If every ocean-going commercial ship were removed from the water, global mean sea levels would fall quickly, but only by a matter of micrometres. According to calculations based on Archimedes’ principle, the mass of water displaced by a floating vessel equals the combined mass of the vessel and everything it carries.
Mike Follows of West Midlands, UK, explains that a ship with a total mass of 100,000 tonnes displaces 100,000 tonnes of seawater, which is equivalent to about 97,560 cubic metres. He notes that while deadweight tonnage (DWT) represents the maximum additional mass a vessel can carry, the total displacement includes the "lightship mass" of the hull and machinery.
Using data from the UN Conference on Trade and Development’s 2025 Review of Maritime Transport, Follows states there are approximately 112,500 propelled seagoing merchant vessels of at least 100 gross tonnage (GT), with a combined carrying capacity of 2.44 billion DWT. Based on typical ratios, Follows estimates the total maximum displacement of the fleet is about 3 billion tonnes, or roughly 2.9 cubic kilometres of seawater.
Given that the ocean covers about 70 per cent of the Earth's surface—an area of approximately 360 million square kilometres—Follows calculates a maximum global mean sea-level rise of about 8 micrometres. He notes the actual figure would be lower because the fleet is never simultaneously carrying its maximum deadweight.
Herman D’Hondt of Sydney, Australia, provides a "back-of-the-envelope calculation" suggesting that removing the ships would remove less than half of one part per billion of the total ocean volume. D’Hondt estimates that removing 3 billion tonnes of ships would drop the surface level by roughly 18 micrometres. He points out that human hairs are typically between 30 and 60 micrometres thick, stating, "I don’t think anyone will notice a sea-level drop of half the thickness of a hair."
Hillary Shaw of Newport, Shropshire, UK, estimates the total global tonnage of ships at around 2.5 billion deadweight tonnes. She notes that a deadweight tonne doesn’t quite equal a tonne, or 1 cubic metre of water, and that some ships will be in dry dock and many won’t be fully loaded. However, she states that approximately 2.5 billion cubic metres of water are displaced by these ships. Shaw concludes that removing them would result in a sea-level drop of just 7 micrometres.
