Option Greeks Explained: Delta, Gamma, Theta and Vega

The Greeks measure how an option's price responds to a change in one input while the others hold still. Delta responds to the underlying's price, gamma to delta itself, theta to time, and vega to volatility. They are sensitivities, not predictions.

This guide covers what each one means, how they change as expiry approaches, which ones actually matter for the strategies most Indian index traders run, and how to read them across a multi-leg position.

The four that matter

Greek	Measures sensitivity to	Typical units
Delta	A one-point move in the underlying	Points of option price per point of index
Gamma	How much delta itself changes	Change in delta per point of index
Theta	The passage of one day	Points of option price lost per day
Vega	A one-point change in implied volatility	Points of option price per IV point

There is a fifth, rho, measuring sensitivity to interest rates. For intraday and weekly index options in India it is small enough that it rarely changes a decision, which is why it is usually left out of practical discussion.

Delta

Delta tells you roughly how much the option price moves for a one-point move in the index. A call with a delta of 0.5 gains about half a point when the index rises one point.

Calls have positive delta, puts negative. At-the-money options sit near 0.5 in absolute terms; deep in-the-money options approach 1.0, and far out-of-the-money options approach 0.

Delta as a rough probability

Delta is also commonly read as a rough proxy for the probability of finishing in the money. A 0.30-delta option is loosely treated as having around a 30% chance of expiring in the money. It is an approximation rather than an identity, but it is a useful intuition when choosing strikes.

Gamma, which is what actually hurts sellers

Gamma measures how fast delta changes. It is the second derivative, and it is the Greek that turns a comfortable short position into an uncomfortable one.

A short option with a small delta feels safe: the index moves a little and your position barely responds. But as the index approaches your strike, delta rises — and gamma is the rate at which it rises. Your exposure grows precisely as the move continues against you.

Why expiry day feels different

Gamma is highest at the money and rises sharply as expiry approaches. A short at-the-money option on expiry day has very high gamma, which is why the same strategy that behaves gently on a Monday can behave violently on a Thursday.

Theta

Theta is the amount an option loses per day purely from time passing, holding everything else constant. It is negative for buyers and positive for sellers.

Theta is the mechanism by which an option-selling strategy makes money when nothing happens. A position that opens and closes with the index unchanged is profitable for the seller because a day has passed.

Why decay accelerates

Decay is not linear. It accelerates as expiry nears, and it is concentrated in at-the-money options — far out-of-the-money options have little premium left to decay. This is why near-expiry at-the-money selling is popular, and why it carries the gamma problem described above.

The trade-off is exact and unavoidable: the options with the most theta to collect are the ones with the most gamma to carry. There is no strike that gives you one without the other.

Vega

Vega measures sensitivity to implied volatility. If IV rises, option prices rise — for calls and puts alike — independently of which way the index moved.

Sellers are short vega. A volatility spike raises the value of options you sold, producing a loss on paper even if the index has not moved against you at all.

Losing money on an unchanged index

This is the source of a genuinely confusing experience: the index sits exactly where you expected, and your short position is losing. Vega explains it, and it usually accompanies the margin requirement rising at the same moment.

A worked reading of one option

Illustrative figures, to show how the numbers combine rather than to represent any current quote.

Suppose a call option shows delta 0.45, gamma 0.004, theta −8 and vega 12, with the index at 24,000.

If this happens	Expected effect on the option price
Index rises 20 points	About +9 points, from delta (0.45 x 20)
Index rises 20 points, then another 20	More than +9 on the second move — delta has risen, because gamma
One day passes, nothing else changes	About −8 points, from theta
IV rises by 1 point	About +12 points, from vega
Index unchanged, IV falls 2 points, one day passes	About −32 points: −8 theta, −24 vega

The last row is the one worth studying. Nothing happened to the index, and the option lost substantial value. For a buyer that is a bad day with no obvious cause; for a seller it is a good one.

How the Greeks change near expiry

Greek	Far from expiry	Near expiry
Delta	Changes gradually	Flips sharply around the strike
Gamma	Moderate	Very high at the money
Theta	Slow decay	Rapid decay
Vega	Significant	Diminishes — less time for volatility to matter

The pattern is that near expiry everything concentrates at the money. Away from the strike, options become nearly inert; at the strike, they become extremely sensitive. That concentration is the defining character of expiry-day trading.

