A pebble you can hide in your fist goes straight to the bottom. A tree-sized log just bobs there, lazy as anything. It isn't about big or small, heavy or light — it's about how tightly the stuff inside is packed. That's density.
Dive inImagine the same lunchbox packed two ways: one with three feathers rattling around inside, one crammed with marbles right to the lid. Same box, same space — but the marble one is heavier, because there's more stuff squeezed in. That "how much stuff per space" is exactly what density means.
This is why a small lump of lead feels surprisingly heavy while a big sponge feels like almost nothing. The lead has loads of matter jammed into a tiny space; the sponge is mostly air. Hold one in each hand and you're feeling density with your own muscles.
By the end of this page you'll have a number for it — density = mass ÷ volume — and you'll use it to predict, every single time, whether something will float or sink the moment it hits water.
Density is built out of two simpler ideas, so let's pin them down first.
Mass is how much stuff — how much actual matter — something is made of. You feel mass as weight: more mass, heavier in your hand. We measure it in grams (g) and kilograms (kg).
Volume is how much space something takes up. A beach ball has a big volume; a marble has a tiny one. We measure it in things like cubic centimetres (cm³) or millilitres (mL).
Density is the deal between the two: how much mass is squeezed into each little chunk of space. The recipe is a single division:
density = mass ÷ volume
Read it like a question: "for every cubic centimetre of space, how many grams of stuff are in there?" Cram lots of mass into a small space and the density is high (lead, gold, a pebble). Spread a little mass over a big space and the density is low (a sponge, a marshmallow, a balloon). The point of dividing is that it makes a fair comparison — it ignores how big or small the lump is and asks only how tightly packed it is, so a chip of lead and a whole brick of lead come out with the exact same density.
Here are two boxes of the exact same size — same volume, every time. Slide marbles into each one. Each marble adds 1 g of mass. Watch what happens to the density when you pack more stuff into the same space.
density = mass ÷ volume → Box A: 6 g ÷ 10 cm³ = 0.6 g/cm³ · Box B: 14 g ÷ 10 cm³ = 1.4 g/cm³
See the trick? The boxes are the same size the whole time, so volume never changes. The only thing you're changing is how much stuff is inside — the mass. When the mass goes up but the space stays the same, the density climbs. That's the whole meaning of "denser": more packed into the same room.
Now slide both boxes to the same number. The densities match exactly — because density doesn't care which box you call A or B, only how tightly each is packed. This little box is a model for real materials: a block of cork is like Box A with hardly any marbles, and a block of lead is like Box B stuffed past the lid.
Here's where density earns its keep. Whether something floats isn't about being big or small, heavy or light. It's about one head-to-head comparison: how dense are you, compared to the water around you?
Water has a density of about 1 gram per cubic centimetre (1 g/cm³) — a tidy, friendly number that makes it the perfect referee. So the rule is beautifully simple:
less dense than water → floats · denser than water → sinks
That's it. If a material packs less stuff into each cubic centimetre than water does, the water can hold it up and it bobs. If it packs more in than water, it loses the contest and sinks. Cork (about 0.24 g/cm³) and wood (around 0.5 g/cm³) float because they're loosely packed — full of tiny air pockets. Steel (about 7.8 g/cm³) and lead (about 11.3 g/cm³) sink like, well, lead, because their atoms are crammed in far tighter than water's. Time to test it.
Every object here is the same size — so size is taken out of the question, and only density is left to decide. Tap an object to drop it in. Watch the readout race its density against water's 1.0, and notice how deep each floater sits.
Densities are rounded, everyday ballparks (g/cm³). Plastics vary a lot — a foam cup floats, a solid plastic ruler can sink — so "plastic" here is just one heavier example.
Did you spot the secret hiding in the floaters? They don't all bob at the same height. Cork rides high with most of itself in the air, because it's so much lighter than water. Ice sits low, with only a sliver poking out — its density (about 0.92) is so close to water's 1.0 that it just barely wins the float. In fact, an object's density tells you the exact fraction that hides underwater: ice is about 0.92 as dense as water, so roughly 92% of an iceberg sits below the surface. That's the famous "tip of the iceberg," and it's pure density.
