The 12-nautical-mile limit marks where a country’s territorial sea ends and international waters begin, and it exists because 168 countries agreed to that distance in a 1982 United Nations treaty. But the number itself has roots going back centuries, to a time when national sovereignty at sea extended only as far as a cannonball could fly.
The Cannon-Shot Rule That Started It All
In the early 1700s, nations needed a practical way to decide how much ocean they could claim. The answer came from Dutch jurist Cornelius van Bynkershoek, who argued in 1702 that “territorial sovereignty ends where the power of arms ends.” In plain terms: you control the water you can defend from shore. At that time, a cannon could fire roughly 3 nautical miles, about one marine league. That became the standard.
The United States adopted this logic early. In 1793, Thomas Jefferson, then Secretary of State, put forward the first official American claim to a 3-mile territorial sea. Most maritime nations followed the same rule for nearly two centuries. Three miles was the global norm well into the 20th century.
Why 3 Miles Became 12
As military technology advanced, the cannon-shot rationale lost its anchor. A coastal defense battery in the 1900s could strike targets far beyond 3 miles, and nations increasingly wanted broader control over fishing grounds, smuggling routes, and security threats near their shores. Several countries unilaterally claimed wider territorial seas, some as far as 200 miles, creating a patchwork of conflicting claims.
The United Nations attempted to resolve this through a series of conferences on the Law of the Sea, culminating in the 1982 UN Convention on the Law of the Sea (UNCLOS). Article 3 of that treaty states that every country has the right to establish a territorial sea “up to a limit not exceeding 12 nautical miles.” The 12-mile figure was a negotiated compromise: broad enough to satisfy countries wanting more security and resource control, narrow enough to preserve freedom of navigation through the world’s oceans. The convention entered into force in 1994 and now has over 160 parties.
The United States, while not a formal party to UNCLOS, extended its own territorial sea to 12 nautical miles in 1988 through a presidential proclamation signed by Ronald Reagan on December 27 of that year. The U.S. treats the 12-mile standard as customary international law.
How the 12 Miles Are Measured
The 12 miles don’t start at the beach. They’re measured from a country’s official baseline, which is typically the low-water line along the coast as marked on large-scale nautical charts. In the United States, NOAA defines this as the mean lower low water (MLLW), essentially the average of the lowest tides. Cartographers use a technique called “envelope of arcs,” rolling a virtual circle along the charted coastline and connecting the resulting points to form a smooth boundary line 12 nautical miles out.
This matters because coastlines are jagged. Bays, islands, and peninsulas can push the baseline (and therefore the 12-mile limit) farther seaward than you might expect from looking at a map.
What Changes at 12 Miles
Within the 12-mile territorial sea, a coastal state has nearly complete sovereignty. It can enforce its criminal and civil laws, regulate shipping, control resource extraction, and restrict access. Foreign vessels do have one key right: “innocent passage,” meaning they can transit through the territorial sea as long as they move continuously and don’t threaten the coastal state’s peace, security, or good order. Submarines, for instance, must surface and show their flag.
Beyond 12 miles, the picture changes in layers. From 12 to 24 nautical miles lies the contiguous zone, where a country can enforce customs, immigration, and sanitation laws but doesn’t have full sovereignty. From the baseline out to 200 nautical miles is the exclusive economic zone (EEZ), where the coastal state controls fishing, drilling, and other resource extraction but cannot restrict navigation. Beyond the EEZ are the high seas, open to all nations.
Freedoms on the High Seas
Once you’re past a country’s claimed zones, UNCLOS guarantees six core freedoms to all nations, whether they have a coastline or not: navigation, overflight, laying submarine cables and pipelines, building artificial islands and installations, fishing (with some conservation limits), and scientific research. No single country can claim sovereignty over the high seas, and all states are expected to exercise these freedoms with due regard for others.
The Special Case of Narrow Straits
The 12-mile rule creates an obvious problem in narrow waterways. When two countries face each other across a strait less than 24 miles wide, their territorial seas overlap, and there’s no strip of international water in between. Choke points like the Strait of Hormuz, the Strait of Malacca, and the Strait of Gibraltar all fall into this category.
UNCLOS handles this through “transit passage,” a right that allows all ships and aircraft to move through straits used for international navigation without being stopped, even though they’re technically inside another country’s territorial sea. Transit passage is stronger than innocent passage: submarines don’t have to surface, and aircraft can fly overhead. The rule exists because blocking these straits would cripple global shipping. Some historically important straits, like the Turkish Straits, are governed by their own long-standing treaties instead.
Why 12 and Not Some Other Number
There’s no formula that produces 12 as the scientifically correct answer. The number is a political consensus. During the decades of negotiation, some nations pushed for 3 miles (preserving maximum freedom of navigation), while others claimed 200 miles (maximizing resource control). Twelve nautical miles, roughly 22 kilometers or 14 statute miles, emerged as the distance most nations could accept. It was large enough to give coastal states meaningful security and enforcement capacity, but small enough that it didn’t carve up major shipping lanes or fishing grounds. The creation of the EEZ as a separate 200-mile resource zone helped close the deal, giving coastal states economic rights over a vast area without granting full sovereignty over it.