Calculating Interest Worksheets - Math Worksheets Center For example, on a private loan of $30,000 concluded six years at 8.40% p.a. and making scheduled repayments: (0.084 ÷ 12) x 30,000 = 210. As we've now begun to wage off our principle, to work out the interest we pay in the following months, we need first to calculate our new balance. So: Principal - (repayment - interest) = new balance.