What is the correct formula for calculating interest on a loan?

The correct formula for calculating interest on a loan depends on whether it is simple or compound interest. For simple interest, the formula is ( I = P \times r \times t ), where ( I ) is the interest, ( P ) is the principal amount, ( r ) is the annual interest rate (in decimal), and ( t ) is the time in years. For compound interest, the formula is ( A = P (1 + \frac{r}{n})^{nt} ), where ( A ) is the total amount after interest, ( n ) is the number of times interest is compounded per year, and the other variables are as previously defined.