How Big Are Hammerhead Sharks? Size by Species, Pups, and Records

Hammerhead sharks range from the scalloped bonnethead, which reaches about 92 cm (3 ft) (opens in a new tab), to the great hammerhead (Sphyrna mokarran), the largest of the family (opens in a new tab), for which 400 cm (13 ft) is the more commonly observed maximum (opens in a new tab). The scalloped hammerhead (Sphyrna lewini) reaches about 4 m (13 ft) (opens in a new tab), the smooth hammerhead (Sphyrna zygaena) 3.7–4 m (12–13 ft) (opens in a new tab), and the bonnethead (Sphyrna tiburo) 1.5 m (5 ft) (opens in a new tab). The 6.1-meter (20-foot) figure often quoted for great hammerheads traces to a 1984 species catalog (opens in a new tab), and the largest great hammerheads measured in the field studies cited here were about 4.5 m (15 ft) (opens in a new tab).

This guide is written by MMF co-founder Dr Simon J Pierce, whose work in the Galápagos brings him into regular contact with hammerheads, and draws on the published research, including Dr. Alex Hearn’s long-term tracking of scalloped hammerheads in the Eastern Tropical Pacific.

This chapter of the guide covers adult size by species, pups, growth, weight, and how scientists measure a free-swimming shark. For telling the species apart, see types of hammerhead shark; for quick answers, see hammerhead shark facts.

How big are hammerhead sharks?

The family has roughly ten species (opens in a new tab). Of the nine assessed on the IUCN Red List, five reach 2 m (6.6 ft) or less and one has an unknown adult size. Only three, the great, scalloped, and smooth hammerheads, grow to about 4 m (13 ft). Researchers group those three as the large hammerhead complex, the species that are over 150 cm (5 ft) at maturity and the slowest growing (opens in a new tab).

The maximum sizes below are those printed in each species’ Red List assessment; they mark the upper end of the range, not a typical adult.

For the scalloped hammerhead, a 2022 study gives an adult maximum of 400 cm (13 ft) (opens in a new tab), citing an earlier species guide. That is close to the Red List’s 420 cm (14 ft), so “up to about 4 m” is the safe summary. For the Carolina hammerhead, the 69 cm (27 in) figure reflects the fact that most specimens have been juveniles collected in shallow coastal bays (opens in a new tab), and the assessment gives no adult maximum.

What is the biggest hammerhead shark ever recorded?

The great hammerhead is the largest hammerhead species (opens in a new tab), and the biggest individuals measured in the field studies cited here were about 4.5 m (15 ft) long. In drumline surveys from Miami to the middle Florida Keys between 2009 and 2021, which caught 238 great hammerheads, the largest shark of any species was a 450 cm (14.8 ft) great hammerhead (opens in a new tab). A tracking study that measured 31 great hammerheads in Queensland and Western Australia between 2010 and 2023 recorded lengths from 125 to 455 cm (4.1 to 14.9 ft) (opens in a new tab). In French Polynesia, where researchers measured free-swimming great hammerheads with lasers at Rangiroa atoll, the largest of 65 total-length measurements was 400 cm (13.1 ft) (opens in a new tab).

The average great hammerhead in those studies was considerably smaller: 270.9 cm (8.9 ft) in south Florida (opens in a new tab) and 288.5 cm (9.5 ft) in Australia (opens in a new tab), while the Rangiroa measurements averaged 288.4 cm (9.5 ft) (opens in a new tab).

What about the 20-foot great hammerhead? The 2019 Red List assessment states that the maximum size “is reported as 550 to 610 cm total length (TL) reported by Compagno (1984), though 400 cm TL is a more commonly observed maximum size” (opens in a new tab). Compagno’s 1984 work is the FAO species catalog of the world’s sharks (opens in a new tab), and more recent papers repeat its upper figure as background. One, for example, notes that the species can grow to over 6.1 m (opens in a new tab), citing that catalog among other compilations. The 6.1 m (20 ft) figure is a reported maximum, not an impossibility. None of the great hammerheads measured in the three field studies cited here exceeded 4.55 m (15 ft), however, and 20 feet describes the most extreme report rather than a typical adult.

How big do scalloped hammerheads get?

Scalloped hammerheads reach about 4 m (13 ft) (opens in a new tab), but the sharks tagged at Darwin and Wolf islands in the Galápagos were much smaller. In one study, the 134 sharks tagged measured an estimated 1.2 to 2.4 m (4 to 8 ft), and 80% were female (opens in a new tab).

