What is meant by Propensity to consume (or Consumption function)?

Meaning of Consumption Function. Consumption function shows the mathematical relation between income and consumption i.e. how much of income is spent on consumption goods. Simply put, consumption function (or Propensity to consume) means proportion of income spent on consumption. Its two features are noteworthy. (i) At zero or very low level of income, consumption expenditure is higher than income because minimum consumption is necessary for survival, and (ii) As income increases, consumption expenditure also increases but increase in consumption is less than the increase in income. Mind, level of household consumption depends directly on the level of income. Thus consumption (C) is a function (f) of income. Symbolically: C = f(Y).
Consumption function (linear, i.e., straight line consumption function) is represented by the following equation.


Thus total consumption (C) comprises two components (i) Autonomous Consumption  not influenced by income, and (ii) Induced Consumption (bY) influenced by income. For example, the consumption equation C = 30 + 0.75 Y means र 30 is autonomous consumption which is incurred without having any income. As income increases, 75% of additional income is spent on consumption. In short, consumption equation  shows that Consumption (C) at a given level of income (Y) is equal to autonomous consumption  times of given level of income. Some numericals for further clarification are given below.