text: anyone know how the volume of a cubic metre of air changes with atmospheric pressure ? (ie, what is the new 'volume' with each millibar increase or decrease) the wisea$$ answer is: a cubic metre is a constant volume. only the amount of air molecules inside that volume will change with pressure.  the correct question would be: what volume would a sample of air occupy at a given pressure that occupied 1 cubic metre at a given starting pressure (at constant temperature)?  k = p*v/t  or p1*v1 = p0*v0.