Emus are large birds. The largest can reach up to 150 to 190 centimetres (59–75 in) in height, 1 to 1.3 metres (3.3–4.3 ft) at the shoulder. Emus weigh between 18 and 48 kilograms (40 and 106 lb) Females are usually larger than males by a small amount, but substantially wider across the rump.
They have small vestigial wings that are around 20 centimetres (7.9 in) long and have a small claw at the tip of this wing. The Emu flaps its wings when it is running and it is believed that they stabilise the bird when it is moving. It has a long neck and legs. Their ability to run at high speeds, 48 km/h (30 mph), is due to their highly specialised pelvic limb musculature. Their feet have only three toes and a similarly reduced number of bones and associated foot muscles; they are the only birds with gastrocnemius muscles in the back of the lower legs. The pelvic limb muscles of emus have a similar contribution to total body mass as the flight muscles of flying birds. When walking, the Emu takes steps of around 100 centimetres (3.3 ft), but at full gallop, a stride can be as long as 275 centimetres (9.02 ft). The Emu's legs are devoid of feathers and underneath its feet are thick, cushioned pads. Like the Cassowary, the Emu has a nail on its toe akin to the blade of a knife, which is its major defensive attribute. This is used in combat to inflict wounds on opponents by kicking. The toe and claw are a total of 15 centimetres (5.9 in). They have a soft bill, adapted for grazing.
The Emu has good eyesight and hearing, which allows it to detect nearby threats. Its legs are among the strongest of any animals, powerful enough to tear down metal wire fences.
The eyes of an Emu are protected by nictitating membranes. These are translucent, secondary eyelids that move from the end of the eye closest to the beak to cover the other side. This is used by the Emu as a protective visor to protect its eyes from dust that is prevalent in windy and arid deserts. The Emu also has a tracheal pouch, which becomes more prominent during the mating season. It is often used during courting, and it has speculated that it is used for communication on a day-to-day basis. The pouch is more than 30 centimetres (12 in), is spacious and the wall in very thin. The width of the opening is only 8 centimetres (3.1 in). The quantity of air that goes through the pouch, as determined by the Emu deciding to open or close it, affects the pitch of an Emu's call. Females typically cry more loudly than males.
On very hot days, emus pant to maintain their body temperature, their lungs work as evaporative coolers and, unlike some other species, the resulting low levels of carbon dioxide in the blood do not appear to cause alkalosis. For normal breathing in cooler weather, they have large, multifolded nasal passages. Cool air warms as it passes through into the lungs, extracting heat from the nasal region. On exhalation, the Emu's cold nasal turbinates condense moisture back out of the air and absorb it for reuse. As with other ratites, the Emu has great homeothermic ability, and can maintain this status from -5 to 45 degrees. The thermoneutral zone of Emus lies between 10–15 degrees and 30 degrees.
As with other ratites, the Emu has a relatively low rate of metabolism compared to other types of birds, but the rate depends on activity, especially due to resulting changes to thermodynamics. At -5 degrees, the metabolism rate of an Emu while sitting down is around 60% of the value for one that is standing, as the lack of feathers under its stomach leads to a higher rate of heat loss when it is standing up and exposing the underbelly.