A unit is a particular physical quantity, defined and adopted by convention, with which other particular quantities of the same kind are compared to express their value. A physical quantity is a quantity that can be used in the mathematical equations of science and engineering. The value of a physical quantity is the quantitative expression of a particular physical quantity as the product of a number and a unit, the number being its numerical value.
Thus, the numerical value of a particular physical quantity depends on the unit in which it is expressed. When making measurements, it is customary to record both the quantity (how much) and the unit (of what).
Science and technology depend largely on the unit of measurement. For example, the value of the height h of building is h = 120 m. Here h is the physical quantity, its value expressed in the unit “meter,” unit symbol m, is 120 m, and its numerical value when expressed in meters is 120.