Maritime Boundaries and Exclusive Economic Zones

Maritime boundaries are the limits of the sea areas where a coastal state has rights, set by the UN Convention on the Law of the Sea (UNCLOS), and the exclusive economic zone (EEZ) is the area extending up to 200 nautical miles (about 370 km) from the coast in which the state has sovereign rights over fishing, oil, gas, minerals and energy production. In brief: from the coast outward, the zones are the territorial sea (12 nautical miles, full sovereignty), the contiguous zone (up to 24 nautical miles), the EEZ (up to 200 nautical miles) and the continental shelf (at least 200 and in some cases up to 350 nautical miles). Beyond lie the high seas, which are open to all. One nautical mile is 1.852 km.

Maritime zones cover about a third of the world’s oceans and contain most of the sea’s fish, oil and gas, so who owns them is a matter of wealth and security. This guide explains the baseline and each zone, how boundaries between neighbors are drawn, the main disputes, compares the zones in a table, works through calculations, outlines the history and corrects common mistakes.

What are the maritime zones?

• Internal waters: landward of the baseline, such as bays, ports and rivers. They are treated as part of the state’s land territory.
• Territorial sea (up to 12 nautical miles): the state has sovereignty over the water, the seabed and the airspace, but foreign ships have the right of “innocent passage.”
• Contiguous zone (up to 24 nautical miles): the state may enforce customs, immigration, health and tax laws.
• Exclusive economic zone (up to 200 nautical miles): the state has sovereign rights over living and non-living resources, and jurisdiction over artificial islands, marine research and environmental protection. Other states may sail, fly over and lay cables and pipelines.
• Continental shelf: the natural prolongation of the land under the sea. The state has rights over the seabed and subsoil resources to 200 nautical miles, and beyond it, up to 350 nautical miles or more in some cases, if it can show that the shelf extends that far, according to a UN commission.
• High seas and the Area: beyond national jurisdiction, open to all states. The seabed there is the “common heritage of mankind” and is managed by the International Seabed Authority. A treaty on biodiversity in areas beyond national jurisdiction was adopted in 2023.

Large EEZs are held by countries with many overseas territories or long coasts. France and the United States each have EEZs of the order of 11 million km², followed by Australia, Russia and the United Kingdom. Small island states, such as Kiribati and the Cook Islands, have huge EEZs compared to their land area.

How are baselines and boundaries between neighbors set?

The zones are measured from the baseline, which is normally the low-water line along the coast. Where the coast is deeply indented or has fringing islands, straight baselines can be drawn. Archipelagic states such as Indonesia and the Philippines may draw baselines around the outermost islands, enclosing the waters between as archipelagic waters. Rocks that cannot sustain human habitation or economic life have a territorial sea but no EEZ or continental shelf (Article 121), a rule that is often at the center of disputes.

When two coastal states are less than 400 nautical miles apart, their EEZs overlap. They must agree on a boundary, often by drawing a median (equidistant) line, with every point equally distant from both coasts, and adjusting for relevant circumstances to reach an equitable result. Where they cannot agree, they may go to the International Tribunal for the Law of the Sea, the ICJ or arbitration.

What are the main disputes?

The South China Sea is the best-known: China claims most of the sea within the “nine-dash line,” while the Philippines, Vietnam, Malaysia, Brunei and Taiwan have competing claims. In 2016, an arbitral tribunal, in a case brought by the Philippines, found that the nine-dash line had no legal basis for historic rights beyond UNCLOS and that none of the Spratly features was an island generating an EEZ, a ruling that China rejects. Other issues include the Arctic, where states claim extended continental shelves as ice retreats, the Eastern Mediterranean, with gas fields near Cyprus, Greece, Türkiye and Israel, the Aegean Sea, and fishing disputes. There are also successful settlements, such as the maritime boundary between Norway and Russia in the Barents Sea agreed in 2010. Illegal fishing, piracy and the protection of marine ecosystems are further reasons for cooperation.

The zones compared

Worked example: distances and areas

1. Conversion: 1 nautical mile = 1.852 km. So 12 nautical miles = 12 × 1.852 = 22.2 km, 24 nautical miles = 44.4 km, 200 nautical miles = 370.4 km and 350 nautical miles = 648.2 km.
2. EEZ of a small island: a small island has an EEZ that is approximately a circle of radius 370.4 km. The area is π × 370.4² = 3.1416 × 137,196 = about 431,000 km², roughly the area of Sweden, even if the island itself covers only a few square kilometers.
3. EEZ along a straight coast: a country with a straight coast of 600 km has an EEZ strip about 600 × 370.4 = 222,000 km² (ignoring the corners), and the territorial sea part is 600 × 22.2 = 13,300 km², which is 6% of the EEZ strip.
4. Median line: two coasts face each other at a distance of 300 nautical miles. Each country’s 200 nautical mile claim would reach 200 nautical miles from its own coast, so the claims overlap by 200 + 200 − 300 = 100 nautical miles. A median line would divide the sea at 150 nautical miles from each coast.
5. Value of fish: if an EEZ yields 2 tonnes of fish per km² per year and has an area of 400,000 km², the annual catch potential is 800,000 tonnes. At a price of 1,200 per tonne, that is 960 million a year (an illustrative figure).
6. Shipping: a vessel crossing a 12-mile territorial sea at 12 knots (12 nautical miles per hour) takes 1 hour.

How did the law of the sea develop?

In the 17th century, the Dutch jurist Hugo Grotius argued for the “freedom of the seas” in his book Mare Liberum (1609), while coastal states claimed a narrow belt of water, often said to be based on the range of a cannon shot, about 3 nautical miles. In 1945, US President Truman claimed the resources of the continental shelf, and several Latin American states, including Chile, Peru and Ecuador, claimed 200 nautical miles in 1947 and 1952. The UN held three conferences on the law of the sea between 1958 and 1982, and UNCLOS was opened for signature in 1982 and entered into force in 1994. It has been ratified by over 160 parties, though the United States has not ratified it, although it follows most of its rules in practice. Since then, claims to extended continental shelves, disputes over islands and growing interest in seabed minerals have kept the law of the sea at the center of international attention.

Which mistakes are common?

Mistake 1: Treating the EEZ as sovereign territory

In the EEZ, the coastal state has sovereign rights over resources, but ships and aircraft of other states keep freedom of navigation and overflight.

Mistake 2: Mixing up nautical miles and statute miles

A nautical mile is 1.852 km, about 15% longer than the statute mile of 1.609 km, so 200 nautical miles are about 230 statute miles.

Mistake 3: Believing that any rock gives a 200-mile zone

A rock that cannot sustain human habitation or economic life has only a 12-nautical-mile territorial sea and no EEZ.

Mistake 4: Assuming that the high seas belong to nobody and have no rules

They are open to all, but subject to international law on fishing, shipping, pollution, safety and the seabed.