Buy one get one calculator for BOGO, buy 2 get 1 and 3 for 2 deals

How to tell which deal is cheaper

The cheaper deal is the one where you spend least to get at least the items you need, not the one with the biggest number on the sign. Every deal is entered as a set of items where some cost less: in buy 2 get 1 free the set is 3 items and the third is free, in BOGO 50% off the set is 2 items and the second is half price. The discount only applies to complete sets, as it does at the register: items left over pay full price.

For each deal the calculator tries buying the items you need and up to one more set, and keeps the lowest spend. When two quantities cost the same it takes more items, since the extra ones are free. Then it ranks the deals from cheapest and flags the ones that only reach their low price per item if you use every item in the set. There is no currency: the price is whatever the shelf tag says.

A worked example: 2 items at 5.00

These are the values the calculator opens with in the US: an item at 5.00, you need 2, and three deals to compare.

Deal	Items to buy	You spend	Price per item
Buy 1 get 1 free (BOGO)	2	5.00	2.50
Buy 2 get 1 free	3	10.00	3.33
Buy 1 get 1 50% off	2	7.50	3.75

BOGO wins: you spend 5.00 instead of 10.00 and save 5.00, 50%. Buy 2 get 1 free makes you take 3 items for the same 10.00 that 2 cost at full price, so on the 2 you need it saves nothing. BOGO 50% off saves 2.50, which is 25% on two items, not 50%.

In UK shops the same calculator in metric opens on 2.50 an item, 2 needed, with 3 for 2, second item half price and 30% off when you buy 2. The 30% off wins at 3.50 against 3.75 for half price and 5.00 for the 3 for 2, which needs a third item you may not use.

Which deal is cheaper for the number of items you need

The same deal is worth more or less depending on how many items you need. Here is what you spend with an item at 5.00; in brackets, the items you end up with when it pays to take more.

Items you need	BOGO free	Buy 2 get 1 free	BOGO 50% off
1	5.00 (2 items)	5.00	5.00
2	5.00	10.00 (3 items)	7.50
3	10.00 (4 items)	10.00	12.50
4	10.00	15.00	15.00
5	15.00 (6 items)	20.00 (6 items)	20.00
6	15.00	20.00	22.50

BOGO free is the cheapest on every line. Between the other two, buy 2 get 1 free wins when you need 3 or 6 items, BOGO 50% off wins at 2, and at 1, 4 or 5 they cost the same.

How to enter the deals in the ad

Each row of the calculator has three numbers: items per deal, how many of those items are discounted, and the discount on them (100% means free).

• Buy 1 get 1 free (BOGO, 2 for 1): 2 items, 1 discounted, 100%. On a full set that is 50% off.
• Buy 2 get 1 free (3 for 2): 3 items, 1 discounted, 100%: 33.3% off. Buy 3 get 1 free: 25% off.
• Buy 1 get 1 50% off: 2 items, 1 discounted, 50%: 25% off.
• Buy 2 get the 3rd half price: 3 items, 1 discounted, 50%: 16.7% off.
• 30% off when you buy 2: 2 items, 2 discounted, 30%: 30% off.

Under the FTC guide on the word "Free", a "Buy 1-Get 1 Free" offer means you pay nothing for the free item and no more than the regular price for the one you buy. Check that regular price before the sale: if it went up for the promotion, the deal is worth less than the calculator says.

Common mistakes

• Looking at the deal's discount instead of what you spend. Buy 2 get 1 free is 33.3% off only if you buy and use 3 items. If you need 2, you spend the same as without the deal.
• Reading BOGO 50% off as 50% off. On two items it is 25% off: you pay for 1.5 items instead of 2.
• Taking extra items you won't use. A free bottle that sits in the pantry until it expires saves nothing. The calculator shows the extra items next to each deal.
• Forgetting that the set must be complete. With buy 2 get 1 free and 4 items, the fourth pays full price: 15.00 at 5.00 each, not 13.33.
• Comparing different regular prices. If one store has the deal on a higher regular price, run two calculations: the deal only wins if the final spend is lower.