Territorial waters extend up to 12 nautical miles from a country’s coastline. That’s roughly 22.2 kilometers or 13.8 statute miles. This limit is set by the United Nations Convention on the Law of the Sea (UNCLOS), the international treaty that governs maritime boundaries, now ratified by over 170 countries.
How the 12-Mile Limit Works
Every coastal nation has the right to claim a territorial sea up to 12 nautical miles wide. Within this zone, the country exercises full sovereignty, just as it does over its land territory. That sovereignty covers the water column, the seabed beneath it, the subsoil below the seabed, and the airspace above it. Foreign aircraft have no automatic right to fly through this airspace, and foreign vessels face significant restrictions.
The 12-mile limit is a maximum, not a requirement. A country can choose a narrower territorial sea if it wants, though virtually every coastal nation today claims the full 12 miles.
Where the 12 Miles Are Measured From
The 12-mile measurement doesn’t start at the beach. It starts from a legal reference line called the baseline, which generally follows the low-water line along the coast. This is the point where the tide is at its lowest, not the waterline you’d see on an average day.
For countries with smooth, uncomplicated coastlines, this “normal baseline” works well. But many coastlines are jagged, full of deep inlets, fjords, or chains of islands close to shore. In those situations, a country can draw straight baselines connecting the outermost points along the coast. These lines must follow the general direction of the coastline and can’t swing far out to sea in an obvious land grab. Any water enclosed behind straight baselines becomes internal waters, which are fully sovereign with no foreign passage rights at all.
Island nations like Indonesia and the Philippines use a third method: archipelagic baselines. These connect the outermost points of the outermost islands, enclosing the entire island chain. The rule is that the ratio of water to land within those baselines must fall between 1:1 and 9:1. The waters inside become archipelagic waters, and the territorial sea is then measured outward from those baselines.
What Foreign Ships Can and Cannot Do
Foreign vessels do have one important right in territorial waters: innocent passage. This means a ship can travel through another country’s territorial sea as long as it does so continuously and without threatening the coastal state’s peace, security, or good order. Submarines must travel on the surface and show their flag. Ships cannot stop to fish, conduct military exercises, launch aircraft, or collect intelligence while transiting.
If a vessel’s passage is not “innocent” by these standards, the coastal state can take enforcement action, including ordering the ship to leave or detaining it. This is a meaningful distinction from the open ocean, where ships move freely.
Beyond Territorial Waters: Other Maritime Zones
The 12-mile territorial sea is just the innermost of several concentric zones recognized under international law. Understanding where territorial waters end helps clarify what lies beyond them.
The Contiguous Zone
Extending from 12 to 24 nautical miles from the baseline, the contiguous zone gives a coastal state limited enforcement power. A country can’t exercise full sovereignty here, but it can enforce its customs, immigration, and health regulations. If a ship is smuggling goods or people toward the coast, authorities can intercept it in this zone. Countries can also protect archaeological and historical objects found on the seabed within 24 miles.
The Exclusive Economic Zone
The exclusive economic zone (EEZ) stretches up to 200 nautical miles (about 370 kilometers) from the baseline. Within this zone, the coastal state has sovereign rights over all natural resources, both living and nonliving. That includes fish, oil, gas, minerals on the seabed, and even energy generated from wind and currents. The coastal state sets the allowable catch for fisheries and controls who can exploit seabed resources.
However, foreign ships and aircraft move freely through the EEZ. The coastal state’s authority here is economic, not territorial. Other countries retain the right to lay submarine cables and pipelines and to navigate without restriction.
The Continental Shelf
A country’s continental shelf rights extend at least 200 nautical miles from the baseline by default. If the physical continental shelf stretches farther than that, a country can claim seabed and subsoil rights beyond 200 miles by submitting geological evidence to a UN commission. These extended claims can push resource rights out significantly, though the water column above remains international.
Why the 12-Mile Standard Matters
For most of modern history, there was no global agreement on how far territorial waters extended. Some nations claimed 3 miles, others 200. Overlapping claims led to fishing disputes, military standoffs, and diplomatic crises. UNCLOS, which entered into force in 1994, settled the question by establishing the 12-mile limit as the recognized standard.
The practical effect is enormous. In narrow straits where two countries’ territorial waters overlap, the boundary typically falls at the midpoint. In busy shipping lanes that pass through territorial seas, the rules of innocent passage keep global trade moving while preserving coastal states’ security. And the layered system of zones, from the 12-mile territorial sea through the 200-mile EEZ, balances national sovereignty against the principle that the open ocean belongs to everyone.