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How we test washing powder and laundry detergents

By Sarah Ingrams

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How we test washing powder and laundry detergents

Find out how we test washing powders, liquid capsules, liquid and gel laundry detergents to find those that clean best overall.

Only the laundry detergents that get clothes cleanest without fading colours or turning whites dull earn our Best Buy recommendation.

We've thoroughly tested washing powders, liquid capsules, liquids and gels in the Which? test lab, so you can be confident that the detergent you buy will not only remove tough stains, but will also leave your whites white and colourful clothes bright.

We do more than 300 washes for each test, so our reviews can answer your most crucial questions about laundry detergents:

  • How well does the detergent work on all types of tough stains?
  • Does it protect white laundry?
  • Can it stop colours fading in your wash?
  • Should I buy it?

Our rigorous tests reveal how brands, such as Ariel, Bold, Daz, Fairy, Persil and Surf, as well as cheaper supermarket own-brand detergents compare. We can then tell you whether it's worth spending more on a big brand for a better wash, or whether you could save money and banish stains with a supermarket own-brand.

Head to our washing powder and laundry detergent reviews to find out which have earned Best Buy recommendations, or read on to find out more about our tests.

How well does it remove all tough stains?

Even the best washing powder or liquid detergent can't remove every last trace of the super-tough stains we use in our tests. So we examine how well each detergent tackles them. We measure stain-removal power using a photospectrometer, which shines light on the stains after washing. The more effective the detergent, the more light is reflected back from the fabric.

And we don't just check a few stains. Our tests assess a huge range of common stains, which are applied in 5cm-diameter circles on swatches of fabric.

  • Drinks: a detergent needs to be good at bleaching to banish drink stains, such as red wine and tea.
  • Food: tomato purée, chocolate ice cream, curry and mustard are some of the colourful challenges we set detergents to work on.
  • Make-up: greasy make-up smears need a good detergent to break them down.
  • Everyday stains: blood, sweat, ink, grass and mud stains are hard to remove once they are dry.
  • Grease and oil: we challenge detergents to break down grimy motor oil and meat-grease stains.

The lowest-scoring products really struggle to banish stains, leaving marks that would be clearly visible on your best white shirt. On the other hand, the top-scoring Best Buy detergents remove almost all evidence of a stain. 

Because we know that these specific tough stains aren't the only items in your washing machine, we wash the stained fabrics with 3kg of cotton sheets, pillowcases and towels. We use the recommended dosage of laundry detergent for a normal soiled load in a 40°C wash cycle - the cycle most used by Which? members.

Whether you’re dealing with muddy gardening clothes or tomato sauce splashes on shirts, we can tell you which products work best – check our full washing powder and laundry detergents results to see the best for each type of stain.

Does the detergent dull whites?

We wash three white pieces of cotton, polycotton, polyester and polyamide to see whether whites go grey after six washes.

A photospectrometer measures how much light is reflected back from these fabrics. The more light reflected back, the better - as this means your white laundry will look bright. The worst detergents will leave your whites looking grey after a few washes, while the best will keep them bright white, wash after wash.

Can it stop colours fading in my wash?

We add 14 different coloured swatches to our test load (like those pictured above), and wash them 20 times with each detergent to see how badly they fade.

All the washing powders we've tested fade clothes a little, but buying a Which? Best Buy will help to keep fading to a minimum. Liquids don't usually fade clothes to the same extent as powders, as they don't contain bleach.

You can see which detergents are best (and worst) at keeping your whites white and your colours bright - go to our full laundry detergent results.

Which detergents are the best value?

You could save £31 a year on your laundry detergent by switching from a branded Best Buy washing powder to a great-value alternative, according to our research. You can also save nearly £25 on liquid capsules and £13 on liquid or gel by switching from the best-scoring brand on test to a great-value alternative.

But watch out for pricier products that don’t wash as well; our tests have revealed that the lowest-scoring washing powder will cost you £7 more per year than a great-value Best Buy.

Find out which products we’re talking about by checking our full washing powder and laundry detergent results. But if you’d rather stick to branded laundry products, make sure you buy only the very best deals with our Best Buy supermarket offers guide.

Should I buy it?

All the tests outlined above contribute to the overall test score that we award to each product. The higher the score, the better the laundry detergent did in our tests. 

Certain assessments are more important than others and so carry different weights. We think a laundry detergent's most important job is to get your clothes clean, so a large proportion of our total test score is based on this. Our overall ratings ignore the price and are based on:

  • 70% stain removal  
  • 20% colour fading  
  • 10% whiteness

A Best Buy laundry detergent won't stop you dropping food onto your clothes, but it will make it easier to clean up the mess afterwards. Top-scoring laundry detergents that stand out earn our Best Buy recommendation. Laundry detergents that score 40% or less are highlighted as Don't Buy detergents to avoid.

The products we've rated elsewhere in this review achieved their scores at the time we first published results, but there's a chance the formulations may have been changed since - laundry detergent manufacturers tinker with these all the time. Plus, we've added four extra stains into our tests from February 2016, and use fresh stains (made from real ingredients). Our liquid capsules results are correct as of February 2016; washing powders as of June 2015 and liquid and gel detergents as of August 2014.

This means that while our reviews can tell you generally about which type of detergent is best for certain stains and which will best preserve coloured clothes, the scores for different types of detergent are not directly comparable.

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