It gets even more complicated than that <img src="/forums/images/graemlins/frown.gif" alt="" /> How warm a jacket will keep you also depends on how it fits (not too much dead air space to try and warm up but not so tight that it cuts off circulation) and how the rest of your clothing fits around it (are there gaps between your down coat and your pants?) My best advice is to go someplace and try on jackets, and then with that information buy the one that fits the best for your layering system with the highest quality of down. The fill weight refers to how much down it takes to fill a given space. If you really want to technical details, check out the backpackerlight.com forum page and search for clo value. Having said that, one of my winter camping buddies bought a marmot down jacket with hood last year and was toasty just standing around at -10F.