overdraft

Q: I am married with a three month old son. My wife and I both work. We live in a very modest lifestyle and Alhamdulliah just about manage to afford our essentials every month (rent, food, electricity, etc). However, once all the expenses are paid, our bank account is in an overdraft. We have been living in a bank overdraft for almost a year now and are trying our best to get out of it due to Allah's ruling on interest. Is Zakaah still obligatory on us? If yes, how do I calculate a percentage on a negative bank balance?