I usually wear size 26 jeans (5'10" tall). Size small from Gymshark was WAY too small on me. I think I'd need a large, but did not order medium or large to try on, because I went off of Gymshark's size chart which I think it is way off.
I was able to put them on, but they were so stretched, they were see-through, and the waist band dug into me, making me look like a snowman. They also rolled down if I moved.
I don't know if they would have been good gym leggings since I got the wrong size and never wore them to the gym. Since trying these, I've noticed when I see others wearing Gymshark leggings, and they usually look good on people who are petite or very muscular with low body fat.
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They should have better size charts. (Maybe each product needs a different size chart?) Or change your sizing to be more standard.