Option Greeks in practice

Greeks measure sensitivities, they do not predict direction

This lesson shows how to read delta, theta, vega, and gamma on a single option position and how to calculate an approximate premium change from those numbers. It is for a reader who already knows option construction and premium decomposition and wants to see what their position reacts to day by day.

The Greeks describe how position value changes under small changes in assumptions. They do not say where price will go. To start, it is enough to track three of them on one position, because the definitions only make sense once you have numbers read from the platform.

Delta, theta, vega, and gamma — what each one measures

A long put has approximately negative delta. A delta of 0.50 is often described intuitively as half the exposure of a stock — a useful simplification, but not an accounting rule. Near expiry, gamma for an ATM option can be sharp: a small price move translates into a large change in exposure, which is why short-dated written options require particular caution. Units can be a trap — theta is sometimes quoted per day and sometimes per year, so check with your broker in which unit the data is presented.

How to calculate the premium change from the Greeks

The first-order formula, that is, an approximation for small changes:

Δpremium ≈ delta × Δspot + theta × Δdays + vega × ΔIV

You count the day increment in sessions, theta is taken per day, and vega describes the premium change per one percentage point of IV. The result applies to one underlying unit; for the contract amount, multiply it by the multiplier.

Example with illustrative numbers: long call, spot 100 USD, premium 3.00 USD, delta 0.50, theta -0.10 USD per day. After two sessions spot rises by 1%, that is, by 1.00 USD. The delta contribution is 0.50 USD, and time takes away 2 x 0.10 USD, that is, 0.20 USD. The approximate premium is therefore 3.00 + 0.50 - 0.20 = 3.30 USD. The platform may at that moment show 3.40 USD with delta at 0.55. The difference is not a measurement error: delta itself changed because gamma acted, and on top of that came a change in IV. The formula shows direction and order of magnitude, not a model price.

Protocol: five sessions of recording and comparison

This sequence turns reading about the Greeks into measuring your own position.

1. Choose one liquid position: an ATM call with 30-45 DTE, sized so small that the premium does not exceed 1% of capital.
2. Before entry, record spot, premium, delta, theta, vega, IV, and the number of days to expiry.
3. Every day after the session closes, record the same fields. Record the session day, not the calendar day — theta accrues in sessions.
4. Calculate the approximate premium change with the formula above and compare it with the actual change on the platform.
5. Record the difference and assign it to one of three sources: gamma, a change in IV, or an event in the market.
6. After five sessions, total up how much of the premium change was explained by spot, how much by time, and how much by volatility.

Without step five, the protocol turns into copying numbers from the platform into a sheet.

Table to fill in: Greeks journal

The DTE column is mandatory. The same delta and theta mean something different 45 days before expiry and 5 days before it — without DTE, the sheet mixes conditions that cannot be compared.

Greeks of one position and of the whole portfolio

Tracking one position teaches the mechanics, but with several open options the Greeks add up. Two long call positions on correlated companies are double delta, not two independent exposures — the relationship between instruments is described by the correlation entry. Before opening the second and third option position at the same time, sum up the delta and vega of the whole portfolio, so you know what total price and volatility move you are exposed to. Individual position cards without that sum give a false sense of control over risk, and the method of setting size based on maximum loss is described in the lesson on managing an option position.

Exchange-traded options and CFD options — are the Greeks the same

KNF publishes the risks of complex products aimed at retail investors — that is an educational warning, not a safety ranking. Taxation and the method of settling profits depend on the product and your situation, so if in doubt, direct the question to a tax adviser. Before the first short position, check three conditions: maximum loss in dollars fits within your adopted threshold, you know whether the position is long or short vega, and the structure itself is not an uncovered short. Structures with a defined maximum loss are discussed in the lesson on basic strategies.

When the Greeks say do not trade

1. You do not know whether you are long or short vega. Without that answer, you do not know whether a rise in volatility helps your position or destroys it.
2. You hold a position with negative gamma near expiry. Then a small price move changes exposure faster than you can react.
3. The passage of time is the only reason you hold the position. Theta works for you only until the first jump in price or volatility.
4. You have not calculated the portfolio’s total delta and vega before adding another position. A third position in the same direction is not three ideas, it is one large exposure.

Exercise

On a liquid underlying instrument, buy one ATM call on demo with 30-45 DTE, sized at less than 1% of capital. For five sessions, fill in the Greeks journal, and each day compare the premium change with the formula. At the end, write one sentence answering what changed the premium most: spot, time, or volatility. Goal: five rows of measurement and one conclusion based on your own numbers. You will find the version of the same discipline for the account as a whole in the lesson on daily limit and drawdown.

Check yourself

Common questions

Which Greek should I start with?

With delta, which describes direction, and theta, which describes the cost of time. Add vega once you notice that the premium changes without a move in the underlying. Three numbers on one position teach more than twelve read without context.

Why is gamma sharp near expiry?

Because the delta of an ATM option then changes quickly on small price moves. Position exposure floats and the risk of a sudden change in result grows. Hence caution with short positions close to expiry.

Is short vega bad?

Not by definition. A short vega position benefits from a fall in IV, and that is the basis of many income strategies. The problem appears when a defined maximum loss is missing, as in an uncovered short. Start with structures that have calculated risk, for example spreads.

Why measure the Greeks when the position result is visible?

Because the position result is a number without a cause. The same loss can come from a price move, from the passage of time, or from a fall in volatility — and each of those causes requires a different reaction. Measurement separates them into three distinct contributions, so you know whether to hold the position, reduce size, or close it early. Without measurement you improve the strategy by feel, and feel leaves no data to compare.