Average propensity to consume (APC) is the percentage of income spent. To find the percentage of income

Income

Income is the consumption and savings opportunity gained by an entity within a specified time frame, which is generally expressed in monetary terms. However, for households and individuals, "income is the sum of all the wages, salaries, profits, interests payments, rents and other forms of earnings...

 spent, one needs to divide consumption

Consumption (economics)

Consumption is a common concept in economics, and gives rise to derived concepts such as consumer debt. Generally, consumption is defined in part by comparison to production. But the precise definition can vary because different schools of economists define production quite differently...

 by income, or
.

Sometimes, disposable income

Disposable income

Disposable income is total personal income minus personal current taxes. In national accounts definitions, personal income, minus personal current taxes equals disposable personal income...

 is used as the denominator instead, so
.

Where C is the amount spent, Y is pre-tax income, and T is taxes

The inverse is the average propensity to save

Average Propensity to Save

The average propensity to save , also known as the savings ratio, is an economics term that refers to the proportion of income which is saved, usually expressed for household savings as a percentage of total household disposable income. The ratio differs considerably over time and between countries...

 (APS).

Average propensity to consume (APC) is the percentage of income people desire to spend.

It is key to note that Average Propensity to Consume (APC) is very different from Marginal propensity to consume

Marginal propensity to consume

In economics, the marginal propensity to consume is an empirical metric that quantifies induced consumption, the concept that the increase in personal consumer spending occurs with an increase in disposable income...

(MPC). These two values are often confused.