Is zakat due on the returns only, or on the principal and the returns together?

Zakat is obligatory on the principal amount and its returns if the principal reaches the nisab and a full hawl (lunar year) passes over it. Thus, profit follows the principal in Zakat. If the money was invested in trade, the profit must be added to the capital and both are subject to Zakat together when the hawl is completed on the principal. If commercial goods were purchased with the money, their value is estimated upon the completion of the hawl, and Zakat is paid at a rate of 2.5% of the total money and profits.