Discount Calculator
Percent off, final price, savings — and stacked discounts done right.
How this is calculated
final = price × (1 − d₁) × (1 − d₂) · stacked discounts multiply, never add
Worked example: R799 at 25% off → you pay R599.25, saving R199.75. Stack a further 10% and you pay R539.33 — an effective 32.5% off, not 35%, because the second discount applies to the already-reduced price. That multiplication is the entire secret of "extra % off" signage.
Stacked discounts do not add up
"30% off, then an extra 20% at the till" is not 50% off. The second discount applies to the already-reduced price: 100 → 70 → 56, which is 44% off. The general rule is to multiply the remaining fractions — 0.7 × 0.8 = 0.56 — and stacked discounts always come out smaller than their sum. It is a legitimate way of advertising, and it is worth being able to do in your head at the till.
The same arithmetic decides the order of a discount and a coupon: applying a percentage discount before a fixed-value voucher is worse for the buyer than applying the voucher first, which is why the terms usually specify the order.
Percentage off versus percentage margin
These are routinely confused and the gap is large. A shop buying at R60 and selling at R100 has a 40% margin and a 66.7% markup — the same two numbers, expressed against different bases. When a discount cuts the price to R80, the margin falls from 40% to 25%, meaning a 20% discount removed 37.5% of the profit. This is why retailers guard discounting so carefully: the price falls a little and the margin falls a lot.
Reading a sale honestly
The reference price is where most sale advertising does its work. A discount from a "recommended retail price" that nobody charged is a discount from a fiction, and consumer-protection rules in many countries require the higher price to have been genuinely charged for a defined period before it can be used as a comparison. South Africa's Consumer Protection Act, the UK's pricing practices guidance and EU rules all address this in different ways.
The practical defence is to ignore the percentage and compare the final price with what you can find elsewhere. A 70%-off sticker on an inflated base is worse value than 15% off a keen price, and the sticker is designed to stop you making that comparison.
Unit price is the comparison that survives
Bulk deals are frequently worse per unit than the standard size, particularly where a "family pack" is priced on convenience. Divide by the quantity — per 100 g, per litre, per item — and compare that number. Many shelf labels are legally required to show it, usually in small print at the corner of the label, and it is the single most reliable defence against a well-designed promotion.
Frequently asked questions
How do I work out a percentage discount?
Multiply the price by the percent and divide by 100: 25% off R799 saves 799 × 0.25 = R199.75, leaving R599.25. The calculator shows both the saving and the final price live.
How do stacked discounts work — 20% off then 10% off?
They multiply, not add: 20% then 10% leaves 0.8 × 0.9 = 0.72 of the price — a 28% total discount, not 30%. Toggle "Stack a second discount" to compute it correctly; the difference is exactly the trick retail signage plays.
Is 50% off then 50% off free?
No — it is 75% off: half of a half is a quarter of the original. Each discount applies to the already-reduced price, which is why stacked percentages never add up the way they read.
How do I find the original price from a sale price?
Divide by (1 − discount): a R599.25 sale price after 25% off was 599.25 ÷ 0.75 = R799. Working backwards is the same trap as VAT — never add the percent back, always divide.