To calculate the required withdrawal from an Individual Retirement Account, use the formula shown on the IRA Required Minimum Distribution Worksheet, according to the IRS. The worksheet is available on the IRS website. There are several tables for use with the worksheet that correspond to marital status and life expectancy.
The minimum distribution applies only if you are at least 70 1/2 years old, according to the IRS. Line one of the worksheet lists your age. To start the calculation, enter the year you reached the age when you are taking the distribution on line two, below your age. Enter the IRA's balance on December 31st of the previous year on line three. Then, refer to the table which corresponds to your marital status. The distribution period numbers in the table are based on age. Find the distribution number that corresponds with your age, and then enter it on line four of the worksheet. The final step is to divide line three by line four. For example, if the balance is $100,000 and you are age 70 1/2, then place $100,000 on line three and place 27.4 on line four. Divide these numbers to calculate the mandatory distribution of $3,649.63.