# Eli5: If water can’t be compressed under normal conditions, then how does water pressure work?

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Eli5: If water can’t be compressed under normal conditions, then how does water pressure work?

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Are you asking about home plumbing or the bottom of the ocean?

Water is compressible, just not by very much. For example 4 km under the ocean, water’s density is increased by just 1.8%. This is so small that saying water is in-compressible make for a good approximation.

Pressure does not depend on how compressible a substance is. It just depends on the total weight per unit area of all the substance above, pressing down.

To “compress” something is to make it smaller by putting it under pressure.

You can put water under pressure just fine, but it won’t get smaller. It stays the same size, now at a higher pressure.

It is “pressurized” but not “compressed”.

If water can’t BE compressed itself, then it would TRANSMIT that pressure forward. That takes into account what’s called “head pressure” where the water on top’s weight presses down (62 lbs per cubic foot) on the water below it. That can add up to a whole lot. It’s why your lungs feel emptier diving to the bottom of a 10ft pool than they do just under the surface.

Lets say you have two syringes, one full of air, one full of water. You seal them so nothing gets out.

If you push on the air plunger, you can move it in, and if you move it in halfway, the pressure is about double what it started at(for more info on that ‘about’ search for adiabatic vs isothermal compression.)

If you push on the water plunger, the pressure rises in proportion to how hard you push, without changing in volume.

Pressure is just how much force there is pushing against an area, compression is about how much stuff you shoved into a given volume.