This includes the amount of money you will be investing or saving, the length of the term and the proposed interest rates. You may have several different interest rates that you want to compare. Divide the number by 100 and then divide this interest rate by 365, the number of days in a year. This will give you the interest rate to use in the formula. An annual percentage rate of .5 percent or .005, when divided by 365, is equal to .00137 percent, or .0000137. If the principle is $10,000, when multiplied by .0000137, it is equal to $.1370. Rounded up, this account will earn approximately $.14 per day, based on these examples. Multiply the principal, $10,000, by the annual percentage rate of .5 percent or .005 to calculate interest manually. The answer is $50.00. Multiply the daily interest amount of $.1370 by 365 days; the answer is also $50.00.

Summary:
Gather the details needed to calculate interest. Convert the percent interest rate to a decimal. Multiply the principal by the daily interest rate. Check your math.