Free second item still has a unit price
Is BOGO a good deal? Only after you divide by the boxes you will actually eat. Buy-one-get-one is 50% off only if you wanted two. Compare that per-ounce number to the size you’d buy anyway.
BOGO is not automatically 50% off
BOGO vs 50% off is not the same math. Half off one box is half off one box. BOGO is full price for the first, $0 for the second — so you only get 50% if both leave the store and both get used. Is BOGO better than half off? Do the unit price. A free second item still has a unit price: shelf price ÷ the ounces you will use, not the ounces they handed you.
The banner is a fake 50% off until that divide wins. Same aisle rule as compare unit price.
Worked example: $5 BOGO cereal vs a 20-oz box already cheaper per ounce
Labeled example, not a current ad. Two brand 12-oz boxes, BOGO at $5. You pay $5, they put 24 oz in the cart.
Labeled example — not a current ad
BOGO, if you eat both: $5.00 ÷ 24 oz ≈ 21¢ per ounce
Store-brand 20-oz, no BOGO: $3.49 ÷ 20 oz ≈ 17¢ per ounce
The “free” box is louder. The 20-oz is cheaper per ounce. Buy one get one unit price lost to the boring box. Read the small type: grocery shelf tags.
If you only wanted one, you paid full price
You needed one box of cereal. BOGO makes you take two. If the second one stales, you paid $5 for 12 oz ≈ 42¢ an ounce. That is not 50% off. That is full price plus a second box you did not want.
Divide by the ones you’ll use. One. Then compare to the 20-oz. The free item is a maybe only if you will finish it before it turns to dust.
Stacking a coupon on BOGO when the store says one coupon per deal
Stores often say one coupon per BOGO, not one per box. A $1 manufacturer coupon on the deal makes it $4 for 24 oz ≈ 17¢/oz — a tie with the 20-oz, not a win, and only if you eat both.
Two manufacturer coupons on one BOGO is two of the same layer. The register will keep one. Only stack when both actually apply. If the coupon excludes BOGO, you have a headline and a full-price first box.
Then the three
Was cereal on this week’s three? Then run the divide. If it was not, BOGO does not hire it. A long list is how impulse wins. Two maybes only if you will use both and the per-ounce number beats the size you’d buy anyway.
Keep the six checks on your phone: save this. Use them on one cart this week.