What is the ruling on zakat for commingled money belonging to a husband, his wife, and their son? Is it obligatory for them to pay zakat for past years, given the uncertainty of having reached the nisab or determining each individual's share? And how is the zakat nisab calculated for gold of varying karats?

Zakat is obligatory on money jointly owned by spouses and a son, for each individual's share, if it reaches the nisab (minimum threshold) and a hawl (one lunar year) has passed. Their wealth should not be combined to complete the nisab because, according to the majority of scholars, commingling is not considered for Zakat purposes except for livestock.

If the exact share of each individual or the amount of money in past years is unknown, estimation and diligence are required to determine each person's share. Whoever's share reaches the nisab and a hawl has passed is obligated to pay Zakat, even if the money is earmarked for specific purposes such as marriage or construction.

If a person does not know or doubts whether the conditions for Zakat are met for the money, the default is that one's liability is cleared.