Idaho uses the Idaho Child Support Guidelines (IRCP Rule 126) to determine child support obligations. Under the Income Shares model, both parents' incomes are combined to estimate what the family would have spent on the child if the household were intact. The obligation is then divided proportionally based on each parent's contribution to combined gross income.

Idaho has several distinctive features in its child support system. Idaho uses 91 overnights (25%) as the threshold for shared physical custody. The state uses a detailed Guidelines Table that is updated periodically and reflects Idaho's cost of living. Idaho imputes minimum wage income ($7.25/hour) for voluntarily unemployed parents.

Idaho bases calculations on gross income — meaning income before taxes and deductions. This includes wages, salaries, commissions, bonuses, self-employment income, and other sources. The self-support reserve in Idaho is $1,200/month, ensuring the paying parent retains enough income to cover basic living expenses.

Idaho recognizes "Shared Physical Custody" when the non-custodial parent has the children for at least 91 overnights per year (approximately 25% of the time). When this threshold is met, the standard child support formula is adjusted to reflect the increased direct expenses borne by both parents.

Under Idaho's Income Shares approach, the shared parenting adjustment typically calculates each parent's obligation based on their income share, then applies a credit or cross-calculation that accounts for the time each parent directly supports the child. The more overnights the non-custodial parent has, the greater the reduction in their payment — but the adjustment is not dollar-for-dollar because fixed costs like housing don't decrease when the child is away.

Below 91 overnights, Idaho generally treats the arrangement as a sole custody situation with standard visitation, and no custody time adjustment is applied to the child support calculation.