How it works
Density is how much mass is packed into a given volume: mass divided by volume. It is the property that decides whether something floats or sinks, feels heavy for its size, or drifts on the breeze. Two objects can be the same size yet wildly different in density — a kilogram of lead and a kilogram of feathers weigh the same but occupy completely different volumes.
Water is the reference everyone carries in their head: 1,000 kg/m³, or a tidy 1 g/cm³. Anything denser than the fluid it sits in sinks; anything less dense floats. That single comparison explains ice cubes bobbing in a drink, oil sitting on vinegar, and hot-air balloons rising.
Because density is mass over volume, you can find any one of the three if you know the other two. Weigh an irregular object, measure the water it displaces, and you have its density — the exact trick Archimedes is said to have used to test a golden crown.
Use it in real life
Cooking and mixing: oil (≈920 kg/m³) floats on water (1,000 kg/m³) because it is less dense — no emulsifier can change the physics, only delay the separation.
Materials and shipping: aluminium (2,700 kg/m³) versus steel (7,850 kg/m³) is a density decision — same strength target, very different weight, which is why aircraft chase low-density alloys.
Diving and safety: seawater is denser than fresh water, so you float more easily in the sea and need extra weight to descend — divers tune buoyancy with density every dive.
Frequently asked questions
What is the density of water?
About 1,000 kg/m³, or 1 g/cm³, at 4 °C where water is densest. It drops slightly as water warms, which is why warm water sits on top of cold in a lake or kettle.
Why do some objects float and others sink?
An object floats if its average density is less than the fluid around it. A steel ship floats because its overall density — steel plus all the air inside the hull — is lower than water, even though solid steel sinks.
How do I measure the volume of an irregular object?
Submerge it in water and measure how much the water level rises. The volume of water displaced equals the object's volume — Archimedes' displacement method — then divide mass by that volume.