You're Gonna Need a Bigger Boat

Chief Brody says the line, and then the film moves on. This works out the number. Give it a shark and it gives you the smallest boat that could reasonably handle one, using a published length-to-weight relationship for white sharks and the displacement math boatbuilders actually use. It draws both of them to scale so you can see the problem, and yes, the Orca is on the chart.

I know the shark I have a boat

Feet Metres

Which shark

Shark length (nose to tail tip, total length)

Your boat's length (overall, bow to stern)

Kind of boat

The boat you actually have (optional, drawn in for comparison)

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The answer, up top

For the shark in the film, you need about 59 feet of boat. The Orca was 42. Chief Brody was right, and he was right by about seventeen feet, which he worked out in roughly four seconds by looking at the animal over the transom. This page takes longer and gets to the same place.

The number is not a guess. A great white's weight can be predicted from its length using a relationship published by the National Marine Fisheries Service, and a hull's weight can be predicted from its length using the ratio boatbuilders have used for a century. Both halves are real. What joins them is a rule we made up, and we will show you exactly where we made it up.

Step one: what a shark that size actually weighs

In 1995 three NOAA researchers published length and weight data for thirteen shark species caught in the western North Atlantic. For white sharks they had 125 animals, and the fit is very good indeed:

W = 7.5763 × 10⁻⁶ × FL^3.0848

W is mass in kilograms and FL is fork length in centimetres, which is measured from the snout to the fork of the tail rather than to the very tip. Since nobody describes a shark that way in conversation, the same paper supplies the conversion, fitted to 112 sharks: FL = 0.9442 × TL − 5.7441. Feed it a total length and it hands back the fork length the weight equation wants.

One detail is worth more than it looks. Push the largest fork length in the study, 493 cm, back through the weight equation and it returns 1,536 kg, against a heaviest recorded specimen of 1,554 kg. The equation reproduces its own sample to within about 1.2 percent. That is the kind of internal check that tells you a formula is describing something real rather than curve-fitting noise.

So how heavy is a 25 foot shark?

Hooper calls it at twenty feet. Quint corrects him: twenty five, and three tons of him. Two numbers, shouted across a deck, at a fish. Let us find out what that shark would really weigh.

Twenty five feet of total length is 762 cm, which converts to 714 cm of fork length, which the equation turns into about 4,800 kg. That is 10,600 pounds, or 5.3 short tons. Which is the delightful bit: the equation says that shark was even bigger than the legend. Quint eyeballed a moving animal from a rolling boat and called it within a couple of tons, which is a good day's work for anybody, and the real number only makes his fish more impressive.

Now let us be equally honest about our own figure. The equation was fitted to sharks up to 493 cm of fork length, and a 25 foot shark is about 45 percent past the biggest animal in the sample. That is extrapolation. The page says so on every result that goes beyond the fit, and no amount of decimal places changes it. It is the best answer available and it is still a projection, so hold it the way you would hold any number that has been asked to reach past its own data.

Step two: what makes a boat big enough

Not its length. Length is the number written on the side of the boat and it is nearly useless on its own, because a 42 foot wooden lobster boat and a 42 foot racing sailboat differ in weight by a factor of four. What matters is displacement: how much water the hull pushes aside, which is the same thing as how much the boat weighs.

The standard way to get from one to the other is the displacement to length ratio:

DLR = displacement in long tons ÷ (0.01 × waterline length in feet)³

An ultralight racing hull sits near 90. A cruising boat runs 200 to 300. A heavy workboat or trawler is 300 to 400, and a tug or a deep sea dragger goes past 400. The Orca, a Cape Island lobster hull planked in wood, belongs firmly in the workboat band, which is why this page puts it at 320 and gets a displacement of about 17 long tons for a 42 footer. That is roughly 39,000 pounds of boat, which is about right for the type.

Step three: the rule we invented, and why

Here is where the honest part ends and the joke begins, so we are going to flag it clearly. There is no published standard for how big a boat has to be for a shark. Naval architecture has never needed to answer that question. So we made one up, and then we did the one thing that makes a made-up rule defensible: we calibrated it against the only full-scale trial that exists.

The Orca displaced about 17 long tons. Quint's shark masses about 4.7 long tons. That is 27 percent of the entire boat, on a rope, off the stern, actively trying to leave. The Orca sank. That is one data point, it is fictional, and it is all anybody has.

