The numbers side by side
Both species are rorquals in the genus Balaenoptera, both strain krill through baleen, and both were hunted hard in the twentieth century. The figures below come from NOAA Fisheries species profiles, the New York State DEC fin whale assessment, and the IUCN Red List assessments for each species. Their menus are not identical, though. NOAA says fin whales take krill, small schooling fish such as herring, capelin and sand lance, and squid, lunging into schools with their mouths open and using 50 to 100 accordion-like throat pleats to gulp food and water.
| Blue whale (Balaenoptera musculus) | Fin whale (Balaenoptera physalus) | |
|---|---|---|
| Maximum length | Up to about 110 ft (Antarctic); about 90 ft in the North Atlantic and North Pacific | 75 to 85 ft |
| Weight | Up to 330,000 lb (Antarctic); over 100,000 lb in northern oceans | 40 to 80 tons |
| Typical speed | Up to about 5 mph feeding and traveling; bursts above 20 mph | Sustains about 23 to 25 mph (Wikipedia); nicknamed "greyhound of the sea" (NY DEC) |
| Lifespan (NOAA estimate) | Around 80 to 90 years | 80 to 90 years |
| Range | All oceans except the Arctic | All the world's oceans |
| US status | ESA endangered (listed December 2, 1970) | ESA endangered throughout its range |
| IUCN Red List | Endangered | Vulnerable |
Two things in that table trip people up. First, the blue whale's famous 110-foot figure belongs to the Antarctic subspecies; NOAA puts northern blue whales nearer 90 feet, which overlaps the upper end of a large fin whale far more than most comparison charts admit. Second, the lifespans are estimates from earplug layers in dead animals, not tracked individuals.
Field marks that actually separate them
Whale-watchers rarely see a whole animal, so the ID comes down to a few visible features. These are the marks that separate them:
1. Look at the head when it surfaces. On a fin whale, the left side of the head is dark gray while the right lower jaw is white or light gray, a lopsided pattern that is the single most reliable mark. A blue whale's broad, U-shaped head is evenly mottled on both sides.
2. Check the skin color. Blue whales are mottled grayish-blue and look blue underwater; their pale underbelly can turn yellowish from diatoms, which gave them the old whalers' name "sulphur bottom." Fin whales are brownish to dark or light gray above and white below.
3. Compare the dorsal fin. The fin whale's is large, hook-shaped and swept back, roughly 24 inches tall. The blue whale's is small and sickle-shaped and sits close to the tail, so it can look tiny against the length of back that rolls past first.
4. Watch the blow. A blue whale's blow rises about 30 to 39 feet, while a fin whale's reaches 13 to 20 feet. Wind flattens blows quickly, so use this as a supporting clue only.
If the head coloring is visible, it settles the question on its own. Otherwise, look for two of the other clues pointing the same way. Size estimates from a boat are the least reliable clue on the list.
How their voices compare
Both species call at frequencies near or below the bottom of human hearing. Blue whale vocalizations have a fundamental frequency of 8 to 25 Hz, with a maximum loudness of 188 dB, and NOAA notes that in the right ocean conditions blue whale sounds may carry to other whales up to 1,000 miles away. Fin whales are known for a pulse near 20 Hz; a 1987 paper by Watkins and colleagues in the Journal of the Acoustical Society of America is titled simply "The 20 Hz signals of finback whales."
NOAA's blue whale page also links a peer-reviewed PLOS One study that describes blue and fin whale calls and where each was detected, a reminder that the two species often share the same stretch of ocean.
They are close enough to interbreed
The two species are not just look-alikes. Blue-fin hybrids are known from both the North Atlantic and North Pacific, likely because the species share prey, overlap in range and often travel in mixed groups. The best-documented case is a 21 m (70 ft) pregnant female caught off Iceland in 1986: molecular tests showed a blue whale mother and a fin whale father, and her fetus had been sired by a blue whale, so the hybrid was fertile. DNA tests on a blue whale killed off Iceland in July 2018 also pointed to a fin whale father and a blue whale mother, though those results are still pending independent testing and verification. DNA testing of whale meat sold in Japan has also turned up blue-fin hybrids.
The genetic gap between them has been compared to the one between humans and chimpanzees, about 3.5 million years of separation, which makes regular hybridization unusual among large mammals. For an ID at sea, the practical lesson is that an odd-looking animal with mixed features may be exactly that.
Why speed shaped their history
The fin whale's speed protected it for a while. The New York State DEC's 2025 species assessment notes that fin whales were not an early whaling target because of their speed and lower oil yield, until late-1800s technology off Norway made them catchable in the 1870s. After that, North Atlantic and Pacific stocks were "quickly overexploited."
Blue whales received full protection from commercial whaling in 1966 under the International Convention for the Regulation of Whaling, two decades before the 1986 moratorium that covered fin whales. Even so, the same NY assessment records that fin whales have continued to be taken under objection, scientific permit or subsistence quotas by Iceland, Japan and Greenland.
How many are left
Neither species has a reliable global count. NOAA says blue whale numbers are "only a small fraction" of pre-whaling levels but are increasing globally. For fin whales, the NY assessment cites an IUCN North Atlantic estimate of 70,000 from 2015 and a western North Atlantic stock estimate of 6,802, while warning there is no trend analysis because survey methods vary.
If you are comparing blue whales with a different rorqual relative, our humpback whale vs blue whale page covers the bulkier, more acrobatic species, and the sperm whale profile covers the largest toothed whale.