When interest is added every six months, one must use the half-yearly interest rate for twice as many periods.
Example: If the interest rate is 4% per annum with semiannual compounding, and the money is invested for 3 years, then the values to use in the calculation are 2% for 6 periods.
When interest is added every month, one must use the annual interest rate divided by 12 for 12 times as many periods.
Example: If the interest rate is 6% per annum with monthly compounding, and the money is invested for 3 years, then the values to use in the calculation are 0.5% for 36 periods.