When interest is compounded every six months, one must use the half interest rate for double the number of periods.
Example: If the interest rate is 4% per annum with semi-annual compounding, and the money is held for 3 years, the relevant information to calculate is 2% for 6 periods.
When interest is compounded monthly, one must use the annual interest rate divided by 12 for 12 times the number of periods.
Example: If the interest rate is 6% per annum with monthly compounding, and the money is held for 3 years, the relevant information to calculate is 0.5% for 36 periods.