How it works
Density is how tightly packed an object's mass is — mass divided by the volume it occupies. The same relationship rearranges to give mass or volume instead, depending on which two values you already know.
density = mass ÷ volume
mass = density × volume
volume = mass ÷ density
Water is a useful reference point: it's almost exactly 1 kg per litre (or about 62.4 lb per cubic foot). Anything denser than water sinks in it; anything less dense floats.
Worked example
A 10 lb block that takes up 2 cubic feet of space.
- Divide mass by volume: 10 ÷ 2.
- That gives 5.
The density is 5 lb/ft³ — lighter than water, so it would float.
Common questions
Why does density decide whether something floats?
An object floats in a fluid when its density is lower than the fluid's — it displaces a weight of fluid equal to its own weight before it's fully submerged. Denser objects sink because they can't displace enough fluid to balance their own weight.
Does this switch to metric units automatically?
Yes — switch the locale in the header and the units change to kilograms, litres, and kg/L. The maths is identical either way; only the labels change.
What's the difference between density and specific gravity?
Specific gravity is density divided by the density of water, so it's a plain ratio with no units — a specific gravity of 2 means twice as dense as water, regardless of whether you're working in metric or imperial.