The formula for compound interest, compounded once per year, is:
A = P(1+r)^t, where r is the annual interest rate as a decimal fraction and t is the number of years.
If interest is compounded twice per year, then the above formula is changed to:
A = P(1+r/2)^(2t), where the '2' indicates twice-per-year compounding.
If interest is compounded n times per year, then the above formula is changed to:
A = P(1+r/n)^(nt).