water displacement

Water displacement is a technique for measuring the volume of an irregularly shaped solid object, like a stone, that doesn't fit a simple length × width × height formula.

• The method is simple in practice: lower the object into a measuring cylinder partly filled with water, and see how much the water level rises — that rise in volume equals the object's own volume.
• If a measuring cylinder reads 50 mL before an object is added and 55 mL after, the object itself must occupy 5 cm³ of space, since the water was pushed aside by exactly that amount.
• This trick works precisely because water (like most liquids) can't be compressed to make room — the only way to fit a solid object into the same container is to push an equal volume of water out of the way.
• Water displacement is often the missing piece needed to calculate an irregular object's density, since density needs both mass and volume, and this is exactly how the volume half gets measured.