Much of what is known about the size of the Galápagos schools comes from the tagging work of Dr. Alex Hearn and his colleagues. In 2007, Hearn’s team, then at the University of California, Davis, and the Charles Darwin Foundation, tagged 71 scalloped hammerheads by free diving among the schools. Their estimated lengths ranged from 1.2 to 2.3 m (3.9 to 7.5 ft) at Wolf and 1.5 to 2.0 m (4.9 to 6.6 ft) at Darwin (opens in a new tab). Of those, 57 were female, only 6 were male, and the rest were not sexed. A larger study, with Hearn as second author, tagged 134 scalloped hammerheads at the two islands between 2006 and 2010. Of those, 80% were females, and the most frequent female sizes were 1.8 m (20%) and 2 m (37%) (opens in a new tab). The authors used a published female maturity length of 2.12 m (7 ft) total length. Because divers’ underwater estimates probably approximate fork length, measured to the fork of the tail rather than its tip, they concluded that at least a third of the females were probably mature (opens in a new tab).

Studies at other offshore islands, both with Hearn among the authors, found sharks of similar size. At Malpelo in Colombia, tagged sharks ranged from 1.3 to 2.5 m (4.3 to 8.2 ft) (opens in a new tab). At Cocos Island in Costa Rica, the sharks mostly appeared to be in the sub-adult and adult size range of 150 to 250 cm (4.9 to 8.2 ft) (opens in a new tab), although that team did not formally estimate lengths.

Fishery landings show the larger end of the range. Scalloped hammerheads landed at Manta, Ecuador, between 2003 and 2009 measured 50 to 310 cm (1.6 to 10.2 ft) total length (opens in a new tab). The largest female among 158 sharks collected for aging in Indonesia was 2,994 mm (9.8 ft) (opens in a new tab). In the southern Gulf of Mexico, the largest female landed by small-scale fisheries measured 354 cm (11.6 ft) in stretched total length (opens in a new tab), taken with the tail pulled straight.

The female bias in the schools has a parallel in the Gulf of California, where female scalloped hammerheads move offshore at a smaller size than males to form schools composed primarily of intermediate-size females (opens in a new tab). The chapter on why hammerheads school takes up that story.

How big are smooth hammerheads, bonnetheads, and the smaller hammerhead species?

Smooth hammerheads grow about as large as scalloped hammerheads, whereas the bonnethead, the winghead, and the other small species stay under 2 m (6.6 ft), as the table above shows. The 2019 assessment gives the smooth hammerhead a maximum size of 370–400 cm (12–13 ft) total length (opens in a new tab). Yet the largest smooth hammerhead in the aging study the assessment relied on was 286 cm (9.4 ft) TL, “considerably less than the known maximum size” (opens in a new tab). A study’s largest shark, in other words, can fall well short of the species’ reported maximum.

The bonnethead reaches 150 cm (5 ft) (opens in a new tab), and its size varies with latitude. On Florida’s Gulf of Mexico coast, bonnetheads at higher latitude were larger and matured at a larger size. The largest observed female measured 1,100 mm (43 in) TL in northwest Florida but 1,000 mm (39 in) in Florida Bay (opens in a new tab). Median female size at maturity, the length at which half the females are mature, fell from 944 mm (37 in) to 770 mm (30 in) (opens in a new tab) between the two regions. Because the northern sharks also grew fastest, the authors presented the pattern as evidence of countergradient variation, in which faster growth may compensate for a shorter growing season (opens in a new tab) at higher latitude.

The other small species are less studied. The scoophead reaches 150 cm (5 ft) (opens in a new tab), and so does the smalleye hammerhead (opens in a new tab); the scalloped bonnethead reaches only 92 cm (3 ft) (opens in a new tab). The winghead shark reaches 186 cm (6 ft) (opens in a new tab). Why its head, and those of the other hammerheads, take the shape they do is the subject of why hammerheads have a hammer.

Are female hammerheads bigger than males?

In the scalloped hammerhead and the bonnethead, females grow larger and mature at a larger size than males; in the great and smooth hammerheads, the published maturity sizes of the two sexes overlap. In the Gulf of California, the smallest female scalloped hammerhead with large eggs measured 217 cm (7.1 ft), whereas males matured from 163 cm (5.3 ft) total length (opens in a new tab). The author, A. Peter Klimley, contended that females move offshore earlier, feed more on open-water prey and with greater success, and so grow faster to the larger body needed to carry embryos (opens in a new tab). On his account, the two sexes reach maturity at a similar age.

