How to Calculate Discounts — and See Through Sale Tricks

Sale season rewards the shopper who can do the math. Here's the 5-second method, plus the pricing games stores hope you won't notice.

The 5-second method

Don't compute the discount and subtract — compute what you pay. 30% off means you pay 70%: price × 0.70, one step. An item at 2,400 → 2,400 × 0.7 = 1,680. For mental math, anchor on 10%: 10% of 2,400 is 240, so 30% is 720, pay 1,680. Same answer, no calculator.

Why "30% + extra 10%" is not 40%

Stacked discounts multiply, they don't add. The extra 10% applies to the already reduced price: 2,400 → 1,680 → 1,512. That's a true discount of 37%, not 40%. The difference looks small on one item and becomes real money on a big cart — and it's precisely why stores advertise stacked offers instead of a single honest percentage.

Four pricing tricks to watch for

• The inflated "original" price. The tag says 50% off 4,000 — but the item sold at 2,400 last month. Judge the final price, not the percentage.
• Charm pricing: 1,999 reads as "one thousand something." Round it up in your head before deciding.
• "Up to 70% off": the "up to" items are usually a handful of leftovers; the rack average is far lower.
• Shrinking pack sizes: the price stays, the grams drop. Compare per-unit price (price ÷ weight), not pack price.

Tax after discount

In most places sales tax or VAT is charged on the discounted price: a 1,512 item with 5% tax rings up at 1,587.60. If a store charges tax on the pre-discount price, that's worth questioning at the counter.

Comparing offer types on unit price

Shops deliberately present offers in formats that resist comparison. Convert everything to price per unit and the ranking becomes obvious.

On a 500 item: 30% off is 350 for one. Buy one get one free is 500 for two, so 250 each. Buy two get one free is 1,000 for three, or 333 each. Buy one get one half price is 750 for two, or 375 each — the weakest, despite sounding generous. Order of value: BOGOF, 3-for-2, 30% off, then BOGO-half-price.

The caveat that undoes all of it: multi-buy only saves money on things you would have bought anyway. Two units you didn't need at 250 each is 500 spent, not 250 saved.

Working backwards from a sale price

To check whether an advertised "was" price is real, recover it: original = sale price ÷ (1 − discount). An item at 1,200 marked 40% off implies an original of 1,200 ÷ 0.60 = 2,000. If you remember it sitting at 1,500 last month, the discount is theatre.

The same formula tells you what discount you are really getting when both figures are shown: 1 − (sale ÷ original). A drop from 1,800 to 1,200 is 1 − 0.667 = 33%, whatever the sign claims.

Clearance timing and price cycles

Discount depth is rarely random. Most retail categories run predictable cycles: seasonal clothing drops hardest 6–8 weeks after a season starts and again at end-of-season; electronics fall when a successor model is announced, not on holidays; furniture and appliances tend to move on public-holiday weekends.

Two practical habits. First, note the price before you want the item — a price you have watched for a month is one you can judge. Second, treat "limited time" as the pressure tactic it usually is; the same offer very often returns. The discipline that saves the most money is simply refusing to let a percentage decide whether you needed something.

Frequently asked questions