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Home & garden.

Updated: 12 Jul 2021

How we test washing-up liquid

Find out exactly how Which? discovers the best washing-up liquid in the land.
Patrick Gallagher
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We've been testing washing-up liquid since 1959. So we know a thing or two about the liquids you can rely on to help you breeze through the washing up.

Back then, most detergents were multi-purpose powders that could be used to wash your clothes as well as your crockery. Today, a host of brands especially designed for dish washing compete for your attention in the supermarket – including Fairy and Ecover – as well as own-brand liquids from supermarkets such as Sainsbury's, Morrisons and Aldi. We've tested all these and more to reveal which liquids are worth buying.

Our reviews answer the most important washing-up liquid questions, including:

  • How well does it remove fat?
  • How long does the foam last?
  • How well does it tackle tough, baked-on grease?

Only the ones that battle valiantly against fat and grime can become a Best Buy washing-up liquid.

How well does the washing-up liquid remove fat?

Removing fat from dirty dishes is the most important part of our test. We count the number of plates a bowl of washing-up liquid can wash before the fat starts to stay on the plates.

Products that wash more plates will last longer, and cut down on the times you need to change the water when scrubbing your way through the Sunday roast dishes.

To score washing-up liquids on fat removal, we pour a total of 1.2kg of animal fat onto 742 plates. We then count the number of plates that each liquid cleans, until the fat begins to be re-deposited, to reveal the best washing-up liquids.

How long does the foam last?

We measure how long the foam lasts, because most of us empty the bowl once the foam runs out. This can be a waste of washing-up liquid, as losing the foam doesn’t necessarily mean that the liquid isn’t capable of cleaning any more dishes.

In our foam duration test, we smear 759 plates with different food, including beef tallow, margarine, lard and flour. We count the number of plates that each liquid cleans before the foam disappears.

How well does the washing-up liquid fight tough, baked-on grease?

Our current washing-up liquid tests are the toughest yet, as we’ve added a new ‘tough-grease’ removal test. Baked-on fat is the most difficult foodstuff to wash off by hand, according to 61% of Which? members who we surveyed. 

Our tough-grease test simulates cleaning leftover fat from grilling meat or roasting potatoes from metal cookware – to find out how products really face up to this tough kitchen challenge.

Should I buy it?

All the tests outlined above contribute to the overall test score which we award each product. The higher the score, the better the washing-up liquid did in our tests.

Certain parts of the test are more important than others, and so carry different weightings. We think a washing-up liquid’s most important job is to bust fat from plates, so a large proportion of our total test score is based on this. Our overall ratings ignore the price and are based on:

  • 90% fat removal
  • 10% tough-grease removal

Although we test for foam duration and include it as a star rating in our reviews, it doesn't contribute to the overall score.

A washing-up liquid needs to earn 81% in our rigorous tests to become a Best Buy. Those with less than 45% become a Don't Buy.

We test washing-up liquids every year. The washing-up liquids we’ve published results for in this review were tested in April 2020. There is a chance the formulations may have changed since then, as washing-up liquid manufacturers tend to tinker with them all the time.