The standard computation of child support under the child support guidelines presupposes that all of the children of the parties will live with the same parent. When the children are divided between the parents, the child support must be adjusted.

American families present courts with a wide variety of relationships. Parents may have two children, with each having custody of one. A family may have four children. After the parents separate one might live with the father, two might live with the mother, and the fourth child may go back and forth between the parents. Another family may have three children, with two spending the school year with the mother and one with the father, reversing the custody arrangement during the summer.

Calculating child support under these circumstances requires a comparison of the theoretical child support each would owe and entering an order requiring a payment of the net difference by the parent with the higher theoretical child support. However, because the guidelines do not explain exactly how the child support should be calculated, it is left to the courts to resolve the issue. Take for example a situation in which there are three children, with two living with the mother and one with the father. One way of calculating child support is to compute the father's support of two children living with the mother, based on the family having only two children, and to compute the mother's support of one child living with the father, based on the family having only one child. If the father's income is greater than the mother's, the father would pay to the mother the difference. A more complicated calculation would start with the total amount the parties should use to support the three children. The court would then prorate this amount between the parents based on the number of children and the amount of income of each party.

Adjustments to the base rate for items such as work related day care, extraordinary medical expenses, or schooling must also be prorated between the parties based on their comparative income. An another adjustment and more complications arise when in addition to split custody of some of the children, there is shared custody of others. In these situations, the court or agency will seek a reasonable approach and will impose child support obligations in a manner that the court believes is fair.