Meal deals, multibuys and BOGOFs: how the maths really works
Promotions are the part of grocery pricing designed to be felt rather than calculated. A big round number — "£10 for 10 items!", "3 for £12!", "buy one get one FREE" — does the persuading, while the till quietly does whatever it does. But every promotion is just arithmetic, and arithmetic can be checked. Here's how the three big types actually work, where the traps are, and why BasketBadger simulates them against your real basket instead of taking the poster's word for it.
Meal deals: the fixed-price bundle
A meal deal charges one flat price for a set combination — the classic is a main, a snack and a drink for a set sum, and supermarkets run bigger cousins for family dinners: a main, two sides and a dessert for £12, say. The maths is simple: add up what your chosen items would cost separately, and compare with the bundle price. The catch is that the saving depends entirely on which items you pick.
Worked example. A "dine in for £12" deal. Pick the premium main (£7.50), the premium sides (£3.25 each) and the premium dessert (£4.50) and the shelf total is £18.50 — the deal saves you £6.50. Pick the standard main (£4.00), standard sides (£2.00 each) and a £2.50 dessert and the shelf total is £10.50 — the "deal" now charges you £1.50 more than the items cost separately. Same deal, same shelf, opposite outcomes. Meal deals reward choosing from the top of the eligible range and quietly tax choosing from the bottom.
There's a second tripwire: the partial bundle. If your basket contains two of the three components, the deal hasn't triggered at all — you've paid full price and received nothing. Sometimes adding the third item costs £2 and unlocks £4 of discount, which is a genuine win; sometimes the third item is something your household will throw away, which makes it a £2 donation to the store's promotion budget. BasketBadger's missed-offer nudge exists for exactly this line — it only fires when the net maths, shown in full, is in your favour on items you'd plausibly use.
Multibuys: "3 for £10" and friends
A multibuy fixes the price for a group: any 3 for £10, 2 for £7, mix and match. The good version spreads evenly — every item effectively costs the group price divided by the count. The trap version is the mixed basket: if the deal is "any 3 for £10" across items priced £4.20, £4.20 and £2.10, the shelf total is £10.50 and you save a grand total of 50p, while the poster implied a third off. Worse is buying up to the deal: needing one £4 item and leaving with three because "it's basically free" turns a £4 shop into a £10 shop. The second item is only a saving if it replaces a future purchase — if it expires in the fridge, it's waste with good branding.
The subtler multibuy is the threshold offer: spend £60, get £6 off; spend £80, get £10 off. These are genuinely valuable when your basket already clears the line — free money on shopping you were doing anyway. They become expensive the moment you pad the basket with £9 of unneeded items to unlock £6 of discount. Rule of thumb: a threshold discount is worth at most the discount itself, and padding is worth it only when you'd buy the padding anyway, soon, at that store, at that price. Four conditions is a lot — which is why most padding fails.
BOGOF and "cheapest free"
Buy-one-get-one-free is the easiest maths and the sneakiest. On identical items it's a true 50% cut — if you use both. The fine print variants matter: "cheapest item free" across a range sounds like BOGOF but isn't. Buy a £6 cheese and a £2 cheese and the free one is the £2 cheese: you've spent £6 for £8 of goods, a 25% saving, not 50%. And on perishables, a BOGOF doubles your stock of something with a use-by date; two lettuces, one eaten, one binned is just an expensive lettuce.
Also watch the shelf price underneath. A product that sits at £2.00 all year and runs BOGOF in December is, fairly, £1 each in December. A product quietly repriced to £2.60 in November and then put on BOGOF is £1.30 each — a discount off a price nobody ever really paid. That's why our fourth promise exists: a deal has to actually be a deal, judged against what the item cost before the poster went up.
How BasketBadger handles all this
When the app compares your basket, it doesn't ask "does this store have offers?" — it runs them. For each store, the engine works out the cheapest legitimate way that store's promotions apply to your exact items and quantities: which items group into the meal deal, whether the multibuy groups help or hurt, what the threshold does at your basket size, and what the total is after all of it. Then it shows its working — tap any line and you can see the deal that was applied to it. Two stores with identical shelf prices regularly finish pounds apart once the promotion maths runs, and a store with higher shelf prices occasionally wins outright. The poster never tells you that. The total does.
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