Calculating discretionary income is simple. There’s no need to get out your protractor or call your friend with a Ph.D. in Economics. It’s a simple three-part formula that requires basic arithmetic. Once you burn this formula into your brain, you’ll begin to see every dollar that flows in and out of your household in a different light.

Discretionary income is calculated by taking your household total income and subtracting two kinds of expenses from it: fixed and variable expenses, which I’ll discuss in a moment.

On paper, it looks like this:
Total monthly income
– Fixed expenses
– Variable expenses
_______________________
= Discretionary income

That’s it!