The multiplier effect refers to the increase in final income arising from any new injection of spending. The size of the multiplier depends upon household's marginal decisions to spend, called the marginal propensity to consume (mpc), or to save, called the marginal propensity to save (mps).

The following general formula to calculate the multiplier uses marginal propensities, as follows:

  1 
1 - mpc

Hence, if consumers save 0.8 and spend 0.2 of every N1 of extra income, the multiplier will be:
 1  =    1    = 5
1 - 0.8 0.2