CNG is made by compressing natural gas (which is mainly composed of methane [CH4]), to less than 1% of the volume it occupies at standard atmospheric pressure. It is stored and distributed in certified (tested) high pressure pressure cylinders or tanks at a pressure of 200–248 bar (2900–3600 psi), usually of a cylindrical or spherical shape. CNG's volumetric energy density is estimated to be 42% of liquefied natural gas's (because it is not liquefied), and 25% of diesel's.