How would a couple calculate Zakat on a joint account?
The answer
Primarily, each spouse is responsible to calculate and pay their own Zakāt. However, one spouse may pay Zakāt on behalf of the other.
In short: each spouse pays on their own share, or on half if shares cannot be determined.
Explanation
If the current share of one spouse in the balance can be determined on their Zakāt paying day, the spouse will be responsible to pay Zakāt on that amount. He/she will add her share of the balance to one’s total gross assets when calculating Zakāt. However, if each spouses’ current share cannot be determined, a husband and wife may treat the balance co-owned equally. Thus, each spouse will be responsible to pay Zakāt on 50% of the balance.1
And Allah knows best.
Mufti Faraz Adam
References
1وعلى هذا ذكرَ في ((الفتاوي الخيريّة))(1: 111-112) أنّه لو اجتمعَ زوجٌ وامرأتُهُ وابنه في دارٍ واحدة، وأخذَ كلٌّ منهما يكتسبُ على حدة، ويجمعان كسبهما، ولا يعلمُ التَّساوي ولا التَّفاوت ولا التَّمييز، فيجعلُ المالُ بينهما على السّوية، وكذلك لو اجتمعَ أخوةٌ يعملونَ في تركةِ أبيهم، ونماءُ المالِ فهو بينهم على السّوية، وإن اختلفوا في العملِ والرأيُّ وهذا كلّه في غيرِالأبِ وابنه، (عمدة الرعاية ج 6 ص 301 دار الكتب)
Based on the Hanafi school and on the question as it was asked. Another mufti may reach a different conclusion. Read the disclaimer.