A formula is an equation that establishes a relationship among the variables involved in it. The subject of a formula is a variable that is expressed in terms of the other variables. The subject of a formula always lies on the left hand side of the equality sign. The coefficient of the subject of a formula is always one. The subject of a formula does not link with any other quantities. Finding the value of the subject by substituting the values of the other variables is called the evaluation of the subject of a formula. Formulae are used to calculate perimeters, areas of closed figures, volumes of solid objects, and so on.