The multiplier effect (K) refers to when the increase in (the change in) income is greater than the increase in investment spending*.
K = change in income / investment spending*
For example, if the change in income is 20 and the change in investment spending is 10, k would be equal to 2.
This can also be written as:
K = 1/1 - (marginal propensity to consume)
K = 1/1-MPC
Or this can be written as
K = 1/ marginal propensity to save
k = 1/MPS
*NB: The multiplier effect works with autonomous changes in spending of all kinds aggregate demand eg: investment, consumption, government, export and import from:
Aggregate Demand = C + I + G + (X-M)