The guideline formula
California child support is calculated using this formula from Family Code section 4055:
CS = K x [HN - (H%)(TN)]
Where:
- CS = child support amount
- K = the income allocation factor
- HN = higher earner’s net disposable income
- H% = higher earner’s timeshare (percentage of time with children)
- TN = total net disposable income of both parents
How K is computed
K is the product of two components:
K = K fraction x timeshare adjustment
K fraction (from income bands)
The K fraction depends on the combined net disposable income (TN) of both parents. It comes from five statutory bands defined in FC section 4055(b)(3), as amended by SB 343:
| Combined net (TN) | K fraction formula |
|---|---|
| $0 to $2,900 | 0.165 + TN/82,857 |
| $2,900 to $5,000 | 0.131 + TN/42,149 |
| $5,000 to $10,000 | 0.250 (constant) |
| $10,000 to $15,000 | 0.10 + 1,499/TN |
| Over $15,000 | 0.12 + 1,200/TN |
For most working families, TN falls above $15,000, giving a K fraction around 0.12 to 0.20.
Timeshare adjustment
The timeshare adjustment makes K larger when the higher earner has less custody time:
- When H% is 50% or less: adjustment = 1 + H%
- When H% is over 50%: adjustment = 2 - H%
At 50/50 custody (H% = 0.50), the adjustment is 1.50. At 20/80 (H% = 0.20), it’s 1.20.
A worked example
Parents with combined net of $20,000 and 50/50 custody:
- K fraction: 0.12 + 1,200/20,000 = 0.18
- Timeshare adjustment: 1 + 0.50 = 1.50
- K = 0.18 x 1.50 = 0.27
This means 27% of the income difference (adjusted by timeshare) becomes child support.
Why it matters
Understanding K helps you understand why your support number is what it is. A higher K means more of the income gap goes to support. K is highest when:
- Combined income is low (the K fraction is higher in lower bands)
- The higher earner has less custody time (timeshare adjustment is larger)
See it in action
The calculator shows the K factor computation with the band highlighted, so you can verify exactly which formula applies to your situation.