So this page sets the limit at ten percent: the shark should mass no more than a tenth of what the boat displaces. Against the one recorded failure that is a safety factor of about 2.7, which is a normal sort of margin for something you would rather not repeat. Ten percent is our number. It is not a standard, it is not an industry figure, and if you quote it at a naval architect they will ask you what you have been reading.

Why the ratio comes out near 2.3 to 1 every time

Run the numbers for a twelve foot shark and you need about 27 feet of boat. Run them for the film's shark and you need about 59. The ratio barely moves, and there is a reason that is not a coincidence.

A shark's mass grows with roughly the cube of its length, since the exponent in the weight equation is 3.08. A hull's displacement grows with exactly the cube of its length, because that is what the displacement to length ratio says. Two cubes divided by each other give something very close to a constant, so the answer lands near 2.3 feet of boat per foot of shark across the whole range. The interesting part is not the ratio. It is that the boat you need scales with the animal's weight, and weight is the thing people are worst at estimating by eye.

What a real great white actually is

Adult males average 11 to 13 feet. Adult females, which are the larger sex, average 15 to 16. The largest white shark anyone has reliably measured was 6.1 metres, or 20 feet, a female taken off Prince Edward Island and verified by the Canadian Shark Research Centre. J. E. Randall, applying a stricter standard to the records, put the largest defensible measurement at 19.5 feet.

Sharks estimated at 23 feet exist in the literature, the specimens known as KANGA and MALTA, and both shrank when their measurements were re-examined. Which puts the film in perspective. The shark in Jaws is not a large great white. It is five feet longer than any great white that has ever been measured, and about four times the mass of the heaviest one ever weighed on a rod and reel.

That is the real reason the Orca was doomed, and it has nothing to do with Quint's seamanship. He was not undergunned for a big shark. He was undergunned for an animal that does not exist.

How honest is the weight figure

Reasonably, with one caveat worth stating. The all-tackle record white shark, caught by Alf Dean in South Australia in 1959, weighed 1,208 kg. Put its length through this equation and the curve predicts around 1,400 kg, about 16 percent heavy. That is not an error in the equation. It is the ordinary scatter you get when two animals of the same length carry different amounts of condition, the same way two people of one height can differ by thirty pounds.

So every weight on this page is shown as a band of plus or minus 20 percent rather than a single confident figure. The boat length moves much less than that, because taking a cube root of a range squeezes it: a 20 percent swing in the shark is only about a 6 percent swing in the boat. Uncertainty about the animal barely reaches the answer, which is a pleasant property for a model to have.

The other two fish in the drawing

Look closely at the scale drawing and there are three animals in it, not one. Both of the extras come from other films, both are drawn to the same scale as everything else, and both are carrying real numbers.

The orange speck near the shark's nose is a clownfish, four inches long, which at the scale of a 25 foot great white is a few pixels. We drew all of him anyway: body, three white bands, tail, one worried eye. The drawing is vector, so pinch to zoom and he is entirely there. The connection runs deeper than a cameo, because the crew of Jaws nicknamed their mechanical shark Bruce, and the great white in Finding Nemo is named Bruce in his honour. Same shark, one generation and one ocean apart.

The faint shape in the deep water at the bottom is Otodus megalodon at 52 feet, the classic scientific reconstruction, drawn to the same scale and left dim on purpose. Here is the part worth knowing: when Gottfried and colleagues published the standard megalodon size estimates in 1996, they derived the mass by scaling up white shark length-to-weight relationships, which is to say, the exact method this page runs on. Push our equation to their 15.9 metre animal and it returns about 46,600 kg against their published 47,690, within about two percent. The same arithmetic that sizes your boat also weighs the megalodon, and it agrees with the palaeontologists. For the record, carrying one would take about 125 feet of workboat. But megalodon has been extinct for about 3.6 million years, no matter what a certain 2018 film implies, which makes it the only shark on this page you genuinely do not need a boat for.

What this page is not

It is not marine safety guidance. It is not a fishing tool, it is not advice about anything you might actually do on the water, and it is emphatically not a reason to go looking for a shark. Great whites are protected in United States, Australian and South African waters, and the sensible number of them to meet from a boat of any size is zero.

It is arithmetic about a film, done properly, because the film asked a question and then cut to the next scene.

