There are two ways of calculating the necessary amount of child support. The simpler way is to refer to the chart created by the Tokyo and Osaka Child Support Research Group in 2003. This chart is used by the Family Court in Japan and may be viewed here in Japanese:
In order to calculate the amount of child support, all you need to know is the incomes of both parents, whether the parents receive a salary or are self-employed, and the number and ages of the children. However, the chart only applies to situations where there are no more than three children.
Where there are four or more children, it is necessary to adopt the second method and calculate the amount of child support by taking into account various expenses. This is an extremely complicated process, and we strongly recommend that you consult an attorney about the calculation.