When you trade stocks on margin, you borrow part of the money needed from your broker and put up the rest. Once you've bought the stock, you must keep a minimum percentage of equity called the maintenance margin. Stock markets set minimum requirements for maintenance margins at about 25 percent. Again, your broker may want more. Since you will get a margin call and have to deposit more money if a stock price decline causes your equity to drop too low, it's important to know how to calculate the maintenance margin for a particular trade.
Calculate the amount of money per share borrowed from your broker for your margin trade. To do this, subtract the margin requirement from 1 and multiply by the market (purchase) price. Suppose you buy stock at $40 per share with a 60 percent margin requirement. The amount you borrow is equal to $40 x (1 – 0.60), or $16 per share.
Calculate the maximum percentage of borrowed money allowed by the maintenance margin. Simply subtract the maintenance margin requirement from 1. For example, if your broker sets the maintenance margin at 25 percent, the maximum allowable percentage of borrowed funds is equal to 1 minus 0.25, or 0.75 (75 percent).
Divide the amount per share you borrowed by the maximum percentage of borrowed funds you are allowed. If you borrowed $16 per share and the maximum percentage of borrowed funds is 75 percent, you have $16.00/0.75 = $21.33. This is your maintenance margin in dollar terms. If the market price falls to $21.33 or lower, your broker will issue a margin call. You must either deposit more money or your broker will close out the transaction to recover the money you borrowed.