Depending on subspecies, the American yellow warbler may be between long, with a wingspan from . They weigh , varying between subspecies and whether on migration or not, globally averaging about but only in most breeding adults of the United States populations. Among standard measurements throughout the subspecies, the wing chord is , the tail is , the bill is and the tarsus is . The summer males of this species are generally the yellowest warblers wherever they occur. They are brilliant yellow below and greenish-golden above. There are usually a few wide, somewhat washed-out rusty-red streaks on the breast and flanks. These markings are the reason for the scientific name "petechia", which roughly translates to "liver spotted". The subspecies in this group mostly vary in brightness and size according to Bergmann's and Gloger's Rule.