate slabs for most goods and services, though the exact rate for any specific item depends on its classification code rather than a general category.
| Rate | Typical Category |
|---|---|
| 5% | Essential goods, some food items, basic household items |
| 12% | Processed foods, some manufactured goods |
| 18% | Standard rate — most goods and services |
| 28% | Luxury and sin goods |
Finding the Correct Rate: HSN and SAC Codes
GST rates are officially assigned to specific HSN codes (Harmonized System of Nomenclature, for goods) or SAC codes (Services Accounting Code, for services) — not to broad product categories. Two items that seem similar in everyday language can fall into different rate slabs depending on their exact classification. Before invoicing, look up the specific HSN/SAC code for your product or service in the official GST rate schedule rather than assuming a rate based on category alone.
A Worked Example
Suppose a Delhi-based business sells goods worth ₹10,000 (before tax) to a buyer also in Delhi, at an 18% GST rate: GST amount = (10,000 × 18) ÷ 100 = ₹1,800. Since this is intra-state, it splits into 9% CGST (₹900) and 9% SGST (₹900), for a total invoice value of ₹11,800. If the same sale went to a buyer in Punjab instead, the full ₹1,800 would be charged as IGST, with no CGST/SGST split.
Common Mistakes in GST Calculation
- Charging both CGST+SGST and IGST on the same invoice — only one path ever applies.
- Subtracting the percentage directly for inclusive calculations instead of using the correct division formula.
- Assuming a product category always maps to one fixed slab without checking the specific HSN/SAC code.
- Forgetting place of supply rules for certain services, which can differ from where the supplier or buyer is physically located in specific cases.
Reverse Charge Mechanism: A Brief Note
In most transactions, the supplier collects GST from the buyer and deposits it with the government. Under the reverse charge mechanism, this responsibility flips — the buyer calculates and pays the GST directly instead of the supplier. This applies to specific categories of goods and services notified by the government, such as certain services from unregistered suppliers. If you're a business that regularly deals with unregistered vendors or specific notified categories, check whether reverse charge applies before assuming the standard forward-charge calculation covers your situation.
GST on Discounts
When a discount is applied before the sale (shown on the invoice itself), GST is calculated on the discounted price, not the original list price. But a discount given after the sale — such as a post-sale rebate or cashback — generally doesn't reduce the GST already charged on the original invoice unless specific conditions around pre-agreed discount terms are met. This distinction matters for retailers and e-commerce sellers calculating GST on promotional pricing.
Using a GST Calculator
- Enter your amount and GST rate into the GST Calculator.
- Choose exclusive or inclusive depending on whether your amount already includes tax.
- Use the calculator's CGST/SGST/IGST split as a reference, then apply whichever one actually matches your transaction using the table above.
Composition Scheme: A Quick Note for Small Businesses
Small businesses below a certain turnover threshold can opt into the GST Composition Scheme, which lets them pay GST at a lower, fixed rate on their total turnover instead of calculating tax on each individual sale at the standard rate. The trade-off is that composition scheme businesses generally can't charge GST separately on invoices or claim input tax credit on their purchases. This calculator and the formulas in this guide apply to standard GST calculation — if your business is registered under the composition scheme, your GST liability works differently and is worth confirming with a tax professional.
Frequently Asked Questions
Do CGST+SGST and IGST ever both apply to the same sale?
No, never both. A sale is either intra-state (CGST+SGST) or inter-state (IGST alone).
How do I calculate GST if the price already includes tax?
Use Original Amount = (Total × 100) ÷ (100 + GST%), then GST = Total − Original Amount. Don't subtract the percentage directly.
How do I find the correct GST rate for a specific product?
Look up the HSN or SAC code in the official GST rate schedule, since rates are tied to that specific code.