Sources

Where the numbers on this page come from. We go to the body that publishes the figure, not to another calculator. See how we verify.

Length-weight relationships for 13 species of sharks from the western North Atlantic, Kohler, Casey and Turner, Fishery Bulletin 93:412-418 (1995), National Marine Fisheries Service, NOAA. MEASURED, and the backbone of this page. The white shark row, combined sexes, gives W = 7.5763 x 10^-6 x FL^3.0848, with W in kilograms and fork length in centimetres, from 125 sharks, r-squared 0.9802. The same paper's length-length table gives FL = 0.9442 x TL - 5.7441 from 112 sharks, r-squared 0.9975, which is how a total length gets converted into the fork length the weight equation wants. Two things about the fit matter enough that the page states them. The weight equation was fitted over fork lengths of 112 to 493 cm, so a 25-foot shark sits about 45 percent beyond the largest animal in the sample and the page says so on every result that goes past it. And the equation reproduces its own sample: at the maximum fitted fork length of 493 cm it returns 1,536 kg against a recorded sample maximum of 1,554 kg, a 1.2 percent miss, which our test suite asserts in both directions so the claim cannot drift. Text recovered by inflating the PDF's own deflate streams, since the scanned page images carry no readable layer.
IGFA all-tackle world record, white shark, International Game Fish Association. MEASURED, and used here as the one independent check available. Alf Dean's 1959 shark from South Australian waters weighed 1,208 kg (2,663 lb) and remains the all-tackle record. Run its reported length through the Kohler equation and the curve predicts roughly 1,400 kg, about 16 percent heavy. That is ordinary scatter for a length-weight relationship, since two sharks of the same length can differ substantially in condition, and it is the stated reason this page shows a weight band of plus or minus 20 percent rather than a single confident figure.
Great white shark size, Wikipedia, summarising Randall (1987), Mollet et al. and the Canadian Shark Research Centre. REPORTED, and the source of the size presets. Adult males average 3.4 to 4.0 m (11 to 13 ft) and adult females 4.6 to 4.9 m (15 to 16 ft). The largest reliably measured white shark is the 6.1 m (20 ft) female taken off Prince Edward Island and verified by the Canadian Shark Research Centre; J. E. Randall's more conservative reading puts the largest defensible measurement at 5.94 m (19.5 ft) from Ledge Point, Western Australia. Specimens estimated at 6.9 to 7.13 m, known as KANGA and MALTA, did not survive scientific re-analysis, which is why nothing above 20 ft appears on this page as a real animal.
Size and skeletal anatomy of the giant megatooth shark (Gottfried, Compagno and Bowman, 1996), In Great White Sharks: The Biology of Carcharodon carcharias, Academic Press; figures as summarised with attribution on Wikipedia's Megalodon article. MEASURED, and the licence for the faint shape in the deep water of the drawing. Gottfried and colleagues reconstructed a 15.9 m (52 ft) megalodon at about 47,690 kg, and the method matters more than the number: they derived megalodon mass by scaling up white shark length-to-weight relationships, which is exactly what this page's Kohler equation is. Our test suite extrapolates our own equation to 15.9 m and asserts it lands within three percent of their published figure (it comes out around 46,600 kg, a miss of about two percent), so the easter egg is running the same arithmetic as the rest of the page. Extinction date of roughly 3.6 million years ago per Boessenecker et al. 2019. Modern estimates of maximum megalodon size run from about 15 to over 20 metres; we draw the classic 52 ft reconstruction.
Displacement to length ratio, Standard naval architecture, as set out by boats.com and the WoodenBoat School. MEASURED convention, EDITORIAL choice of values. The ratio is DLR = displacement in long tons divided by (0.01 x waterline length in feet) cubed, and it is the standard way to turn a hull length into a displacement. Published bands run from about 90 for an ultralight hull to over 400 for a tug or a deep-sea dragger, with modern trawlers and workboats quoted at 260 to 320. This page assigns 180 to a centre console, 250 to an express cruiser, 320 to the lobster boat and trawler class that the Orca belongs to, and 420 to a commercial dragger or tug. Those four numbers are our editorial picks inside published bands, not measurements of any particular boat, and the page says so. Waterline length is taken as 0.90 of overall length, a normal working figure for this kind of hull.
Jaws (1975), screenplay, Peter Benchley and Carl Gottlieb, Universal Pictures. The premise, quoted from the script rather than from memory. Hooper: 'That's a twenty footer.' Quint: 'Twenty five! And three tons of him!' Brody's line, ad-libbed by Roy Scheider and now the reason this page exists, reads simply 'You're gonna need a bigger boat.' The Orca was a 42-foot Cape Island style lobster boat, wood planked over frame, originally named Warlock, with a single diesel making 9 to 10 knots. This page is not affiliated with, endorsed by, or connected to Universal Pictures in any way. It is arithmetic about a film.

