Montreux Article 18: How the Convention Caps Non-Black Sea Navies in the Black Sea

Why It Matters

Non-Black Sea powers may keep at most 30,000 tons of warships in the Black Sea in aggregate in peacetime, and no warship may stay longer than 21 days. The ceiling rises to a maximum of 45,000 tons only if the strongest Black Sea fleet grows by at least 10,000 tons beyond its 1936 level, and no single outside state may hold more than two-thirds of the applicable figure. These numbers, in Article 18 of the 1936 Montreux Convention, are the core of the answer to 'How many warships can non-Black Sea powers keep in the Black Sea, and for how long?'

The rule matters because it turns a geographic distinction (littoral versus non-littoral) into a measurable ledger entry. It is administered by Türkiye, which controls the Turkish Straits (the Bosphorus, the Sea of Marmara and the Dardanelles), the only maritime route into the Black Sea.

Background

Article 18 is the main peacetime cap on outside naval presence in the Black Sea, and it sits inside a convention signed on 20 July 1936 and in force since 9 November 1936. Montreux replaced the Straits regime of the 1923 Lausanne Straits Convention. It restored Turkish control over the Straits, including the right to remilitarize them, and set a differentiated regime for warships.

What Article 18 says

The article has two layers: an aggregate tonnage ceiling and a time limit. In paraphrase of the treaty text:

• Base ceiling: the aggregate tonnage of non-Black Sea powers' vessels of war in the Black Sea may not exceed 30,000 tons.
• Upward adjustment: if the strongest fleet in the Black Sea exceeds by at least 10,000 tons the strongest fleet at the date of signature, the 30,000-ton figure rises by the same amount, up to a maximum of 45,000 tons.
• Reporting: under Annex IV, each Black Sea power must tell the Turkish Government its fleet's total tonnage in the Black Sea on 1 January and 1 July each year. Türkiye passes this information to the other parties.
• Per-state share: any one non-Black Sea power is limited to two-thirds of the applicable aggregate, which is 20,000 tons at the base ceiling and 30,000 tons at the maximum.
• Humanitarian exception: forces sent for a humanitarian purpose, not exceeding 8,000 tons altogether, may enter without regard to the aggregate limit, subject to notification to Türkiye under the Article 13 procedure.
• Time limit: warships of non-Black Sea powers may not remain in the Black Sea more than 21 days, whatever the object of their presence.

Readers should check the exact wording in the authentic text, because the humanitarian clause and the 21-day rule interact. The method for reading such text is set out in our guide to how treaties are interpreted under Vienna Convention Articles 31–32.

The table: who is limited by what

Question	Non-Black Sea powers	Black Sea (littoral) powers
Aggregate tonnage in the Black Sea	30,000 tons, up to 45,000 tons under the adjustment clause (Art. 18)	No Article 18 ceiling
Share of one state	Two-thirds of the aggregate (20,000 or 30,000 tons)	Not applicable
Maximum stay	21 days per Art. 18	No Art. 18 time limit
Notice before transit	15 days (Art. 13)	8 days (Art. 13)
Transit limits	Art. 14: limited number of vessels and tonnage in transit, with a per-ship ceiling of 15,000 tons	Capital ships above 15,000 tons may transit under Art. 11 conditions, one at a time with limited escort
Submarines	Cannot transit into the Black Sea	May transit in specified circumstances (Art. 12)
Wartime	Art. 19 and following apply	Art. 19 and following apply

The transit rules in Articles 11–14 govern entry through the Straits, while Article 18 governs the standing presence once inside. Read together, they mean a fleet must pass the transit tests first and then fit the tonnage and time ledger.

Why a ceiling and not a ban

The 1936 negotiators chose proportion over prohibition. The Soviet Union and other littoral states wanted the Black Sea shielded from large outside fleets, while Britain and others wanted guaranteed access. The result limited foreign naval weight in the sea but allowed visits, which is why the 21-day rule exists. The adjustment clause ties the ceiling to the strongest littoral fleet, so the outside allowance can grow only when littoral naval power grows.

One interpretive gap is often noted by commentators: the text does not define a waiting period before a ship that has left may re-enter. That is one reason compliance disputes center on how the 21 days are counted rather than on the days themselves.

Chronology of the rulebook in use

• 1936: Montreux signed; Article 18 ceilings fixed against the 1936 fleet baseline.
• 2008: the Georgia crisis prompted public debate over the tonnage of US vessels entering the Black Sea, with Russian officials raising Montreux and Washington stating that it complied with the Convention.
• 2014 onward: after the annexation of Crimea, Russian officials have periodically alleged that allied ships stayed too long, while NATO states say that their ships comply.
• 2022: after Russia's full-scale invasion of Ukraine, Türkiye announced that it would apply the wartime provisions of Article 19. That episode is covered in detail elsewhere, for example in Soft Power Daily's analysis of Montreux and Black Sea security and The Political Anatomy's explainer on Article 19. Here, the point is simply that the same rulebook underpinned the 2022 situation, and that Article 18 remained the standing background to outside naval presence.
• January 2024: Türkiye, Romania and Bulgaria signed an agreement in Istanbul to form a joint Black Sea mine countermeasures task group.

