How to Calculate a Discount

The one-step method

Rather than calculating the discount and subtracting it, multiply by the percentage you actually pay. It is faster and less error-prone.

• Sale price = original price × (1 − discount ÷ 100)
• Example: 250 with 30% off → 250 × 0.70 = 175
• Saving = original − sale price = 75

Stacked discounts are not additive

If a shop offers 20% off and then a further 10% off, the total discount is not 30%. The second percentage applies to the already-reduced price.

• 100 with 20% off → 80
• 80 with a further 10% off → 72
• Total discount: 28%, not 30%

Multiply the complements: 0.80 × 0.90 = 0.72, so you pay 72% of the original price.

Discounts plus tax

Tax is charged on the discounted price, not the original one. Apply the discount first, then add tax.

Example: 200 with 25% off is 150; adding 16% tax gives 174, not 208.

Working backwards to the original price

If you know the sale price and the discount, divide by the complement to recover the original.

• Original = sale price ÷ (1 − discount ÷ 100)
• Example: a 72 sale price after 28% total discount → 72 ÷ 0.72 = 100

Spotting a genuine bargain

• Compare against the price you would have paid elsewhere, not against the "was" price.
• Check the unit price — larger packs are not always cheaper per unit.
• Include delivery and returns costs in the comparison.