Zakaat on a joint loan

Q: Myself and my husband had taken a home loan of 12 Lakhs (Joint loan, interest based) in 2006. We got rid of the loan in 2010. To calculate my Zakat for these years, should I deduct my assets from 12 lakhs or 6 lakhs? For 3 years I was paying whole of the installment, 4th year onwards both of us paid half and half.