Monthly Child Support Calculator. This calculator provides an estimate for a single source of income.These forms allow parents, families, and employers to provide the Child Support Division with additional information so we can better serve you. Under Texas law, child support is actually calculated as a percentage of monthly net resources, not a percentage of income. There are multiple ways to open a child support case. The Texas Family Code outlines a percentagebased formula, contingent on the number of children, to determine the monthly support obligation. What Is the Percentage of Child Support in Texas? For instance, fluctuations in the income of a 1099 independent contractor can significantly impact child support modifications. How Many Children Are Supported? In Texas, the family code dictates the factors that are used to calculate child support payments.