How we test washing powder and laundry detergents

Only the laundry detergents that shift stains without fading colours or leaving whites dull earn our Best Buy recommendation.

We've thoroughly tested washing powders, capsules, liquids and gels in the Which? 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 test each detergent six times, totalling more than 210 washes, so our reviews can answer those crucial questions about laundry detergents, including:

• How well does the detergent work on different types of tough stains?
• Does it protect white fabrics?
• Will it stop colours fading in the wash?
• Should I buy it?

Our tests reveal how brands such as Ariel, Persil and Bold compare to cheaper supermarket own-brand detergents, as well as detergents from brands designed to be more eco-friendly such as Ecover, Smol and Splosh. 

We can tell you whether it's worth spending more for a better wash, whether you could save money and banish stains with a supermarket own-brand, and which ones are least damaging to the environment. 

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How well will my detergent remove tough stains?

We apply stains to laundry before washing, and then measure stain-removal power after washing using a photospectrometer. This shines light on the stains. The more effective the detergent, the more light is reflected back from the fabric.

We test 17 different types of stain, applied in 5cm-diameter circles.

• Drinks: a detergent needs to be good at bleaching to banish drink stains, such as red wine, coffee, fruit juice and tea
• Food: tomato purée, chocolate ice cream, curry and mustard are some of the colourful challenges we set for detergents
• 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 dry
• Grease and oil: we challenge detergents to break down grimy motor oil and greasy stains from meat

The lowest-scoring products really struggle to banish stains, leaving marks that would be clearly visible on your best white shirt. The best washing powder and laundry detergents remove almost all evidence of a stain. 

Not all items in your washing basket will be severely stained, of course. So we wash the stained fabrics in a load 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, read all of our washing powder and laundry detergent reviews to find the one that will work best for you.

Will my detergent dull whites?

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

Our 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.

Will my detergent stop colours fading in the 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.

To find out which detergents are best (and worst) at keeping your whites white and your colours bright, take a look at our full laundry detergent results.

If the problem is actually your washing machine, head over to our monthly round-up of the best washing machine deals.

What is its environmental impact?

We now examine the eco-credentials of each laundry detergent that we test, so that you can make a more informed buying decision. 

We look into the ingredients used and what their environmental impact is, including whether they are biodegradable, harmful to aquatic life or persistent (whether they are likely to accumulate and linger in waterways). 

We consider how transparent manufacturers are being about the ingredients they use. 

We also take into account how easy the packaging is to recycle and how much information about recyclability is given on the packaging. 

Plus, we look into the packaging utilisation of each detergent (whether or not the packaging is under-filled).

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What are Which? Best Buy, Don't Buy, Eco Buy and Great Value recommendations?

Top-scoring laundry detergents that achieve 73% or higher earn our Best Buy recommendation. Laundry detergents that score 45% or less are highlighted as Don't Buy detergents to avoid.

Great Value products are a selection of the cheapest products per wash that score 60% or above. Which? recommends these as a good choice for those who want to spend less, without sacrificing too much in the way of performance.

An Eco Buy needs to be better for the environment than alternatives on the market, and it needs to perform well at its core job, so Eco Buy products must have a minimum overall score of 68% plus four stars for sustainability.

Learn more about how choosing a Which? Eco Buy can save you money and benefit the environment. 

Should I buy it?

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

Certain assessments are more important than others and so carry different weights. 

Of course, 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. But we also look at other aspects, including sustainability – an important consideration for the future of our planet. 

Our overall ratings ignore the price and are based on:

• 60% stain removal 
• 20% colour fading
• 10% whiteness
• 10% sustainability

Don't forget, top-scoring laundry detergents that achieve 73% or higher earn a coveted Which? Best Buy recommendation.

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