When you’re trading options, it’s important to keep in mind that options are derivatives. This means they trade based on the price action of an underlying security. Understanding how option prices will react to changes in the underlying security is key.

At first glance, option pricing can appear relatively simple. The price of a call option increases as the underlying stock (or ETF) goes up. The price decreases as the price of the stock declines. (The price of a put option increases as the underlying stock’s price falls and decrease as the stock rises.)

But, of course, there are a lot of other factors in play. While the primary driver of option prices still remains the action in the underlying stock, option prices are also affected by volatility, time decay, and interest rates, among other forces.

Let’s take put selling for example. With this strategy, you need to estimate the potential return and the amount of capital at risk. Our primary means for doing this is derived from assuming a worst-case scenario. (This would be where your puts are assigned and are required to buy the underlying stock.) You would then determine how much capital would be at risk if the stock traded to a level where our assigned position was stopped out and sold.

This type of assessment is helpful for option contracts that are actually converted into a stock position. But it doesn’t help us get an idea where an option contract will be trading before the assignment actually takes place. Let’s get into that today…

Delta — The Primary Measure of Option Price Movement

The options Greek delta refers to the degree to which an option contract reacts to a $1 movement in the underlying stock. The values range from 0 to 1 for call options and 0 to -1 for put options.

For example, a call option with a delta of 0.5 would be expected to increase $0.50 for every dollar that the underlying stock rises. If a call has a delta of 1, the price would be expected to move in lockstep with the underlying stock price.

A put option with a delta of -0.5 would be expected to increase $0.50 for every dollar that the underlying stock fell. And a put with a delta of -1 would move in line with the underlying stock price.

Delta is calculated for individual option contracts. It’s typically available as part of any standard brokerage platform, option charting, or quote service.

The important thing to realize is that delta can change rapidly depending on the dynamics for a specific option contract. The two primary factors affecting delta are how far in or out of the money an option is trading and how much time is left until expiration.

In the Money vs. Out of the Money

For most of us, the majority of put contracts that we sell are out of the money. This means the strike price for the put option is below the current stock price, and the stock price must fall before we become obligated to purchase shares.

Delta decreases (moves closer to zero) for option contracts the further they are out of the money. This is because options that are significantly out of the money have a much lower probability of being exercised. So the less the likelihood that the put contract will be exercised, the less it is worth in absolute terms, and the less it matters if the stock moves up or down by a small amount.

In contrast, the further in the money an option contract is, the higher the delta (i.e., the closer to 1 for a call or -1 for a put) and the more in line the option will trade with the stock. If the stock is trading well below the strike price for your put contract, there is a high probability that the put will be exercised and we will be required to take delivery of the stock.

Of course, if the stock is trading fairly close to the strike price, there will be a material amount of uncertainty as to whether the option will be assigned. This is when it is particularly important to pay attention to the options delta to determine how much the underlying option contract will react to a change in the stock price.

The following table illustrates the point with a fictional “XYZ” stock trading at different prices in relation to its call and put options at the $75 strike price:

The Amount of Time Until Expiration

The second important factor affecting delta is how long the option contract has until expiration. If an option is set to expire very soon, then there will be much more certainty as to whether the contact will be exercised.

In this case, if the option is out of the money, its nominal price and its delta will naturally migrate toward zero as expiration approaches. Because the option is unlikely to be exercised, it has very little value. Therefore, a small fluctuation in the underlying stock price won’t affect that value very much.

If the option is in the money, the delta will naturally migrate toward 1 (call) or -1 (put) as time runs out. This means that pricing for the option contract will fluctuate more in line with the underlying stock because the probability of the option actually being converted into a stock position is very high.

All else being equal, the longer the amount of time until an option expires, the lower the delta measurement will be. This is because a small fluctuation in the day-to-day pricing of the stock does not necessarily have a significant effect on the long-term expectation for the stock.

So, from a timing perspective, delta is higher when there is less time until expiration and lower when there is more time until expiration.

Putting Delta Into Practice

It is important to keep an eye on the delta of option contracts. It helps you know what to expect in terms of profit and risk from your positions.

For example, when you are selecting a put option to sell, the closer the delta is to zero, the more likely the put will expire worthless. This allows you to keep the premium you collected with no obligation to buy the underlying stock.

Once you have sold a put, if the delta for the contracts rises unexpectedly, this can be a good signal to take a close look at your position to make sure everything is still trading as expected. In this type of scenario, you are more likely to have the put contract assigned, leaving you with a long position in the underlying stock.

If you’re familiar with selling puts, then you know this doesn’t have to be a bad thing. But only if you’re selling puts on stocks that you wouldn’t mind owning at that price. But you’ll need to monitor the situation carefully in case the stock is breaking down for an unexpected reason.

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