Black Friday Math: How to Tell If a Deal Is Really a Deal

Black Friday banners shout "Up to 70% off!", "Extra 20% at checkout!" and "Buy one, get one half price". Some of those deals are great; some are ordinary prices in a red font. A few seconds of maths tells them apart.

Shopping bags with sale tags

1. Stacked discounts don't add up

"30% off, plus an extra 20% at checkout" is not 50% off. The second discount applies to the already-reduced price:

$100 − 30% = $70
$70 − 20% = $56 → you save 44%, not 50%.

The quick way: multiply what's left. 0.70 × 0.80 = 0.56, so you pay 56%.

Enter both numbers in the Discount Calculator — it has an Extra discount field for exactly this.

2. Check the "was" price

A "was $129.99, now $89.99" tag looks like a big saving. Work out the real percentage with the Discount Calculator's discount % from two prices mode: 30.8% off.

Then ask whether $129.99 was ever the real price. If an item sat at $99 all autumn, the true Black Friday saving is only 9%. Screenshot prices you're watching from early November so you can compare.

Have only the sale price and the percentage? See How to Find the Original Price Before a Discount.

3. Percent off vs dollars off

"25% off" or "$15 off" — which is better depends on the price:

On an $80 item, 25% is $20 — better than $15.
On a $50 item, 25% is $12.50 — the $15 coupon wins.

The break-even point is where the two are equal: $15 ÷ 0.25 = $60. Above $60, take the percentage. The Percentage Calculator does the "what is 25% of 80" step instantly.

Two shoppers carrying Black Friday bags

4. Buy one, get one half price = 25% off

"BOGO 50%" sounds like half off. It's half off one of two items, so the pair costs 1.5 × the price instead of 2 × — a 25% saving, and only if you wanted two.

"3 for 2" is a 33% saving per item. "Buy 2, get 1 free" is the same thing.

5. Compare price per unit

Multi-buys hide the unit price. "3 for $10" is $3.33 each; if the single item is $3.49, you save 16 cents per item — fine if you'd use all three.

6. Don't forget tax and shipping

In the US, sales tax is added to the discounted price, so a discount also cuts the tax. Shipping isn't discounted, and a $7.99 delivery fee can erase a small saving. Add it before you compare.

A simple Black Friday plan

Make a list now, with today's prices.
Check the dates — the Black Friday countdown shows how long you have.
When deals go live, run each one through the Discount Calculator.
Set a total budget and stick to it.

FAQ

Are Black Friday prices really the lowest of the year? Often, for electronics and big brands — but not always. Some items are cheaper in January sales or on Prime Day.

What is the exact date this year? The day after US Thanksgiving. See the Black Friday countdown.

Try it now

Check any deal with the free Discount Calculator.