Other regions show the same pattern. Off Ecuador, the median size at maturity was 178.1 cm (5.8 ft) TL for males and 219.4 cm (7.2 ft) TL for females (opens in a new tab). In Indonesia, the median sizes at maturity were 2,285 mm (7.5 ft) total length for females and 1,756 mm (5.8 ft) for males (opens in a new tab). In the northwest Atlantic and Gulf of Mexico, female scalloped hammerheads grew more slowly than males but to a larger theoretical maximum length. That growth-model estimate was 233.1 cm (7.6 ft) fork length for females, compared with 214.8 cm (7.0 ft) for males (opens in a new tab).

The bonnethead follows the same rule: males are smaller than females (opens in a new tab), and in northwest Florida the largest female was 1,100 mm (43 in) against 930 mm (37 in) for the largest male (opens in a new tab). The 2019 assessments, however, give great hammerhead males as maturing at 225–269 cm (7.4–8.8 ft) TL and females at 210–300 cm (6.9–9.8 ft) TL (opens in a new tab). Smooth hammerhead males mature at 250–260 cm (8.2–8.5 ft) TL and females at about 246–265 cm (8.1–8.7 ft) TL (opens in a new tab). These overlapping maturity sizes do not show whether adult females grow larger than males.

How big is a baby hammerhead shark?

The smallest birth size suggested in the Red List assessments is about 23 cm (9 in) (opens in a new tab), for the scalloped bonnethead, and the largest is 50–70 cm (20–28 in) (opens in a new tab), for the great hammerhead. In the Arabian Gulf, back-calculation from birth marks in the vertebrae of young great hammerheads gave a mean size at birth of 71.2 cm TL (28 in) (opens in a new tab), larger than reported for the northwest Atlantic and northern Australia.

Scalloped hammerhead pups are smaller, and their size at or near birth has been measured in several regions:

• Ecuador: 47–55 cm TL (opens in a new tab) (19–22 in), the range between the smallest free-swimming newborn and the largest embryo.
• Galápagos: Yasuní Chiriboga-Paredes and colleagues, including Dr. Alex Hearn of the Universidad San Francisco de Quito’s Galápagos Science Center, sampled Puerto Grande, a shallow bay on San Cristóbal Island. They caught pups every time they set a beach seine (a net drawn through the shallows). The 54 they measured averaged 52.13 cm (20.5 in) TL, from 42 to 78 cm (17 to 31 in) (opens in a new tab), sizes typical of newborns and young of the year rather than a birth-size estimate. The bay is a putative nursery, one suspected but not yet confirmed, and the first identified at an oceanic island in the Eastern Tropical Pacific (opens in a new tab).
• Southern Gulf of Mexico: newborns in small-scale fishery landings measured 35.5–57 cm (14–22 in) stretched total length (opens in a new tab).
• Indonesia: birth at about 400 mm (16 in) total length (opens in a new tab).
• Hawaii: in Kāne’ohe Bay, pups with open umbilical wounds measured 32.0–41.0 cm (12.6–16.1 in) precaudal length (opens in a new tab), a measurement that stops short of the tail.

The smooth hammerhead’s pups are born at 49–63 cm (19–25 in) TL (opens in a new tab), and the winghead’s at 32–50 cm (13–20 in) TL (opens in a new tab). Among the small species, the 2021 assessment gives the bonnethead’s size at birth as 35–40 cm (14–16 in) TL (opens in a new tab). Near-term embryos (those close to birth) from three Florida regions, however, averaged only 21.5–29.7 cm (8.5–11.7 in) TL (opens in a new tab), smallest in Florida Bay and largest in northwest Florida. Scoophead young measure 34 cm (13 in) (opens in a new tab), and smalleye hammerhead young 30 cm (12 in) (opens in a new tab), while Carolina hammerheads are thought to be born near 39 cm (15 in) (opens in a new tab). The scalloped bonnethead is suspected to give birth to two pups of about 23 cm (9 in) (opens in a new tab).

Litter sizes, pregnancy, and nurseries are covered in baby hammerhead sharks.

How fast do hammerhead sharks grow?

Hammerheads grow quickly as pups, then slow down, and the large species take several years to more than a decade to mature. Young great hammerheads in the Arabian Gulf were estimated to grow 83.3 cm (33 in) a year just after birth, slowing to 22.7 cm (9 in) a year by 2.9 years (opens in a new tab). Near Miami, one juvenile great hammerhead grew 25 cm (10 in), from 103 to 128 cm (41 to 50 in), in 146 days (opens in a new tab). The researchers suggested that published growth rates may therefore underestimate growth potential in that habitat.

