John LewisUltra JL 2082JLU

How we test protection from heat damage

The laboratory sets the hair straighteners to their highest temperature setting and repeatedly flat irons test hair to replicate regular use. The hair is then assessed via a tensile strength test to check how easily the hair shaft breaks. The lower the measurement, the more damage there is.

How we test temperature accuracy

Our laboratory uses a special thermometer probe to check the top, middle and bottom of the heating plates to see if the highest and lowest temperature claims for each device are accurate.

How we test straightening power overall

We get a panel of 30 people to try out the straighteners and rate them on how well they smooth hair, and leave it shiny and frizz-free. We also check in with the triallists 24 hours later to see if the straighteners can also provide style longevity, or if you're they started to see kinks or frizz appearing by the end of the day.

How we test style retention after 24 hours

The user triallists that try each set of hair straighteners are contacted by us 24 hours after using them to see how well their style remained and whether or not frizz or curls started to appear as the day went on.

How we test ease of use

Our panel of user triallists rate the hair straighteners of a range of aspects including how heavy they feel, whether they're easy to grip the hair and if the controls are simple to use. We also ask them to provide their thoughts on whether the build quality is good, that is, whether they'd survive if you knocked the straighteners off your dresser now and again.

How we test time taken to heat up

The laboratory times how long it takes for the straighteners to reach their highest setting and do this three times to get an average heat up time.

How we test time taken to cool down

The laboratory times how long it takes for the straighteners to cool down once you've switched them off from their highest temperature until they reach 40°C.