The problem with price per unit food comparisons

Price per unit is a great way to find the best value at the supermarket – but it can also be frustrating and potentially misleading.

Logic says anything in the supermarket labelled “Value pack” or part of a special offer is going to be the cheapest option. Sadly that’s not always the case, which is why I’ll often recommend using the price per unit to compare exactly how much you’re paying. 

But at the same time, inconsistencies can make that comparison difficult. Call me cynical, but as you’ll see from the list below, I can’t see many decent justifications for not making it simpler – other than a deliberate attempt to confuse shoppers.

How price per unit helps you find the best value item

We generally use the same standard units in UK supermarkets. Weight is always in grams or kilograms – no need to worry about pounds and ounces. Volume is almost always in millilitres and litres.

Simply by checking the shelf label in the aisles or the slightly smaller print online you can quickly see just how much the pack your buying costs per standardised unit – normally something like per 100g or per 100ml.

So there’s no need to work out in your head whether the 1.25kg pack at £2.60 is actually better value than two 500g packs at £2.15. The maths has been done and is displayed right there below the price. (It’s actually the bigger pack at £2.08 per 100g vs £2.15 per 100g).

And with an increase of shrinkflation, where brands make pack sizes smaller but keep the prices the same (or just have a load of dead space in the box), you’re also better able to spot when the pack that looks like it’s a litre is now 875ml.

It really is a great tool – possibly the best one out there – to help you get as much or as little as you need at the supermarket for the lowest price. Great.

When price per unit doesn’t work

But sadly it’s not perfect. In fact, as useful as price per unit is, it can at times be misleading and confusing.

To help you avoid getting caught out, I’ve compiled this list showing where you need to watch out, and a few tools to help you get the right numbers.

Comparing special offers

This is the biggest fail by supermarkets with price per unit. More often than not the shelf label isn’t updated to reflect special offer pricing. And this makes it tricky to work out the cheaper of two similar products on different offers.

I’ve found this online too. In the past I’ve noticed the selling price has been lowered but the old price per unit hadn’t been changed. So watch out!

The app MySupermarket can help here though as it handily displays both the normal price per unit and the special offer price per unit. So you can see exactly what the different the deal makes.

Comparing conflicting units

From time to time you’ll see different measures used for similar items.

A good example is mayonnaise. At Tesco you can currently buy an 600g jar of Hellmann’s mayo for £2. There’s also a 750ml pack of mayo for £2 – also made by Hellmann’s. So which is best value? Well there’s no point checking the price per unit as the first is given per 100g and the second per 100ml.

I’ve also seen milk priced per pint next to milk priced per litre. Again, not a like-for-like comparison.

There’s no real reason for the differing units, so it feels like a deliberate attempt to confuse shoppers.

Not comparing like with like

Some items are similar but not the same, so comparing price per unit doesn’t really help. I’ll give you an example.

Apart from quality, toilet and kitchen rolls are pretty much the same right? So comparing price per roll seems a fair enough. Except it’s often pretty pointless. What you can’t always tell from the packaging is how many sheets are on the roll. So a pack at 45p a roll might actually be better value than one priced at 35p a roll.

However price per roll is still useful when comparing the same brand in different sizes.

Comparing processed vs non-processed

We recently needed some pistachios for a recipe. The nuts still in their shells were far cheaper at £1.50 per 100g than those that had already been shelled, at £2.86 per 100g. Of course, the price per 100g also includes the weight of the shells. But how much?

We bought the cheaper pack of nuts in shells and I weighed out 100g. Then I removed the shells. That left me with 40g of edible pistachio. So the real price per 100g was £3.75 – almost a quid more than the other pack.

It similar with meats. OK, two packs of chicken breasts might look the same, but the unknown ingredient is how much water has been added in. You might actually be getting more meat for your money with the pricier option once it’s been cooked.

Obviously you’re unlikely to be able to work this out in the shops, making the price per unit figured not much use. At best you can use them to guess.

Comparing confusing units

This problem occurs when one pack is labelled, for example, as price per 100 grams, while the neighbouring pack is in kilograms. Ok, so you only need to add a zero to the price per 100 grams to make them comparable, but really this shouldn’t happen.

You see this with volume too, with an item priced per 100ml next to a similar item priced per litre.

Comparing price per items rather than weight

Let’s say you want to buy some bananas. You’ve a choice between loose or prepackaged. To find out which is cheapest I’d check the price per unit. But more often than not, the loose bananas are by priced by weight and the prepackaged bundles by the number of bananas. This leads to the frustrating “price per item”.

This is pretty common across fruit and veg. Sadly the only way to work out for sure is to weigh the single, loose item to find out how much it costs. This will give you a comparable price per unit. But it is a bit of a faff – I certainly wouldn’t bother.But that could well mean you’re paying more than you need to for the practically the same item. And there’s no rule to say loose is always cheaper or visa versa.

Elsewhere it’s more difficult to compare when this happens. You sometimes see price per biscuit, while eggs are all per egg, regardless of size.

11 thoughts on “The problem with price per unit food comparisons

  1. This is an interesting and relevant article. Supermarkets clearly do this to confuse us in paying more. That is why I always try and buy fresh produce when it is reduced with a yellow sticker as much as possible unless I really need the item

    1. Thanks Joy

  2. I dislike that supermarkets put varying standards. I find it deceptive.

  3. I tend to notice that checking the same products on the tesco/asda/sainsburys website usually has more consistent “units” than the labels on the shelves. Not sure if anything regulates it but that’s what I fact check if something in the shop is hard to compare. Always check if the deal really is the same as on the shelf tho.

  4. I guess sometimes this kind of thing cannot be helped, but other times it’s supermarkets trying to confuse their customers, plain and simple.

    The other week in my local Morrisons they had 12-pack Cadbury Cream Eggs for £5 and right next to them 5-packs for £2.

    On the shelf labels, Morrisons had helpfully broken down the units;

    The 12 packs were priced at 41.7 pence per egg.

    The 5 packs… well those were £1.02 per 100g.

    Now if that wasn’t intentional to hide the fact that the 5 packs were only 40p per egg, then someone in that business is making some very conveniently profitable errors.

  5. Interesting article. I agree that usually the price per unit is the best way to go BUT does not reflect like you say when you get an offer of 2 for £x

    Also loose v packaged for example in Tesco loose onions are cheaper than packaged ones but with carrots the opposite is true.

    1. Re the veg. Wouldn’t it just be better if there was consistency. So frustrating when they flip it around

    2. I have used the MySupermarket App for a few years, but it’s not been updated on Android since October 2017 and does feel unloved.

      Using the key board is best described as sticky and the maths it produces is often incorrect.

      Any motivation to get the app updated would be appreciated or perhaps a competitor us needed.

  6. Useful post.
    It can be so easy to get caught out by the inconsistent pricing models at the supermarket. The cynic in me says they intentionally mislead the consumer.
    This has given me a couple of things to look out for.

    1. I know, it feels cynical but what other reasonable explanation is there?


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