Not every nursery supports fast growth. Scalloped hammerhead pups in Kāne’ohe Bay, Hawaii, grew about 9.6 cm (3.8 in) a year in precaudal length (opens in a new tab), and most of those recaptured within 60 days had lost weight. Starvation appears to be a primary cause (opens in a new tab) of the low number that remain in the bay through their first year; the feeding chapter explains why.

Growth slows sharply as sharks approach adult size. In the northwest Atlantic and Gulf of Mexico, scalloped hammerheads of both sexes grew rapidly until about age 12, after which male growth slowed considerably (opens in a new tab). In Indonesia, half of male scalloped hammerheads were mature by 8.9 years and half of females by 13.2 years (opens in a new tab), although the male estimate rests on only 23 sharks. Female great hammerheads mature at 5.5 and 8.3 years in the northwest Atlantic and western central Pacific (opens in a new tab), respectively, and bonnetheads in the eastern Gulf of Mexico at 2–3 years (opens in a new tab).

Because growth slows so much, length is a poor guide to the age of an adult. The oldest scalloped hammerheads aged in the northwest Atlantic were 30.5 years old at fork lengths of 241 cm (7.9 ft) for a female and 234 cm (7.7 ft) for a male (opens in a new tab). In Indonesia, the oldest was a 35-year-old female of 2,773 mm (9.1 ft) (opens in a new tab), shorter than the largest female in the same sample. Great hammerheads have been aged at up to 44 years (opens in a new tab), and bonnetheads have validated maximum ages of 16.0 years for males and 17.9 for females (opens in a new tab). These ages come from counting growth bands in the vertebrae, like tree rings. The authors of the Atlantic study cautioned in 2007, however, that annual band formation had not yet been validated in adult scalloped hammerheads (opens in a new tab).

How much does a hammerhead shark weigh?

Scalloped hammerhead pups measured in swim-tunnel experiments in Hawaii weighed less than 1 kg (2.2 lb) (opens in a new tab). At the other end of the scale, a mature male scalloped hammerhead tagged off Texas, in the Gulf of Mexico, was estimated to weigh 82 kg (181 lb) (opens in a new tab). That male measured 243 cm (8.0 ft) fork length (opens in a new tab), and none of the studies cited in this guide reports a measured weight for an adult great hammerhead.

Weights are easier to obtain for pups. The Hawaiian experimental pups, 50.5 to 59.0 cm (19.9 to 23.2 in) TL, weighed between 506 and 927 g (opens in a new tab), roughly 1 to 2 lb. Researchers in Kāne’ohe Bay estimated that an average one-year-old pup would be 50.3 cm (19.8 in) in precaudal length and weigh 2,000 g (4.4 lb) (opens in a new tab). Bonnethead embryos close to birth averaged 35 g (1.2 oz) in Florida Bay and 97 g (3.4 oz) in northwest Florida (opens in a new tab), consistent with the larger size of bonnetheads at higher latitude.

How do scientists measure a hammerhead shark?

A shark that has been caught is measured with a tape. Paired lasers and stereo cameras allow researchers to measure free-swimming hammerheads without capturing them, which matters because divers’ estimates of length are prone to error (opens in a new tab). Which length is meant matters too. Total length runs from the snout to the tip of the tail, and fork length stops at the fork of the tail. Precaudal length stops at the precaudal pit (opens in a new tab), the notch in front of the tail, and stretched total length (opens in a new tab) is taken with the tail pulled straight. The conversions differ by species and region. For northwest Atlantic scalloped hammerheads, for instance, total length equals 1.296 times fork length plus 0.516 cm (opens in a new tab), so a fork length of 200 cm (6.6 ft) corresponds to a total length of about 260 cm (8.5 ft).

Estimating length by eye underwater is difficult. At Cocos Island, a team including Hearn did not formally estimate the length of the sharks it tagged, given the errors associated with multiple divers estimating in-water length (opens in a new tab). At Darwin and Wolf, the tagging team judged that its visual estimates probably approximate fork length (opens in a new tab) rather than total length.

Two camera-based methods avoid the guesswork:

• Stereophotography. Two cameras photograph the same shark, and its length is calculated from the pair of images. In 1980 and 1981, Klimley used this method to measure 511 free-swimming scalloped hammerheads in offshore schools in the Gulf of California (opens in a new tab), the data behind his account of female schooling.
• Laser photogrammetry. Two parallel laser pointers project dots a known distance apart, 30 or 50 cm (12 or 20 in) in one study (opens in a new tab), onto the shark’s flank, giving a scale bar in the photograph. At Rangiroa, researchers used precaudal length as their reference size because the tail flexes during swimming, shortening total length (opens in a new tab), and also measured the width of the head from above. Their system built on earlier designs (opens in a new tab), including one MMF scientists published for whale sharks. In Mozambique, a team led by MMF’s Dr. Chris Rohner found that laser measurements were more precise than visual size estimates by experienced researchers (opens in a new tab).

