Tonnage is a measurement of how much a ship can carry or how big it is, though it doesn't always mean actual tons. When sailors talk about a ship's tonnage, they're describing its size and capacity: how much cargo it can hold, how much space it has below deck, or sometimes how heavy it is when fully loaded.

Over time, tonnage became a common way to describe a ship's size. A small fishing boat might have a tonnage of 50, while a massive container ship could have a tonnage of 200,000 or more.

Tonnage matters because it helps determine what a ship can do. A ship with greater tonnage can carry more cargo across the ocean, which can make it more valuable for trade. Harbors and canals often have tonnage limits: if your ship's tonnage is too high, it might be too big to enter safely.

During wartime, nations sometimes measure their naval power by the total tonnage of their fleet. Having more tonnage can mean having bigger ships or more of them, which can translate to greater strength at sea.