Is Zakat on money to be paid after deducting various family expenses such as school installments, children's expenses, and other household needs?

Zakat (definition) is only obligatory on wealth that has completed a full year (hawl) and reached the minimum threshold (nisab (definition)). What is spent during the year on household expenses or other needs is not included in the zakat calculation. As for school installments paid during the year, they are not counted. However, if they become due at the completion of the hawl, they are to be deducted from the wealth on which zakat is due, provided you do not possess other non-zakat-eligible wealth sufficient to cover the debt. Otherwise, they are not deducted, and you pay zakat on the entire wealth, then settle the debt.