The calculation formula of the sum of principal and interest in primary accounting is: sum of principal and interest = principal+principal × annual interest rate× deposit period. The sum of principal and interest refers to the sum of principal and interest, in which interest is the reward obtained by the owner for lending funds, and the calculation formula is: interest = principal × interest rate× deposit period.

Principal is the original amount of a loan, deposit or investment before interest is calculated. As a whole, the principal of an enterprise (including fixed capital and working capital) is simultaneous, and it is spatially juxtaposed at different stages, showing different forms of principal occupation. Under the condition of market economy, due to the capitalization of various factors of production, the principal is often called capital.