Why Early Exercise of an American Call Is Usually Wrong

Early exercise of an American call only makes sense right before a dividend large enough to clear a precise interest rate threshold.

Exercising an American call before expiry throws away whatever time value remains in the option in exchange for intrinsic value the holder can capture just as well by selling the option in the market, so early exercise is almost always a mistake on a stock that pays no dividend. The one real exception is a call on a stock about to pay a dividend large enough that missing it costs more than the remaining time value is worth, and that condition can be checked with a precise formula rather than a guess.

A call option's price is always at least equal to its intrinsic value plus some amount of time value, as long as there is time remaining and the stock could plausibly move further in the holder's favor. Exercising converts the option into stock, capturing only the intrinsic portion and forfeiting the time value outright. Selling the call in the market instead captures both pieces at once, which is strictly better whenever the option still has time value left to sell, and as the Black-Scholes mechanics show, that time value only reaches zero exactly at expiry.

There is also an interest rate argument working against early exercise of a call. Exercising means paying the strike price now instead of later, and paying money sooner rather than later is a cost, not a benefit, when interest rates are positive. Both effects, the loss of time value and the loss of interest on the deferred strike, push in the same direction: hold the call, do not exercise it.

A dividend flips part of this logic because holding the call through an ex-dividend date means missing the dividend entirely, while exercising the day before captures it by converting into actual shares that receive the payment. The decision comes down to comparing the size of the dividend against the value of the option's remaining time, and the interest that would otherwise be earned by not exercising.

The standard condition says early exercise just before an ex-dividend date can be worth considering only when the dividend exceeds the strike price multiplied by one minus the discount factor covering the period from that ex-dividend date to the next dividend or to expiry, whichever comes first. In plain terms, the dividend has to be bigger than the interest the holder would otherwise earn by deferring payment of the strike over that remaining stretch.