Why coastlines keep changing length depending

British King Charles III last month inaugurated a new hiking route that will eventually run around the entire coast of England.

Yet while the length of the newly designed path is precisely defined, the coastline it traces is anything but—highlighting a long-standing scientific puzzle over how coastlines are measured.

The project is around 80% complete and, when fully opened later this year, the “King Charles III England Coast Path” will become the longest managed coastal walking route in the world. Stretching 4,327 kilometres, it will allow walkers to trace England’s shoreline step by step, passing everything from granite cliffs and sand dunes to iconic white chalk cliffs.

England’s coastline is typically counted as part of the wider UK shoreline, but as the BBC notes, official figures vary widely depending on the source. The CIA World Factbook puts the UK coastline at 12,429 kilometres, while the World Resources Institute estimates 19,716 kilometres—a difference of more than 7,242 kilometres.

“The thing is, no one really knows exactly how long England's coastline is, or the United Kingdom's or most coastlines around the world, for that matter,” said Victoria Braswell, a researcher and member of the Royal Geographical Society. “It's all in how you measure it.”

The discrepancies become even larger when looking at countries such as the United States. Estimates range from 19,924 kilometres according to the CIA World Factbook, to 135,185 kilometres from the US Army Corps of Engineers, and as much as 153,646 kilometres according to the National Oceanic and Atmospheric Administration (Noaa), the federal agency responsible for mapping the US coastline.

Why coastlines don’t have a fixed length

The reason for these dramatic differences—and why none of them is strictly incorrect—can be traced back to a curious mathematical problem identified nearly a century ago by a British pacifist studying war patterns.

In 1921, Quaker mathematician and physicist Lewis Fry Richardson examined whether countries sharing longer borders were more likely to go to war. In the process, he noticed inconsistencies in how countries reported their own boundaries: Spain recorded its border with Portugal as 987 kilometres, while Portugal measured it at 1,214 kilometres. Belgium and the Netherlands showed a similar mismatch of 69 kilometres, with comparable differences appearing across Europe.

Richardson found that countries typically measured borders and coastlines using rulers placed end-to-end along maps. But because natural boundaries are rarely straight, the smaller the ruler used, the more detail it captured—and the longer the measured line became.

This became known as the coastline paradox, and it applies to almost all irregular borders and shorelines, including England’s.

More recently, in December 2024, India drew attention when it announced its coastline had effectively lengthened by more than 3,500 kilometres without gaining any new territory—an increase of nearly 50%. The change reflected the use of higher-resolution mapping, effectively using “smaller rulers” to capture finer geographical detail such as inlets, bays and offshore islands.

Barriers to unified system

All of this raises a simple question: why don’t countries agree on a single standard for measuring coastlines?

“It’s complicated,” said Hyam. One issue is tides: measuring a coastline at high tide versus low tide can significantly alter its length, meaning a truly standardised system would require identical tidal conditions everywhere—an approach that is practically unworkable.

Scale is another challenge. Hyam explained that Britain maps its coastline at a 1:10,000 scale, consistent with national land mapping and the country’s relatively compact geography. But in larger countries such as the US, Canada or Australia, vast sparsely populated areas make such high-resolution mapping far less practical.

Even if a global standard were agreed, questions would remain over what that scale should be. While smaller units produce more precise measurements, they also risk pushing coastline length toward infinity—raising the question of whether such precision brings us closer to reality, or further away from a usable definition.

By Nazrin Sadigova