Frequently asked questions

How big a boat do you need for a great white shark?

For the 25-foot shark in Jaws, about 59 feet of workboat. For a real great white, less. The largest reliably measured white shark was 20 feet, which works out to about 47 feet of boat, and a typical adult female at 15 to 16 feet needs about 36 feet. The rule this page uses is that the shark should weigh no more than a tenth of what the boat displaces, which is a safety factor of about 2.7 against the only full-scale trial anyone has on film.

Was Chief Brody right that they needed a bigger boat?

Yes, and it is possible to say by how much. The Orca was 42 feet. Running the film's own stated shark, 25 feet long, through a published length-to-weight relationship for white sharks gives about 5.3 short tons, which is roughly 27 percent of what a 42-foot wooden lobster boat displaces. A quarter of your entire displacement, on a rope, off the stern. By the standard applied on this page the Orca was about 17 feet short. Brody had it right within about four seconds of looking at the animal.

How much would the shark in Jaws actually weigh?

About 5.3 short tons, which is even more than the three Quint calls out. Feed 25 feet of total length into the Kohler length-weight equation for white sharks and it returns roughly 4,800 kg, or 10,600 lb. So the shark comes out heavier than the legend, which is a fun thing for a movie to be. Quint sized a moving animal by eye from a rolling deck and landed within a couple of tons, and the equation just gives his fish a bit more credit. Hold our figure loosely too: a 25-foot white shark is well beyond the largest animal in the study, so this is an extrapolation, not a measurement.

Has a 25-foot great white shark ever been recorded?

No. The largest reliably measured white shark is 6.1 m, or 20 feet, a female taken off Prince Edward Island and verified by the Canadian Shark Research Centre. J. E. Randall's stricter reading puts the largest defensible measurement at 19.5 feet. Sharks estimated at 23 feet, the specimens known as KANGA and MALTA, did not hold up when the measurements were re-examined. The shark in Jaws is not a big great white. It is five feet longer than any great white anyone has been able to measure.

Why does boat length depend on the shark's weight and not its length?

Because weight is what actually loads the boat. A hull's ability to absorb a shock, resist a heeling pull, or carry a mass on one quarter comes from its displacement, and displacement grows roughly with the cube of length. A shark's mass grows with roughly the cube of its length too, which is why the answer lands near a constant ratio of about 2.3 to 1 for most sizes. Length alone is the wrong measure. A 25-foot shark and a 25-foot sailboat share a number and nothing else.

Where does the 10 percent rule come from?

From us, and the page labels it as ours. There is no published standard for how big a boat has to be for a shark, because it is not a question anyone in naval architecture has needed to answer. What does exist is one documented full-scale trial: the Orca, carrying a shark at about 27 percent of its displacement, which sank. Setting the limit at 10 percent is a safety factor of roughly 2.7 against the only data point on record. That is a joke with the arithmetic shown, not a standard, and it should not be mistaken for one.

What does displacement to length ratio mean?

It is the standard way naval architects turn a hull's length into a weight. DLR is displacement in long tons divided by the cube of one hundredth of the waterline length in feet. A light racing hull sits near 90, a cruising boat around 200 to 300, and a heavy workboat or trawler runs 300 to 400, with tugs and deep-sea draggers above that. It is why a 42-foot wooden lobster boat weighs several times what a 42-foot performance sailboat does, and why picking your boat type on this page changes the answer so much.

Is this real science or a joke?

Both, and neither half is faked. The length-to-weight equation is from a peer-reviewed NOAA study of 125 white sharks and is used exactly as published. The displacement math is the ordinary ratio boatbuilders use. What is invented is the 10 percent rule tying the two together, which is clearly labelled as ours. So the shark weight is real, the boat weight is real, and the thing joining them is a gag with its working shown. This is not marine safety guidance, and nobody should use it to plan an encounter with a shark.