The groundhog is by far the largest sciurid in its geographical range, except in British Columbia where its range may abut that of its somewhat larger cousin, the hoary marmot.

Male groundhogs are slightly larger than females on average and, like all marmots, they are considerably heavier during autumn (when engaged in autumn hyperphagia) than when they emerge from hibernation in spring. Adult males average year-around weight 3.83 kg (8 lb 7 oz)