Illinois has established a specific formula for calculating the amount and duration of spousal maintenance obligations or awards. This Illinois alimony calculator makes Illinois maintenance calculations using the Illinois alimony formula.Estimate Alimony Payments. Illinois uses a formula outlined in the Illinois Marriage and Dissolution of Marriage Act to calculate spousal maintenance. To calculate Illinois guideline maintenance, you must enter gross income, child dependent, and marriage length information into the calculator. , your trusted Chicago divorce attorney. To get an estimate based on your specific situation, use our Alimony Calculator for Illinois Couples. Our Illinois divorce attorneys can discuss spousal maintenance and any other aspects of your divorce at no cost. The support amount varies depending upon many factors, each situation is different. Is alimony (maintenance) calculated off Gross or Net income in Illinois?