Calculate your average portfolio size. For a given period, add the beginning and ending value of your portfolio, then divide the number by two. For example, suppose you want to calculate a monthly turnover in which the value is $22,000 on April 1 and $22,900 on April 30. The average portfolio size is $22,000 plus $22,900 divided by 2, or $22,450.
Figure your purchases for the period. Add together the amounts you spent during the period to buy securities. Say, for this example, that you spent $2,000.
Add up the total value of securities you sold during the period. For example, you might have sold $1,400 of securities in April.
Divide the lesser of purchases and sales by the average portfolio value. In this example, you bought more than you sold, so divide the sold amount, $1,400, by the average value, $22,450. The result, 6.24 percent, is your monthly portfolio turnover. You can figure weekly or annual portfolio turnover in a similar way.