Parametric modelling (or parametric design) is the creation of a digital model based on a series of pre-programmed rules or algorithms known as 'parameters'. That is, the model, or elements of it are generated automatically by internal logic arguments rather than by being manually manipulated.
Typically, parametric rules create relationships between different elements of the design. So for example, a rule might be created to ensure that walls must start at floor level and reach the underside of the ceiling. Then if the floor to ceiling height is changed, the walls will automatically adjust to suit. Other examples might include the height of window cills above floor level, the relationship between walls and a pitched roof, the relationship between floor area and the size of windows or number of luminaires and so on.
The same algorithm might be used throughout a model so that if a particular element or rule is changed, it changes throughout the model. In effect the model is a representation of all the rules that the user has defined.
Historically, parametric modelling has tended to be used for fairly specialist design tasks. For example, the Beijing National Stadium constructed for the 2008 Summer Olympics used specialist parametric modelling software to develop a bowl geometry optimised for athletics that would also work well for football after the Olympics.
Building information modelling (BIM) has introduced a certain amount of parametric modelling into mainstream building design. BIM is a very broad term that describes the process of creating an managing digital information about a building or other facility (such as a bridge, highway, tunnel and so on). BIM software is generally based on the definition of objects, but these can have parametric attributes attached, such as colour. So for example if a colour scheme for a building is changed, every object that has that colour attribute will also change. Other parameters might include; positional data, dimensions, manufacturer's data, algorithms describing form and so on.
However whilst BIM objects and their attributes can be changed, the relationship between objects defined, and the model updated automatically, the objects themselves tend not to be designed or modelled parametrically. The model is an assembly of objects, rather than the a composition generated by logic, and there is some suggestion that this object-based approach is incompatible with true parametric design.
BIM is helping the move away from design based on the laborious and error prone manual manipulation of 'dumb' objects, and increasingly it includes the capability of using 'smart' parametric logic, but there is some way to go before it is able to encompass the wide range of rules that govern the way buildings are designed, and enables design possibilities to be tested based on complex interrelationships.
See also: Parametricism.