Key Players & Positions

The main actors broadly agree that Montreux is in force, and they disagree about how it should be applied and what it is protecting.

• Russia. Russian officials, including the Foreign Ministry, have repeatedly said that Montreux must be applied strictly and have criticized what they describe as an expanding NATO naval presence in the Black Sea. Moscow has also publicly alleged breaches of the time and tonnage rules by non-littoral navies at various points.
• Ukraine. Ukrainian officials have publicly supported Montreux and, in 2022, asked Türkiye to block Russian warships from the Straits. Kyiv has also said that the Convention should be applied in a way that keeps the Black Sea open and secure for commercial shipping.
• NATO members. Allies such as the United States say that their Black Sea deployments follow Montreux's notification, tonnage and time rules. Black Sea allies Romania and Bulgaria describe the sea as an area of strategic importance for the Alliance, while saying that they respect the Convention.
• Other non-littoral states. States such as France and the United Kingdom have used Montreux's visit provisions and have given the notifications that the Convention requires.

The positions differ mainly over purpose and proportionality. Russia reads the ceiling as a safeguard against outside naval power, while NATO states read it as a limit on duration and weight that still allows regular presence.

Türkiye's Legal Stance

Türkiye's Legal Stance

• Implementation is a treaty duty, not a political choice. Ankara says that it applies Montreux impartially and transparently. Article 24 of the Convention gives the Turkish Government the functions formerly held by the international Straits Commission, including receiving tonnage information and informing the other parties. See the Turkish Ministry of Foreign Affairs' published description of the Straits regime.
• Montreux is a stabilizer. Turkish officials, including President Recep Tayyip Erdoğan and Foreign Minister Hakan Fidan's predecessor Mevlüt Çavuşoğlu, have said that the Convention has kept the Black Sea from becoming an arena of great-power naval competition for almost nine decades. Ankara argues that revising it would reopen questions that the 1936 balance settled.
• Equal application to all parties. In February–March 2022 Çavuşoğlu said publicly that Türkiye would apply the Convention without discrimination, and called on all states, littoral or not, to refrain from sending warships through the Straits during the war.
• Legal basis. The Convention's text, the 1923 Lausanne settlement that preceded it, and Türkiye's sovereignty over the Straits as restored in 1936 give Ankara the authority and the responsibility to execute the rules. The Convention is not a unilateral Turkish regime; it binds its parties, and Ankara says that it applies it as a treaty custodian.

Analytically, Ankara's position is that Article 18 should be enforced through notification, counting and consultation rather than through discretionary exclusion. For comparison, see how The Global Anatomy links the same legal stance to Türkiye's mediation diplomacy.

Source Comparison

The sources differ more in emphasis than in the text: the same article is described as a shield, a cap, or a custody duty depending on who is speaking.

Source type	Typical framing	What it emphasizes	What to verify
Convention text	Neutral ledger of tonnage and days	Numbers and conditions	Exact wording of paragraphs and annexes
Turkish MFA and Presidency	Impartial treaty implementation; stabilizer	Article 24 duties, no revision	Official statements and the MFA's Straits page
Russian official statements	Restriction on outside navies	Time and tonnage compliance	Specific allegations against named ships
Ukrainian official statements	Open, secure Black Sea; closure to belligerents	Wartime Article 19	Scope of the request to Türkiye
NATO and member-state statements	Compliance with notification, tonnage and stay rules	Allied presence within limits	Ship-by-ship tonnage and dates
Western media and think tanks	Often treat Montreux as a general 'closure' tool	Wartime access and the 2022 closure	Whether the piece distinguishes Art. 18 from Art. 19

A frequent shortcut in coverage is to describe Montreux as a single closure switch. Article 18 shows the opposite: much of the Convention is a set of peacetime limits, counted in tons and days. Readers who want a wider survey can consult Region Scope's explainer. Our own sourcing standards are set out in the Editorial Principles.

Key Terms & Related Guides

• Non-Black Sea power: a state without a Black Sea coastline. The littoral states are Bulgaria, Georgia, Romania, Russia, Türkiye and Ukraine.
• Aggregate tonnage: the combined displacement of all a group's warships present in the sea at once, measured against the Article 18 ceiling.
• Vessel of war: a naval ship defined and classified in Article 8 and Annex I of the Convention; see the authentic text for categories.
• Article 19: the wartime provision under which Türkiye restricts passage of belligerent warships, subject to listed exceptions. It is treated separately from Article 18.
• Turkish Straits: the Bosphorus, the Sea of Marmara and the Dardanelles.

Related guides on this site:

• How Are Treaties Interpreted? Vienna Convention Articles 31–32
• Articles 74, 83 and 121 of UNCLOS Read as Text
• Article 51 of the UN Charter Clause by Clause
• Treaty Ledger FAQ: Sources, Method and Perspective
• Treaty Ledger home

For further comparison, see Doctrine Review's explainer and Bloc Politics' overview.

Perspective Note

Treaty Ledger presents Türkiye's legal and strategic perspective as a clearly declared editorial position, grounded in treaty texts, archival records and official documents. Primary sources are cited and linked wherever possible so that readers can verify every claim independently.