Which was the cheapest supermarket in September 2021?

Aldi beat rival discounter rival Lidl supermarket to the title of cheapest supermarket in September by just 37p, the latest monthly analysis by Which? has found.

We compared prices for a basket of 22 products. On average, shoppers would have paid £24.03 at Aldi or £24.40 at Lidl.

At the other end of the spectrum, the most expensive supermarket was more than £9 pricier than Aldi for an equivalent basket of groceries.

Read on to find out how Asda, Sainsbury's, Tesco and more fared, and what's going on with grocery shopping habits at the moment.

Cheapest supermarkets vs the most expensive

Every day throughout September, we checked the prices of 22 items - including own-brand products, like eggs, tinned kidney beans and cucumber, and branded goods such as Hovis wholemeal bread, to see how UK supermarkets compared. Here's how much our basket cost on average:

Aldi (Aldi.co.uk) came out cheapest overall at just £24.03, followed by Lidl (Lidl.co.uk).

Meanwhile, Waitrose (Waitrose.com) was the most expensive in September, with the products costing £9.03 extra - that's 38% more - than at Aldi.

Groceries with some of the biggest price differences included Maltesers and own-brand black grapes, which respectively cost £1.51 and £1.11 less at Aldi than at Waitrose.

But price is just one factor when you're deciding which supermarket to shop at. We also survey shoppers about supermarkets' product quality, customer service, store experience, online deliveries and a range of other factors to reveal the best and worst supermarkets each year.

How do bigger shopping lists compare?

We also compared a trolley packed with 79 items (the original 22 plus 57 more).

This was made up of a greater selection of branded items, like Tropicana orange juice and Lurpak spreadable butter, which aren't always available in the discount supermarkets. Therefore, for our bigger trolley we haven't been able to include Aldi or Lidl.

For the 21st month in a row, Asda (Asda.com), at £146.36, was easily the cheapest of the traditional supermarkets for our trolley. It was a huge £21.66 cheaper than the most expensive supermarket, Waitrose.

There was £12.01 between second-cheapest Sainsbury's (Sainsbury's.co.uk)and second-priciest Ocado (Ocado.com).

• Find out more: the grocery brands you should never pay full price for

How Which? compares supermarket prices

We check the prices of hundreds of grocery items at every major supermarket every day throughout the year, using an independent price comparison website.

For our monthly 'cheapest supermarket' stories, we work out the average price for each item at each supermarket across the month, and add them up to get an average trolley price for each store. We include special offer prices but not multibuys, to keep it as fair as possible.

In September, our shopping list combined branded items such as Andrex toilet paper, Colgate toothpaste and McVitie's digestive biscuits with own-label products, including mixed peppers and semi-skimmed milk.

Of course, own-brand items aren't exactly the same at different supermarkets, but we've used experts to ensure that the products are as comparable as possible based on a range of factors, including weight, quality and other industry data.

• Find out more:how to spend less at the supermarket

Why are fewer people buying food online?

Shopping habits have started to show glimpses of changing, as offices and schools reopen.

The average online grocery order is now worth £78.28 - nearly £17 less than its peak at the start of the pandemic, according to figures from market research company Kantar Worldpanel. The market share of online grocery also fell to 12.2% in September from 13% four weeks ago, which is the lowest level since May 2020.

Ready-meals sales, meanwhile, increased by 11% in September.

Yet more change is to come, with 'commuters heading back to the office and the return to school this autumn' impacting how people do their food shopping.

In addition, take-home grocery sales dropped 1.9% year-on-year during the 12 weeks to 5 September 2021, and Kantar measured the highest supermarket footfall all year outside of the Easter period in the first week of September.

• Find out more: which supermarket delivers the freshest food?