A worked example: 20% off ₹1,500
A ₹1,500 item with a 20% discount saves ₹300 (₹1,500 × 20 ÷ 100), bringing the final price to ₹1,200 (₹1,500 − ₹300).
The formula: discount amount, then final price
Discount amount = original price × discount% ÷ 100. Final price = original price − discount amount. Two simple steps, but the order matters for what comes next if a second discount is applied.
Why 20% + 10% off doesn't mean 30% off
Stacking a second discount on top of an already-discounted price compounds rather than adds. Taking the ₹1,200 final price from the first 20% discount and applying an additional 10% off saves a further ₹120, landing at ₹1,080 — a combined saving of ₹420 off the original ₹1,500, which is 28%, not the 30% that simply adding 20% + 10% would suggest. Each discount percentage applies to whatever price it's actually offered against, not the original price.
Where GST fits relative to a discount
In India, GST is normally calculated on the discounted (final) price, not the original MRP, when a discount is shown on the invoice — so a discount calculation typically comes first, with GST applied to its output afterward.
The two other retail-pricing calculations this doesn't answer
A discount calculation starts from a price a seller already has and reduces it. It doesn't answer how that starting price was set in the first place from a known cost — that is the separate job of the markup and margin calculators, which build a selling price upward from a cost price instead of reducing an existing one.