Every liquid is perfectly compressible but for most use cases *assuming* the liquid hasn’t compressed is sufficiently accurate.
Water has a similar squishyness to chalk. Compressing water is about as easy as compressing a lump of chalk – so not particularly easy, but compared to a lump of steel water is very easy to squish.
Very few applications get to the pressures where liquid compressibility is significant. It compresses about 1% per 200 atmospheres of pressure. This is about where hydraulics operate, and honestly that 1% doesn’t matter calculation wise.
However in a water jet cutter, operating at 4000 atmospheres, the water has reduced in volume quite a bit.
In a reasonably modern diesel high pressure fuel system, which operates around 2000 bar (or higher) the fuel compresses by about 30% and accounting for this effect is important in correctly designing the system. It’s one of the few common systems where this effect matters.
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