How we test hair straighteners

The best hair straighteners will straighten curls or kinks out of your hair and leave it looking sleek, shiny and healthy. They will be easy to use without feeling too unwieldy or cumbersome. The worst will drag your hair, leave it looking damaged and feel heavy to use or fiddly to operate.

When we test hair straighteners, we don't just find out if they're good at straightening your hair or whether they're easy to use like reviews on other websites. We also carry our scientific checks in a proper laboratory to find out if the straighteners will damage your hair and leave it looking straw-like and dry.

We’ve tested and rated hair straighteners from the biggest brands, including Babyliss, Cloud 9, Dyson, GHD and Remington, and our test is made up of two main parts; lab-based checks and a user trial to see how well they work on different hair types. 

Head straight to our hair straighteners reviews to find the perfect model for you or read on below to find out more about the details of how we test.

What are Which? Best Buy, Don't Buy and Great Value hair straighteners?

Best Buys

Best Buy straighteners excel in our tests and score 70% or above. However it's still worth reading our Best Buy reviews to check if there are any factors that might impact your buying decision.

Don't Buys

Any hair straighteners that score 45% or less is a Don't Buy. This means the hair straighteners performed so poorly in our tests that we don't recommend you buy it.

Great Value

Hair straighteners can really vary in price, and with some costing hundreds of pounds, we've also made Great Value recommendations. These hair straighteners score 60% or more in our tests and are at least 20% cheaper that the average cost of straighteners that we've tested. Some of our Great Value hair straighteners also score high enough to be Best buys.

Will these hair straighteners effectively straighten my hair?

Our panel of 30 triallists each tried out two different brands of hair straighteners chosen at random. They used each set of straighteners on different sides of their head so they can compare them.

Each one was rated on smoothness, shine, frizz control and overall straightening ability. We checked in with the triallists 24 hours later and asked them for a rating on the style longevity following the test. We wanted to know if their hair went frizzy as soon as they went outdoors or after a night's sleep.

The lab also checked the accuracy of the heat setting at three spots along the plate to make sure that it's even and it's the temperature it claims to be. Products with inaccurate temperatures could mean the straighteners are less effective or could cause more damage to your hair.

Will these hair straighteners damage my hair?

The lab sets each hair straighteners to their highest heat setting. 

It then repeatedly flat irons test hair (medium brown Caucasian hair) in blocks of 10 passes with each stroke lasting five seconds and a time gap to allow the hair to cool down. This is done up to 20 times to replicate repeated use of the straighteners and the maximum amount of damage that could be caused when using them.

The hair is then assessed with a tensile strength test. This is where the test hair (50 hair fibres) is clamped in a tensile tester, the hair fibres are measured with a micro laser and then a force is applied to the hair to determine at what point it breaks. 

This is known as the break stress measurement. The lower the measurement, the higher the damage that has been done to the hair shaft by the straighteners.

All measurements are compared against a control sample, which hasn't been exposed to any heat treatments. Straighteners with low break stress measurements will have their score limited.

How easy are they to use?

Our panel of triallists provided scores and feedback on using the straighteners, rating them on the heaviness of the device, comfort to hold, simplicity of the controls and setting the temperature, plus ease of gripping the hair.

We also asked people to rate the build quality of the straighteners to find out if any models felt cheap or flimsy.

The lab times how long it takes for the straighteners to reach the highest possible temperature point. Some devices will reach this in around 30 seconds, while others take closer to six minutes, which could be frustrating if you're getting ready in a rush.

It also checks the temperature of the outer casing using an infrared thermometer to make sure it doesn't get so hot that you might burn yourself.

Should I buy them?

Each of the above assessments go towards making up a total test score, which is the overall percentage figure awarded to each set of hair straighteners.

Hair straighteners need to score at least 70% to earn our Best Buy recommendation.

Certain elements of the test carry more weight for the total test score than others. 

It's made up of the following:

• 45% panel test (styling ability, ease of use, style longevity, build quality)
• 35% damage performance (based on measurements from the tensile strength test
• 20% temperature measurements (accuracy of heat setting, heat-up and cool-down time, and casing temperature).

Any hair straighteners that get 45% or less will be a Don't Buy and not a model that we would recommend.