A pressure vessel is a structure designed to contain a fluid at a different pressure to the pressure surrounding the structure without changing volume.

Examples of pressure vessels are: diving cylinder, recompression chamber, nuclear reactor vessel, habitat of a space ship, habitat of a submarine, pneumatic reservoir and hydraulic reservoir.

In industrial sector, pressure vessels are designed to certain pressure and temperature, both technically referred to as "Design Pressure" and "Design Temperature". Because the pressure exceeds normal pressure which people can handle in manual operation, the design of pressure vessels are governed by design codes such as ASME (American Society of Mechanical Engineering), PED (Pressure Equipment Directive of the EU), JIS (Japan), and other international standards.

No matter what shape it takes, the minimum mass of a pressure vessel scales with the pressure and volume it contains. For a sphere, the mass of a pressure vessel is

Where M is mass, P is pressure, V is volume, d is the density of the pressure vessel material, and s is the maximum working strain that material can tolerate. Other shapes besides a sphere have constants larger than 3/2.

So, for example, a typical design for a minimum mass tank to hold helium (as a pressurant gas) on a rocket would use a spherical chamber for a minimum shape constant, carbon fiber for best possible d/s, and very cold helium for best possible mass/PV. There is no theoretical efficiency of scale to be had in a pressure vessel.