Appearance 
The Sei Whale’s gray-blue coloration means that it is often mistaken for a Fin Whale or a Blue Whale, although the Sei is much smaller than either of those two giants. Adult Sei Whales are 40-50 feet (12-16 meters) long, and weigh 20-30 tons (20,320-30,480 kilograms). The Sei Whale’s flukes are fairly small compared to the size of its body.