Shopping Maths: Deals, Unit Prices and Shrinkflation

Shops are designed to make prices feel simple and deals feel generous. A few quick calculations show you what you are really paying, so you can choose calmly rather than react to a big red sign. (For budgeting and shopping habits, see the Money Basics module.)

Watch the 1-minute overview

Watch the full video guide for this module

Discounts: check the final price, not the headline

• Use a multiplier: 25 percent off means you pay 75 percent, so AED 160 x 0.75 = AED 120 (the multiplier method explained by Tutorful).
• 'Extra 10 percent off sale prices' is applied to the already reduced price. AED 200 with 20 percent off and then an extra 10 percent off is 200 x 0.8 x 0.9 = AED 144.
• That is a total saving of 28 percent, not 30 percent. Stacked discounts multiply; they do not add.
• A discount only saves money if you would have bought the item anyway. Spending AED 120 on something you did not need is not a saving of AED 40.

Multi-buys and 'buy one get one free'

• 'Buy one get one free' is the same as 50 percent off each item, but only if you will actually use both.
• 'Buy 2, get a third free' works out at one third (about 33 percent) off each item. 'Second item half price' is only 25 percent off the pair.
• Multi-buys are not always cheaper. In a case Which? raised with the UK Competition and Markets Authority (CMA), four cans of sweetcorn cost 2 pounds (50p a can) while a six-can multi-buy cost 3.56 pounds, about 59p a can (lovemoney, 2015).
• The CMA found that some supermarket pricing practices could 'confuse or mislead' shoppers, although it said the problems were not widespread (lovemoney, 2015).
• Before you grab a multi-buy, divide the deal price by the number of items and compare it with the single price.

Unit prices: the fairest comparison

• A unit price is the price per standard amount, such as per 100 g or per litre. New Zealand's consumer protection agency says it lets you compare the same item across different sizes and packaging.
• Bigger is not always cheaper: the same agency says buying in bulk can save money, but not always, so check the unit price for each size.
• Worked example: 500 g of rice for AED 12.50 is AED 2.50 per 100 g. 750 g for AED 17.25 is AED 2.30 per 100 g, so the bigger bag is cheaper per gram.
• How to calculate it: divide the price by the weight in grams, then multiply by 100. Or divide by the number of 100 g portions (750 g is 7.5 portions).
• Consumer NZ's tips include checking the unit price of specials, comparing fresh with frozen, and doing your own maths where the label does not help, for example cost per wash for laundry detergent.

Shrinkflation: same price, less product

• The UK Office for National Statistics (ONS) defines shrinkflation as manufacturers reducing the package size of household goods while keeping the price the same.
• The ONS gives the example of Toblerone bars, which shrank by about 10 percent in November 2016. Food and drink saw the most package-size reductions between 2012 and 2017, with sugar, jam, syrups, chocolate and confectionery a notable contributor.
• Worked example: a 200 g pack at AED 10 is AED 5.00 per 100 g. If it shrinks to 180 g at the same price, it becomes about AED 5.56 per 100 g, an 11 percent price rise you might not notice.
• Notice that a 10 percent cut in size gives an 11 percent rise in unit price, because the price is now shared over less product.
• The unit price is your best defence: it changes even when the sticker price does not.

A 30 second routine in any shop

• Look for the unit price on the shelf label; if it is missing, calculate a rough one on your phone.
• For multi-buys, work out the price per item and compare with buying one.
• For 'percent off' signs, multiply to find the final price and ask whether you planned to buy it.
• Check the pack size of products you buy often, so you notice when they shrink.