The Système International d'unités - the SI system - is the coherent system of units adopted and recommended by the General Conference on Weights and Measures (CGPM). It is based on seven base quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance and luminous intensity.

Pressure is not a base quantity but a derived quantity, with dimensions of length (L), mass (M) and time (T). This can be demonstrated by considering the two fundamental ways of measuring pressure: directly in terms of force-per-unit-area measurements and with liquid columns.

(i) Pressure is defined as force per unit area but force = mass × acceleration (Newton's second law of motion) and acceleration is rate of change of velocity. Thus if pressure is force/area, it equates to (mass × rate of change of velocity)/area. This gives pressure the dimensions of mass × length/(time² × length²) which simplifies to mass/(length × time²) or M.L^-1.T^-2. Thus, from the definition, it can be shown that pressure is derived from three base quantities; mass, length and time.

(ii) The pressure at the bottom of a fluid column is calculated by multiplying together the density of the fluid, the acceleration due to gravity and the height of the column (p=dgh). Since density is mass/volume it has dimensions mass/length³. Acceleration is rate of change of velocity so it has dimensions length/time². The vertical distance is simply length so the product dgh has dimensions mass/length³ × length/time² × length, which simplifies to M.L^-1.T^-2 and is dimensionally identical to the force/area calculation in (i) above.