Hold on — you just watched a solid lump of steel plummet to the bottom of the tank. Steel is way denser than water. So how does a 200-metre cargo ship, made of thousands of tonnes of the stuff, sit happily on the ocean?
The answer is one clever word: shape. A ship isn't a solid block of steel. It's a giant hollow bowl, and most of what's inside that bowl is air. To decide if something floats, you don't ask "how dense is the metal?" — you ask "how dense is the whole object, air pockets and all?" That's its average density: the total mass of everything spread across the total volume it takes up.
Spread all that steel out into a huge, mostly-empty hull, and the average density of the ship-plus-air drops below water's 1.0. So it floats. Crush that same ship into a solid cube — squeeze all the air out — and its density shoots back up, and down it goes. It's the very same trick as a sponge or a life jacket: trap enough air inside, and you pull your average density below water.
Once you're watching for it, density is everywhere — quietly running the show.
Ice floats, and that's a really big deal. Most things get denser when they freeze, but water is a rebel: it gets less dense as it turns to ice, which is why ice cubes bob in your drink instead of sinking. Out in the world, that means lakes freeze from the top down — a lid of floating ice forms on the surface while liquid water stays below it. Fish and other creatures can survive the winter in that liquid layer underneath. If ice sank, lakes would freeze solid from the bottom up, and that would be very bad news for everything living in them.
Oil floats on water. Tip cooking oil into a glass of water and it gathers in a shiny layer on top, refusing to mix in. That's because oil is a little less dense than water, so it wins the float and rises to the surface. It's the same reason an oil spill spreads across the top of the sea instead of vanishing into the deep — and the same reason a salad dressing separates into layers if you leave it standing.
Liquids will even stack into layers by density when you pour them carefully: honey at the bottom, then water, then oil on top, like a little rainbow in a jar — densest on the bottom, least dense floating up high. Each layer is just losing or winning its own float contest with the layer below.
Let's predict before we peek. Suppose you've got four same-sized cubes, and a friend tells you their densities:
Cube 1 — 0.3 g/cm³. Less than water's 1.0, so it floats — and floats high, like cork. Cube 2 — 0.9 g/cm³. Still under 1.0, so it floats — but only just, sitting low in the water like ice. Cube 3 — 1.0 g/cm³. Exactly the same as water: it hovers, drifting in the middle, neither rising nor sinking. Cube 4 — 2.6 g/cm³. More than 1.0, so it loses the contest and sinks.
Notice you never needed to know what the cubes were made of, how big they were, or how heavy they felt. One number each — the density — and the comparison with water settled every case. That's the power of turning "how tightly packed?" into an actual number: prediction becomes almost automatic.
This is the trap almost everyone falls into, so let's spring it carefully. The myth says weight decides floating — heavy sinks, light floats. But think about that giant log from the very start of this page. It's enormously heavy; you could never lift it. And yet it floats, no problem. Meanwhile a single steel paperclip — light enough to balance on your fingertip — sinks straight away.
So weight alone is a liar, and so is size. A huge log floats; a tiny pebble sinks. What actually matters is the combination — how much mass and how much space, packed together into a density. The log is heavy, but it's also huge and full of air-filled wood fibres, so its density stays low. The pebble is light, but it's also tiny and tightly packed, so its density is high. Heavy-but-spread-out floats; light-but-crammed sinks.
This is exactly why the tank demo gives every object the same size. By freezing the volume, it strips away the "but it's bigger!" and "but it's heavier!" distractions, and leaves only the thing that truly decides the contest: density. Whenever someone insists heavy things must sink, just point at a ship.
A mystery block appears above the water with its density printed on it. Before it drops: will it float or sink? Remember the rule — race it against water's 1.0. Tap your call, then watch it fall.
Score: 0 right out of 0
After a few rounds, you'll feel it instantly: your eyes jump to the number, compare it to 1.0, and you just know. Under 1.0 floats, over 1.0 sinks — no guessing, no lifting, no getting fooled by how big the block looks.
Density = mass ÷ volume — how tightly matter is packed into a space.
Less dense than water (1.0) floats; denser than water sinks.
Trap air inside and your average density drops — that's how a steel ship floats.