Britannica Sources used in this answer

Why does a submerged object seem to weigh less than it does in air?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

differential manometer, Torricellian barometer, and siphon — fluid mechanics Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.

A submerged object appears to weigh less than it does in air because of a physical law known as Archimedes’ principle. When an object is immersed in a fluid—whether a liquid or a gas—it is acted upon by an upward force called buoyancy.12

This buoyant force is caused by the pressure of the fluid, which increases with depth due to the gravitational weight of the fluid above. This pressure exerts an upward force on the submerged object that opposes the downward force of gravity. According to Archimedes' principle, the magnitude of this upward force is exactly equal to the weight of the fluid displaced by the object.12

Consequently, the object's apparent weight is its true weight minus the weight of the displaced fluid. Because the buoyant force effectively offsets a portion of the object's gravitational weight, the object feels lighter while submerged.314