Start with the formula
Every California child support number comes out of one equation in Family Code section 4055:
CS = K x [HN - (H%)(TN)]
The pieces:
- CS is the child support amount
- K is the income allocation factor
- HN is the higher earner’s net disposable income
- H% is the higher earner’s timeshare, the share of time the kids are with them
- TN is the two parents’ combined net disposable income
K is the part most people have never heard of, and it’s the part that does the heavy lifting.
Where K comes from
K is two numbers multiplied together:
K = K fraction x timeshare adjustment
The K fraction
The K fraction depends on the parents’ combined net (TN). It’s set by five income bands written into 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 |
Most working families sit in that bottom row, above $15,000 combined, which puts the K fraction somewhere around 0.12 to 0.20.
The timeshare adjustment
The second number pushes K up when the higher earner spends less time with the kids:
- H% of 50% or less: adjustment = 1 + H%
- H% above 50%: adjustment = 2 - H%
So 50/50 custody (H% = 0.50) gives a 1.50 adjustment, while a 20/80 split (H% = 0.20) gives 1.20.
Working an example
Take two parents with $20,000 of combined net and equal 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
A K of 0.27 means 27% of the timeshare-adjusted income gap flows through as child support.
Why bother understanding it
K is the reason your number is what it is. The bigger it gets, the more of the income gap turns into support, and it climbs in two situations: when combined income is low (the lower bands carry a higher K fraction) and when the higher earner has less custody time (a larger timeshare adjustment). Knowing that tells you which levers actually move your number and which don’t.
The calculator shows the K factor as it computes it, with your income band highlighted, so you can see exactly which row of the table applies to you. That link opens the worked example above.