I use these under jeans when I work outside all day (about 8 hours outside) in weather ranging from 30's to 50's and they are a great midweight product. In the 30's it can still be a little cold if I'm just standing around. Size is great. I usually wear a 0 or 2 size jean (at Old Navy- just so there's a reference), but I also have really thick legs for my size and these fit great so if your legs are smaller, an XS might work better for you.