The price of the product is Rs. 1000 and you paid 18% tax i.e. Rs. 180 on it. So the purchase cost is Rs. 1180 for you. You will get Rs. 180 as input credit. Now if you add Rs. 100 as your profit/margin, the product cost becomes Rs. 1100 (1000+100), and you will charge 18% GST from the consumer so the sale price becomes. GST Calculator India for Goods and Services Tax. Rs. 1100 + 198 (18% of 1100) = Rs. 1298.