How a margin call actually happens, and the price you can compute in advance

Two requirements, and only the second one bites

Regulation T governs the initial extension of credit and generally limits it to 50% of the purchase price, so 50,000 of stock takes 25,000 of your own money. That rule is the one everybody knows and it almost never causes a problem, because it applies at the moment you buy, when nothing has gone wrong yet.

Maintenance is the rule that matters. FINRA Rule 4210 requires equity of at least 25% of the current market value of long positions, and brokers set house requirements above that, commonly 30% to 40%, higher on concentrated, volatile, or low-priced names. Equity here means market value minus the loan. As the price falls, the loan stays fixed and the equity falls faster than the value does, which is the whole mechanism.

The trigger price, worked

Buy 1,000 shares at 50 for 50,000, borrowing 25,000. At a 30% house maintenance requirement, the call arrives when equity divided by market value falls to 0.30. Market value minus 25,000 must be at least 0.30 of market value, so 0.70 of market value must cover the 25,000 loan, which puts the threshold at 35,714 of value, or 35.71 per share. That is a fall of 28.6%.

The general form is short enough to keep in your head: trigger price equals the loan divided by the shares, divided by one minus the maintenance rate. At the FINRA floor of 25% the same position triggers at 33.33, a fall of 33.3%. Five percentage points of house requirement moved the trigger by 2.38 a share, almost 5% of the purchase price, which is why the house number is the one to ask for rather than the regulatory minimum.

• Get the house maintenance requirement for the specific security, not the general one.
• Compute the trigger price before entering the position.
• Compare it against a normal one-year decline for that asset.
• Recompute it whenever you add to the position or the loan grows.

Three things people learn the expensive way

The broker does not have to call you. Margin agreements generally permit liquidation without prior notice, and the broker chooses which positions to sell, which will not be the ones you would have chosen. The word “call” implies a conversation and a deadline; the contract usually implies neither.

House requirements change during the position. A name that becomes volatile, or heavily shorted, or the subject of a pending event can have its requirement raised, sometimes to 100%, and the trigger price you computed at entry moves up underneath you. This is the mechanism behind most of the forced-selling stories that get told as conspiracies.

And the loan accrues daily. A position that goes sideways for a year on margin has lost the interest, which means the break-even is not the entry price, it is the entry price plus carry. That is the least dramatic item on the list and the one that quietly does the most damage.

Size it so the call cannot arrive

Our position: if the trigger price sits inside the range the asset routinely travels in a year, the position is too large, and no amount of conviction fixes that. Leverage does not improve an edge. It multiplies the edge and the variance together, and it is the variance that reaches the threshold where you stop being allowed to hold the trade.

The forced sale is what makes leverage qualitatively different from a big position. An unleveraged holder who is wrong for eighteen months and right afterwards collects. A leveraged holder with the same view gets sold at the bottom and collects nothing. Sizing is the only defence, because by the time the call arrives every remaining option is bad.