Child Support Formula

The child support formula is also known as the basic formula and formula 1. It is a complex calculation that uses an income shares model to make a child support assessment.

The formula determines how much the children cost to raise based on age and number and the combined income of the parents. Each parent’s individual financial responsibility is calculated based on their share of combined income. The annual child support payment represents the difference between a parent’s financial responsibility and what they contribute through direct care.

Formula structure

The child support formula is the core calculation used to produce an assessment. It brings together income, care, and the calculated cost of children into a single annual amount. Each step builds toward that outcome.

The formula follows an income shares approach. Both parents are responsible for the total cost of raising their children. That total is calculated within the formula, then divided between the parents based on income and adjusted for the costs of care.

The full calculation runs through eight steps. These steps convert income and care into percentages, determine each parent’s share of responsibility, and then apply that share to the calculated cost of the children.

The formula is called “basic” because it applies to a single case with only two parents. Other situations adjust the inputs before this formula runs, but the structure remains the foundation of every assessment.

Example

Daniel and Sarah have 3 children aged 9, 7 and 5. The children live mostly with Sarah, who provides care for around 80% of the year, while Daniel provides care for the remaining 20%. Daniel has an adjusted taxable income of $90,000 and Sarah has an adjusted taxable income of $80,000.

Step 1 – Child support income
Each parent’s income is reduced by the self-support amount. This ensures each parent keeps a base level of income for their own living costs.
Daniel: $90,000 − self-support amount = child support income
Sarah: $80,000 − self-support amount = child support income

Step 2 – Combined income
The two child support incomes are added together to form the combined child support income.

Step 3 – Income percentages
Each parent’s share of the combined income is calculated.
Daniel has the higher share of income.
Sarah has the lower share of income.

Step 4 – Care percentages
Care is measured based on nights per year.
Sarah provides about 80% care.
Daniel provides about 20% care.

Step 5 – Cost percentages
Each care level is converted into a cost percentage using the care bands.
Sarah’s high level of care results in a high cost percentage.
Daniel’s lower level of care results in a lower cost percentage.

Step 6 – Child support percentages
Each parent’s income percentage is compared to their cost percentage.
Sarah’s cost percentage exceeds her income percentage, so she has a negative child support percentage.
Daniel’s income percentage exceeds his cost percentage, so he has a positive child support percentage and becomes the payer.

Step 7 – Cost of children
The formula calculates the total annual cost of raising 3 children of these ages based on the parents’ combined income.

Step 8 – Annual rate
Daniel’s positive child support percentage is applied to the calculated cost of the children. This produces the annual child support amount he must pay to Sarah.

This step-by-step process shows how the formula moves from income and care to a final payment. The result reflects the gap between each parent’s financial responsibility and the care they actually provide.