Reading the Greeks for a multi-leg position

Individual-leg Greeks are less useful than the position's net figures, and the netting is simply addition with the right signs.

A worked reading

For a short straddle, taking both legs together:

Net delta near zero at entry. The short call's negative delta and the short put's positive delta roughly cancel at the money.
Net gamma strongly negative. Both legs are short, and both contribute negative gamma. This is the position's main exposure.
Net theta positive. Both legs decay in your favour.
Net vega negative. A volatility rise hurts both legs.

Read that list and the strategy explains itself: you are being paid theta to carry negative gamma and negative vega. That is the trade, stated in Greeks rather than in words.

How protection changes the picture

An iron condor has the same signs with smaller magnitudes, because the bought legs offset part of each exposure. That is what the protection is buying, and it is why a condor's margin is lower than an equivalent strangle's.

What the Greeks do not tell you

Direction. They measure sensitivity to a move, not whether one is coming.
Whether an option is cheap. Vega tells you the sensitivity to IV, not whether current IV is high or low in context.
What happens in a gap. Greeks assume small, continuous moves. A gap open is exactly the case they describe least well.
Liquidity. A theoretically attractive position on a strike nobody trades is not attractive.

Do you need the Greeks to trade options?

Not to place a trade, no. Plenty of people run rule-based option strategies without computing a single Greek, and a structured builder does not ask you for them.

What the Greeks give you is an explanation. When a short position loses on an unchanged index, vega explains it. When a position that was comfortable all morning becomes uncomfortable in minutes, gamma explains it. Without that vocabulary those events look random, and things that look random are hard to plan for.

The useful level of understanding

The practical level of knowledge is: know which Greeks your strategy is short, and know which market condition hurts each one. That is enough to anticipate most of what will happen.

Where to see them

Arthalab's Options Analysis screen shows payoff and greeks modelling alongside the option chain, so you can see a structure's net exposure rather than computing leg by leg. Reading the option chain covers the raw data the Greeks are derived from.

The short version

Delta is sensitivity to price; gamma is how fast delta changes
Theta is what pays an option seller; gamma is what they pay it with
The strikes with the most theta have the most gamma — there is no free version
Vega explains losing money on an unchanged index, and tracks margin rises
Near expiry everything concentrates at the money
Greeks describe small continuous moves; a gap is where they help least

Frequently asked questions

No. They are estimates valid for small changes, and they shift as the inputs move. Using them to predict the effect of a large move is where they are most often misused.

Yes, and routinely. A day passing costs theta, and a fall in implied volatility costs vega. Both can act while the index is unchanged.

For a short options position, gamma — because it determines how fast your exposure grows if the index approaches your strike. Theta is working for you in the background and needs no watching.

Both, but the net figures across the position are what matter. Individual leg Greeks rarely tell you what you need to know about a multi-leg structure.

Measures of how an option's price responds to one input changing while the others hold still — the underlying's price (delta), the rate delta changes (gamma), time passing (theta), and implied volatility (vega).

Gamma. Theta is what you are paid, but gamma is what you are carrying — it is the reason a short position accelerates against you rather than losing at a steady rate.

Usually vega. A rise in implied volatility raises the value of options you sold regardless of direction, and it typically raises your margin requirement at the same time.

It is a rough approximation, not an identity. A 0.30-delta option is loosely treated as having around a 30% chance, which makes delta a useful strike-selection heuristic.

Gamma is highest at the money and rises sharply as expiry approaches. The same strategy that behaves gently earlier in the week can move violently on expiry day.

No. You can run rule-based strategies without computing any. What the Greeks give you is an explanation for why a position behaved the way it did, which makes the behaviour predictable rather than mysterious.

They are two sides of the same trade. The options with the most time decay to collect are the ones whose delta changes fastest. There is no strike offering one without the other.

For short-dated index options, sensitivity to interest rates is small enough that it rarely changes a decision. It matters more for long-dated options.

Net them across legs with the right signs. The net figures describe the position; individual legs rarely tell you what you need to know.

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