Find the final invoice price after both a discount and GST are applied.
The discount is applied first, then GST is calculated on the discounted price — matching how real invoices are raised.
Final price: (Price − Discount) × (1 + GST rate)
When a product is sold at a discount and also carries GST, the order of the two steps changes the final price. This calculator applies the discount first and then charges GST on the reduced price, which is how a discount shown on the invoice is treated under GST. It shows the discount, the price after discount, the GST and the final amount.
Retailers use it during sales to set festive offer prices, buyers use it to check that a shop has charged tax on the right amount, and students use it to practise successive percentages. A shopper in Delhi who buys a ₹3,500 jacket at 20% off wants to know the real bill. The calculator answers: after the discount and the tax, what do I actually pay?
1. Note the original price P, the discount rate d and the GST rate R.
2. Discount amount = P × d ÷ 100.
3. Price after discount = P − discount, or P × (1 − d ÷ 100).
4. GST = price after discount × R ÷ 100.
5. Final price = price after discount + GST, which equals P × (1 − d ÷ 100) × (1 + R ÷ 100).
6. Your saving is the discount amount; the GST saved is R% of that discount.
GST is charged on the transaction value, which is the price actually payable. A discount given before or at the time of supply and shown on the invoice reduces that value. So the tax is calculated on the reduced price, and the buyer saves both the discount and the tax that would have been charged on it. Discounts given later, such as a year-end volume rebate, follow separate conditions and may not reduce the GST on the original invoice.
A discount of d% followed by tax of R% is a multiplication by (1 − d/100) × (1 + R/100). The two steps do not cancel. A 20% discount followed by 5% GST gives 0.8 × 1.05 = 0.84, so the final price is 84% of the original. Because multiplication is commutative, the order of the multipliers does not change the answer mathematically. What changes on real bills is which value the tax is declared on, and that must be the discounted value.
Some goods and services carry no GST. The calculator includes a 0% option for this case, and the final price is then simply the discounted price. It is a useful reminder that the tax saving from a discount depends on the rate. On an 18% item, every ₹100 of discount also saves ₹18 of tax, while on a 5% item it saves only ₹5, and on an exempt item nothing.
Gurpreet in Delhi buys a jacket tagged at ₹3,500 before tax during an end-of-season sale that offers 20% off. The shop bills GST on the jacket at 18%.
Answer: Final price ₹3,304; GST charged ₹504
Charging GST on the original price and then subtracting the discount.
Adding the discount and GST percentages together, for example treating 20% off with 18% GST as 2% off.
Assuming a post-sale cash-back reduces the GST on the original invoice.
Using the GST-inclusive tag price as the pre-tax price.
Setting festive sale prices in a shop.
Checking a bill for a discounted purchase.
Comparing two offers with different discounts and tax rates.
Practising successive percentage problems for exams.
Is GST calculated before or after the discount?
After. When a discount is shown on the invoice at the time of sale, GST is charged on the lower, discounted price — not the original price.