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How we test dishwasher detergents

By Sarah Ingrams

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How we test dishwasher detergents

Find out how Which? tests dishwasher tablets – including cleaning, drying and shine – to find the best for your dirty dishes, cutlery and glassware.

Only the dishwasher tablets that shift stubborn food and drink stains, and keep dishes clear and watermark-free after multiple washes can call themselves the best dishwasher tablets. Read on to find out more about our tough tests.

We’ve put dishwasher tablets from big brands and supermarket own-brands through our tough independent tests to reveal how well each remove dried-on food and drink, prevent glasses from going cloudy and losing their shine and stop streaks and watermarks.

We put on more than 400 loads of dirty dishes for each test so our reviews can answer the most important questions about dishwasher tablets:

  • How well does the detergent remove different food and drink?
  • Will it stop my glasses going cloudy?
  • How well does it prevent watermarks?
  • Will it keep my dishes shiny?
  • Should I buy it?

Our rigorous tests reveal how brands, such as Fairy, Finish and Ecover dishwasher tablets, compare with cheaper supermarket alternatives. So you can use our results to discover if it’s worth spending more on a big brand or if you could save money and still get stain and streak-free dishes.

Take a look at our best dishwasher tablets to find out which detergents deliver first-class results across all our tests, or read on to find out more about our testing.

How well does it remove different foods and drinks?

We use a range of foods and drinks in our dishwasher tablet tests to represent the different types of residue you’d expect to wash off in the dishwasher. But to put dishwasher tablets through their paces, we use some of the toughest foods we can find:

  • Tea – a bleachable stain, so we use it to test how effectively detergents remove coloured stains
  • Starch – starchy marks show how well a detergent can clean residue from pasta, rice and potatoes
  • Egg-yolk – dried-on, this stain tests how well detergents wash off protein-rich foods, including meat
  • Dried-on milk – this is really tough to shift, so we use it to give a good indication of overall cleaning power

In our lab we use cups, glasses, plates and baking sheets covered in carefully measured amounts of these substances. After washing them at 50°C we measure how much residue remains.

We find big differences between the best and worst dishwasher tablets we test – as you can see in the gallery below.

We know that the cups, plates and cutlery you put in your dishwasher are encrusted with more than the specific stains in our test. So we also add a mixture of other common foods to each load, including margarine, flour and sauces.

In our test results, a five-star cleaning score means that all the stain is removed. One star means that half the food or drink applied still remains on the dishes - which means you'd have to wash up by hand. 

Will it stop my glasses going cloudy?

We wash glasses, plates, cutlery and plastic food-storage containers multiple times to see whether there’s evidence of residue, cloudy or coloured film or streaks starting to build up on them. We check them after five washes and then again after 30 washes, to reveal how your dishes will fare over time.

Hard water causes some cloudy or coloured filming, but alkali conditions also cause an oily or coloured film to appear on items. These deposits can often be removed by rinsing or wiping with a cloth. But if they aren’t removed after the first wash, subsequent washes can cause further build-up that’s far more difficult to shift. 

So it's worth using our full dishwasher tablet results to check you're using decent dishwasher tablets, as it'll help prevent build-up and save you having to give all your crockery, cutlery and glasses a good scrub by hand.

Your dishwasher also plays a part in keeping glasses clean, clear and watermark-free.

How well does it prevent watermarks?

Watermarks on your best dishes, cutlery and glasses doesn't look great, particularly if you're cooking a special meal and want to impress your guests. So our experts also check whether watermarks appear on the crockery after multiple washes. This indicates how good the rinse aid element of the all-in-one dishwasher tablets is and whether it enables dishes to dry without leaving visible outlines of water spots. 

We test all-in-one dishwasher tablets on normal wash cycles without adding extra salt or rinse aid, as the packaging recommends for medium water hardness.

Will it keep my dishes shiny?

After five and 30 washes, we also inspect plates and glasses to see if they have been dulled through washing – we call this loss of brilliance. Dishes that once were bright and glossy start to appear duller and less shiny (as you can see in the photo of the black plate, below).

Our lab experts use a reflectometer to detect any loss of brilliance. The dishwasher tablets that score top marks on this will keep your glasses sparkling wash after wash.

Should I buy it?

All the tests outlined above contribute to a total test score. The higher the score, the better the dishwasher tablet did in our tests.

Products achieved their scores at the time of publication, but there’s a chance the formulations may have been changed since – detergent manufacturers tinker with these all the time.

Certain assessments are more important than others and so carry different weights. We think a dishwasher tablet’s most important job is to get all the items you put in your dishwasher clean, so the largest proportion of our total test score is based on this. Our overall ratings ignore price and are based on:

  • 60% cleaning
  • 15% filming and cloudiness
  • 15% watermarks
  • 10% brilliance

Top-scoring all-in-one dishwasher tablets that stand out in our tests earn our Best Buy recommendation. Dishwasher tablets that score 40% or less are highlighted as Don’t Buy detergents to avoid.

Before you buy your next box of dishwasher tablets, check our best dishwasher tablets.

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