Compare buy 2 get 1 free deals against other discounts.
A buy-two-get-one-free offer is a 33% discount only when your basket fills whole sets of three. Buy four items and you pay for three, which is a 25% discount. Counting complete sets and the leftover items gives the real amount paid, and turning that into a percentage lets you compare the offer against a plain percentage-off promotion.
Effective discount on a multi-buy
Items paid = complete sets x paid per set + min(remainder, paid per set); effective discount = (1 - items paid / items wanted) x 100
Only when the quantity you buy is a multiple of three. Any leftover items are charged in full, so the effective discount falls below 33% for baskets that do not fill complete sets.
Round the basket up to a whole number of sets, provided you will use the extra item. Buying an item you do not need to unlock a free one is not a saving.
It depends on quantity, which is what the comparison here resolves: at six items the multi-buy gives 33%, but at four it gives 25% and the coupon wins.