GST Calculation Formula: Add & Reverse GST Explained with Examples

The Goods and Services Tax (GST) is a unified multi-stage tax system used in many countries, most notably India, to simplify tax structures. For business owners, accountants, and consumers alike, understanding how to calculate GST is essential.

Whether you need to add GST to a base price or reverse-calculate a price to remove the tax component, simple mathematical formulas make the process easy. In this guide, we will break down the formulas, explain CGST, SGST, and IGST, and walk through manual examples.

Formula 1: How to Add GST to a Base Price

If you have a product’s base cost and want to calculate the final price after adding tax, use this method:

$$\text{GST Amount} = \text{Base Price} \times \left(\frac{\text{GST Rate}}{100}\right)$$ $$\text{Total Price} = \text{Base Price} + \text{GST Amount}$$

Alternatively, you can compute the total price directly: $$\text{Total Price} = \text{Base Price} \times \left(1 + \frac{\text{GST Rate}}{100}\right)$$

Example: Adding 18% GST

If a service costs ₹10,000 and the GST rate is 18%:

1. Calculate the tax: $\text{₹}10,000 \times 0.18 = \text{₹}1,800$
2. Calculate the total: $\text{₹}10,000 + \text{₹}1,800 = \text{₹}11,800$
3. Result: The customer pays ₹11,800.

Formula 2: How to Remove GST (Reverse GST Calculation)

If a product’s price is already inclusive of GST and you need to find the original base price before taxes, use the reverse calculation formula:

$$\text{Original Base Price} = \frac{\text{Total Inclusive Price}}{1 + \left(\frac{\text{GST Rate}}{100}\right)}$$

Once you have the base price, the GST amount can be found by subtraction: $$\text{GST Amount} = \text{Total Inclusive Price} - \text{Original Base Price}$$

Example: Reversing 12% GST

If you bought an item for a total price of ₹5,600 (inclusive of 12% GST):

1. Apply the formula: $\text{Original Price} = \frac{5,600}{1 + 0.12} = \frac{5,600}{1.12}$
2. Calculate base price: $\text{Original Price} = \text{₹}5,000$
3. Subtract to find the tax: $\text{₹}5,600 - \text{₹}5,000 = \text{₹}600$
4. Result: The base price was ₹5,000 and the tax was ₹600.

Understanding CGST, SGST, and IGST

Under the Indian tax regime, GST is divided to distribute revenue between state and central authorities:

• CGST (Central Goods and Services Tax): Levied by the Central Government on intra-state supply.
• SGST (State Goods and Services Tax): Levied by the State Government on intra-state supply.
  • Rule: For sales inside a state, CGST and SGST rates are equal and divide the total tax. For an 18% GST rate, 9% is CGST and 9% is SGST.
• IGST (Integrated Goods and Services Tax): Levied by the Central Government on inter-state supply (where the seller and buyer are in different states).
  • Rule: The full tax rate (e.g. 18%) is applied as IGST.