Formula

PED = % change in Quantity Demanded             PED = (New QD – Old QD) x100

                      % change in Price                          ,                  (New P – Old P) x 100

Inelastic  : The PED value is between 0 and -1. It tends to have few substitutes, is necessities, and/or can be

addictive, e.g., alcohol, cigarettes, or petrol.

Elastic     : The PED value is between -1 and -∞. It has a lot of substitutes and may be an inferior good.