Lasers also allow sharks to be re-measured without catching them. One study combined dorsal-fin photo-identification with laser photogrammetry to identify 16 great hammerheads in 59 days, with a mean precaudal length of 220.82 cm (7.2 ft) and a mean head width of 71.38 cm (28.1 in) (opens in a new tab). The authors suggest the approach may work over several years to measure growth.

Does a hammerhead’s head grow with its body?

The head grows as a hammerhead grows, but in scalloped hammerheads it becomes proportionally smaller, although studies disagree on whether its width keeps pace with the body. Biologists describe a body part that keeps its proportions as growing isometrically, and one that grows relatively more slowly than the body as showing negative allometry. In museum specimens of 32 to 130 cm (13 to 51 in), the scalloped hammerhead’s head area scaled with negative allometry, while the eye-to-eye width was isometric (opens in a new tab). In other words, head area increased less than it would if the head kept its proportions, while its width kept pace with body length. A study of 129 freshly landed scalloped hammerheads in Baja California Sur, Mexico, by contrast, found strong evidence of negative allometry in the cephalofoil diameter (opens in a new tab), the eye-to-eye width of the hammer-shaped head, across the size range sampled.

Why the head should be relatively larger in pups is unresolved. The Mexican study suggests that a wider head, with its larger search area and greater maneuverability, may give an advantage to newborns and young juveniles hunting small, agile prey on the seafloor (opens in a new tab). Older sharks feeding in open water are unlikely to benefit as much. The museum study suggests that the change in head shape might increase maneuverability to aid in the capture of more evasive prey, or might relate to sexual maturity (opens in a new tab).

That second idea has a precedent. In male bonnetheads, a prominent bulge develops along the front edge of the head at the onset of sexual maturity (opens in a new tab), a change not seen in adult females, juveniles, or embryos. The functions of the hammer itself are covered in why hammerheads have a hammer.

Are hammerhead sharks getting smaller?

In at least one region monitored for decades, yes. In Queensland, Australia, the average size of great and scalloped hammerheads caught by the state’s shark-control program declined significantly between 1997 and 2017 (opens in a new tab). Over those two decades, the average great hammerhead fell from 274 to 215 cm (9.0 to 7.1 ft), and the average scalloped hammerhead from 204 to 177 cm (6.7 to 5.8 ft) (opens in a new tab). Over the same period, the probability of recording a mature female scalloped hammerhead fell from 54% to 14%, and a mature female great hammerhead from 85% to 59% (opens in a new tab).

Fisheries elsewhere catch mainly young sharks. Off Ecuador, 82.7% of the female scalloped hammerheads recorded in landings between 2003 and 2009 were immature (opens in a new tab). In Indonesia, virtually all gillnet-caught scalloped hammerheads were below the size at maturity (opens in a new tab), which led the authors to suggest the species is likely to be prone to overfishing there.

Effective fisheries management can help these populations recover. Great and scalloped hammerheads are prohibited species in Florida’s state waters (opens in a new tab). A global analysis published in 2021, with an MMF co-author, reported that hammerhead shark populations are rebuilding in the Northwest Atlantic, owing to strictly enforced quotas throughout their US range (opens in a new tab).

Why it matters for conservation

The large hammerheads are vulnerable to fishing because females mature late and at about 2 m (6.6 ft) or more (opens in a new tab), so a fishery that catches mostly smaller sharks removes them before they have bred. Off Ecuador, half of female scalloped hammerheads are mature only at 219.4 cm (7.2 ft) TL (opens in a new tab), and in Indonesia only at 13.2 years of age (opens in a new tab). For the great hammerhead, the 2019 assessment notes that a long lifespan and breeding only once every two years, combined with high bycatch mortality, make it susceptible to depletion (opens in a new tab); see overfishing for the wider picture.

The same biology points to solutions. Scalloped hammerhead pups occupy identifiable nursery habitats, such as the putative nursery at Puerto Grande (opens in a new tab), and adults show high site fidelity to particular islands and seamounts (opens in a new tab). That combination makes protection through well-placed marine protected areas feasible (opens in a new tab). The chapter on threats and conservation sets out the protection that exists, and the Galápagos page describes MMF’s work there with Dr. Alex Hearn’s team. To support MMF’s shark research in the Galápagos, visit the silky shark tracking project.