The groundhog is the largest sciurid in its geographical range. Adults are 16 to 20 inches (40–50 cm) long, including a six-inch (15 cm) tail. A large woodchuck thought to weigh twenty pounds when carried was exactly half that weight when weighed by scale. Woodchuck weight ranges from five to twelve pounds. Extremely large individuals may weigh up to 15 pounds. Seasonal weight changes indicate circannual deposition and use of fat. Progressive higher weights are attained each year for the first 2–3 years after which weights plateau. Groundhogs have four incisor teeth which grow 1⁄16″ (1.5 mm) per week. Constant usage wears them down again by about that much each week. Unlike the incisors of other rodents, the incisors of groundhogs are white to ivory white. Groundhogs are well adapted for digging, with short, powerful limbs and curved, thick claws. Unlike other sciurids, the groundhog's tail is comparably shorter — only about one-fourth of body length.
Groundhogs are one of the few species that enter into true hibernation, and often build a separate "winter burrow" for this purpose. This burrow is usually in a wooded or brushy area and is dug below the frost line and remains at a stable temperature well above freezing during the winter months. In most areas, groundhogs hibernate from October to March or April, but in more temperate areas, they may hibernate as little as three months. Groundhogs hibernate longer in northern latitudes than southern latitudes. To survive the winter, they are at their maximum weight shortly before entering hibernation. When the groundhog enters hibernation, there is a drop in body temperature to as low as 35 degrees Fahrenheit, heart rate falls to 4–10 beats per minute and breathing rate falls to one breath every six minutes. During hibernation, they experience periods of torpor and arousal. Hibernating woodchucks lose as much as half their body weight by February. They emerge from hibernation with some remaining body fat to live on until the warmer spring weather produces abundant plant materials for food. Males emerge from hibernation before females. Groundhogs are mostly diurnal, and are often active early in the morning or late afternoon.